Section Ii.—Articulata.

Order 3. PROTREMATA.—This group, formerly very abundant, is now almost extinct, the Family Thecidiidæ representing the Order at the present day. Thecidium mediterraneum (Fig. 9) forms little oval boxes about a third of an inch in length, shaped somewhat like a pear cut in half (peduncle valve), and with a semicircular lid (brachial valve) working on a hinge on the upper flat surface. The foramen and peduncle are absent; but between the pointed end of the peduncle valve and the hinge is an area filled in by a calcareous plate characteristic of the Protremata.

Fig. 9.

Thecidium mediterraneum. A, natural size. B, section through shell. Magnified. ]

The brachial valve opens like the lid of a snuff-box, and shuts down on the least alarm with the rapidity of lightning. The peduncle valve is fixed on the rocks by its convex surface. The species is common in the Mediterranean in from 30 to 300 fathoms, and is also found in the West Indies.

Order 4. TELOTREMATA.—This group, which at the present day contains the largest number of species, includes the Lamp shells, so called from their resemblance to an ancient lamp. The valves are joined by a well-marked hinge, the peduncle passes through the peduncle valve through a foramen completed by two plates secreted by the mantle edges, and the brachial valve has attached to it a calcareous scaffolding of processes or loops for the support of the “arms.”

The shells in this group are frequently ridged. Their colour is usually white, but sometimes red or yellow; deep-sea forms are generally vitreous.

Rhynchonella psittacea has a black shell with a pointed incurved beak; each of the arms forms a many coiled spiral and can be protruded beyond the shell; the brachial skeleton is comparatively small and simple, consisting of two separate processes.

In Terebratulina the brachial skeleton forms a simple loop; in Magellania the loop is reflected on itself (Fig. 2).

The beautiful and unique specimen of Dyscolia wyvillii, from 390 fathoms W. Indies, is remarkable for its size, being over two inches in length. The small vitreous specimens of Terebratula wyvillii were obtained off Chili from a depth of 2160 fathoms; specimens of the same species were obtained also from a depth of 2900 fathoms in the North Pacific.

TUNICATA.

The Tunicata are marine animals, the majority of which live, in their adult stage, a stationary life, fixed to the rocks or sea-bottom, but a comparatively small number are free-swimming.

Fig. 1.

Ascidia mentula from the right side. at, atrial aperture; br, branchial aperture; t, test.

They occur in the form of cartilaginous or leathery sacs, fleshy incrustations, solid fleshy masses, free-swimming, barrel-shaped animals, solitary or united into chains or hollow cylinders; or, lastly, of minute free-swimming tadpole-shaped organisms. To explain briefly the structure of a Tunicate, Ascidia mentula (Fig. 1), is selected. The animal, which lives on a muddy bottom, in from five to twenty fathoms, resembles a conical sac fixed by the broader end, of grayish green colour and about 4 inches in height. At the narrower end are two orifices, one terminal—the branchial orifice or mouth, and the other a little lower—the atrial orifice: the former has eight lobes and the latter six.

When the Ascidian is undisturbed, the orifices are wide open, and currents enter by the branchial and leave by the atrial orifice. On the least alarm, the orifices close, jets of water being at the same time squirted out; hence the popular name “Sea-squirts” given to these animals.

Fig. 2.

Diagrammatic section of Ascidia representing the three sacs, and the branchial sac as the pharynx or throat.

a, branchial; and b, atrial orifice; c, tunic or test; d, mantle; e, branchial sac; f, gullet; g, stomach; h, anal orifice; i, dorsal lamina; dotted line indicates the endostyle. ]

The Ascidian is orientated as follows: hold the animal with the branchial orifice pointing forwards and the atrial upwards; the branchial orifice will be anterior and the opposite end posterior; the atrial orifice will lie on the upper or dorsal aspect, the opposite aspect being lower or ventral, and the sides right and left. The aspects, in fact, correspond with those of a vertebrate animal. A vertical section roughly shows the animal to be formed of three concentric sacs (Figs. 2, 3). The outermost, which is tough and membranous, is called the Test or Tunic, the whole group owing its name to the presence of this protective covering.

Fig. 3.

Diagrammatic dissection of A. mentula.

at, atrial orifice; br, branchial orifice; a, anal orifice; brs, branchial sac; dl, dorsal lamina; end, endostyle; m, mantle; ng, nerve ganglion; oea, orifice of gullet; pbr, peribranchial cavity; st, stomach; t, test; tn, tentacles.

(After Herdman: Tunicata, Encyc. Britannica.) ]

The middle sac, termed the Mantle, which almost corresponds in shape to the outer, is composed of connective tissue, muscle-fibres, blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from the test except at the orifices and at a point behind, where vessels enter the test.

The innermost or Branchial Sac is attached behind the branchial orifice and along the ventral edge, but otherwise hangs free in the interior, the space around and outside of the sac being termed the atrial or peribranchial cavity.

The delicate walls of the branchial sac, which resemble fine muslin, are perforated by innumerable vertical slits, termed stigmata, arranged in transverse rows (Fig. 4).

Fig. 4.

Ascidia mentula. Part of wall of branchial sac showing stigmata. Magnified. ]

The margins of the stigmata are lined with cilia which set up currents; and the water which enters by the branchial orifice, passes through the stigmata into the atrial cavity, and thence out through the atrial orifice. The walls of the branchial sac are chiefly composed of a sieve-like meshwork of fine blood-vessels arranged in transverse and longitudinal rows. The currents of water passing through the stigmata aërate the blood in the vessels. Besides the stigmata, the branchial sac has two relatively large orifices, viz., the branchial orifice or mouth, and, at the opposite end, the opening into the gullet. The branchial sac is, in fact, a capacious throat or pharynx (Diagram Fig. 2 and Fig. 14). Inside the branchial orifice is a circle of fine tentacles, which guard the entrance to the branchial sac. The food of the animal consists of minute animal and vegetable organisms.

It may be wondered how this food is secured, seeing that the currents of water are continually passing through the sieve-like walls of the branchial sac to the exterior again. Within the branchial orifice and above the branchial sac are two circular ciliated ridges with a groove between, which is full of viscid secretion; the cilia on the ridges direct particles into the groove where they are retained by the mucus.

Passing backwards along the ventral edge of the branchial sac is a thick-lipped furrow, which appears like a rod in the thin-walled sac, and hence is called the endostyle. This organ secretes the mucus which is carried up by ciliary action to the circular groove in front of the branchial sac, and thence to the gullet along a fold or crest, termed the dorsal lamina, situated along the dorsal edge of the branchial sac.

The gullet opens into a large stomach situated posteriorly on the left side of the branchial sac. The stomach opens into the intestine, which, after forming a loop, terminates in the anal orifice or vent opening into the atrial cavity.

The tubular heart lies below the stomach, a remarkable feature in the circulation consisting in the periodic reversal of the blood current. An elongated nerve ganglion is situated between the branchial and atrial orifices.

Fig. 5.

Ascidian Tadpole with part only of the tail C. Magnified section.

N, nervous system with enlarged brain in front and narrow spinal cord behind n; N′, cavity of brain; O, the single cerebral eye lying in the brain; a, auditory organ; K, pharynx; d, intestines; o, rudiment of mouth; ch, notochord or primitive backbone.

(From Gegenbaur’s ‘Elements of Comparative Anatomy.’) ]

Ascidia mentula is hermaphrodite. The egg develops into a minute tadpole-like larva which swims about by means of its tail. Water entering by the mouth passes out through the gill-slits. A nerve-tube extending along the back and tail is swollen in front into a brain-vesicle; and underneath the long nerve-tube behind the brain is a stiff skeletal rod or axis—the notochord—which constitutes the rudiment of a backbone. Inside the brain are two unpaired sense organs, an eye and an organ of hearing (Fig. 5). After swimming freely for a few hours, the larva settles down head foremost and fixes itself by papillæ on the anterior end (Figs. 6, 7). Presently the tail becomes absorbed, and the posterior end of the nerve-tube, and the brain with its eye and hearing organ, undergo atrophy, the nerve ganglion of the adult alone representing the cerebrospinal axis of the larva. The branchial sac and intestines develop greatly, and growth proceeds in such a manner that the mouth is pushed round to a position opposite to the fixed area, and gradually the animal becomes the adult ascidian.

Fig. 6.

Degeneration of Ascidian Tadpole to form the adult. The black pieces represent the rock or stone to which the Tadpole has fixed its head. ]

Fig. 7.

Very young Ascidian with only two gill-slits.

(Figs. 6, 7, from Lankester’s ‘Degeneration.’) ]

This wonderful metamorphosis presents a striking example of DEGENERATION resulting from the adoption of a fixed mode of life. The active free-swimming larva with its brain, eye, hearing organ, and muscular tail becomes transformed into a comparatively inert sac.

Fig. 8.

Tadpole of Frog and Ascidian. Surface view. (Lankester’s ‘Degeneration.’) ]

Fig. 9.

Tadpole of Frog and Ascidian. Diagram representing the chief internal organs. (Lankester’s ‘Degeneration. A chapter in Darwinism.’) ]

The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), not merely superficially, but also in its general structure and mode of development. The Tunicata are now generally regarded as a degenerate offshoot from the ancestral stock of the Vertebrata, in that the larva possesses a skeletal rod (rudimentary backbone) separating the dorsally situated nerve-tube (cerebrospinal axis) from the ventrally situated intestinal tube, the existence of the cerebral eye in the Ascidian tadpole further tending to confirm the truth of this theory. Apart from a knowledge of the course of their development, Tunicata would have been classed among the Invertebrata, but the structure of the larva clearly reveals the affinities of the group to the backboned animals.

Ascidia mentula belongs to the group of SIMPLE ASCIDIANS which are all fixed, and are either solitary or joined into colonies in which each individual or ascidiozooid has a distinct test of its own. In the COMPOUND ASCIDIANS, which form colonies by budding, the ascidiozooids are buried in a common investing mass and have no separate tests. In a third group, the SALPA-LIKE ASCIDIANS, the ascidiozooids are united to form free-swimming colonies shaped like hollow cylinders open at one end. The above three groups belong to one great Order—the ASCIDIACEA. A second Order, THALIACEA, includes the free-swimming Salpa and Doliolum, which exhibit alternation of generations in their life history. A third Order LARVACEA, includes very minute free-swimming forms which possess a tail in the adult stage. There are sixteen families of Tunicata.

The following is a tabular view of Prof. Herdman’s classifications:—

Order I. Ascidiacea.│Sub-order 1. Ascidiæ Simplices, 4 Families. „ │Sub-order 2. „ Compositæ, 7 Families. „ │Sub-order 3. „ Salpiformes, 1 Family. Order II. Thaliacea │3 Families. Order III. Larvacea │1 Family.

Order I.—ASCIDIACEA.

The Ascidiacea include the great majority of species. With the exception of the one genus Pyrosoma, they lead a fixed or stationary life.

Sub-order 1.—ASCIDIÆ SIMPLICES.

The Simple Ascidians are mostly solitary; in a few forms, however, colonies arise by budding from stolons, but each individual has a distinct test. The four families into which the sub-order is divided are chiefly characterised by the nature of the test, the number of lobes round the branchial and atrial orifice, and the character of the branchial sac.

In the family Molgulidæ the tough membranous test is often coated with sand; the branchial aperture is six-lobed, the atrial four-lobed, the branchial sac has long folds or pleats, and the stigmata are curved or arranged in spirals.

Molgula gigantea, which is one of the largest of the Ascidians, and which attains a length of over thirteen inches, forms a tough conical sac; the branchial and atrial orifices at the upper end have six and four lobes respectively. The test is leathery, smooth above, but coated with sand below. The exhibited specimen, which comes from the Straits of Magellan, has several specimens of the stalked Boltenia legumen attached to the lower part of the test.

The curious Molgula oculata (Fig. 10) has a soft oval or rounded body coated with sand. The branchial and atrial orifices have respectively six and four lobes. Specimens grow attached to the rocks and also live free in the sand. The surface of the test is provided with hairs, which adhere to the rocks and collect particles of sand. The adhesion not being very firm, specimens are easily detached by currents and collected into heaps by the eddies; when living in the sand only the two dark orifices are visible. The sand coating has been supposed to confer protection by mimicry of the environment; but Professor Lacaze Duthiers found, much to his chagrin, that the sandy tests of his specimens were of no avail in securing them from being devoured by crabs who seemed to scent their prey from afar.

Fig. 10.

Mogula oculata. a, branchial; b, atrial orifice. ]

In the family Cynthiidæ the test is usually leathery, the branchial and atrial apertures four-lobed, and the branchial sac folded into longitudinal pleats.

The genera Boltenia and Culeolus include species in which the body is attached to a peduncle.

The large exhibited specimen of Boltenia pachydermatina is 28 inches in length, the head being 4 and the stalk 24 inches long. The two four-lobed apertures are along one edge, the branchial being the lower; the body is marked with long deep furrows, and the stalk with transverse wrinkles. Culeolus perlucidus, from 1600 fathoms in the Southern Ocean, is in the form of a small pear-shaped head on a slender stalk, the total length being 4½ inches. The branchial orifice forms a transverse slit with raised lips near the stalk, the slit-like atrial orifice being near the rounded end of the body. Culeolus moseleyi, another slender-stalked form, was obtained from 2425 fathoms in the Central Pacific.

Fig. 11.

A. Styelopsis grossularia on shell. B. Tadpoles of same, × 9. a, branchial; b, atrial orifice. (B, after Sir J. Dalyell.) ]

The little Cynthiid Styelopsis grossularia (Fig. 11), popularly known as the “Currant Squirter,” occurs in the form of bright red hemispherical blobs on stones and shells; when undisturbed, the branchial and atrial orifices expand and project upwards. The eggs are brilliant red in colour. Sir John Dalyell was the first to discover the tadpole form, which is about ⅒ inch long (Fig. 11, B), and to observe the tadpoles become fixed and develop into fixed Ascidians. He calls the active little swimming larvæ “Spinulæ,” from their resemblance to small pins.

The family Ascidiidæ includes forms with a gelatinous or cartilaginous test; the branchial and atrial orifices usually have 8 and 6 lobes respectively; the branchial sac is without folds.

Ascidia mentula, described above, belongs to this family.

Chelyosoma is characterised by the test forming tortoise-like horny plates on the upper surface. The exhibited specimen of C. macleayanum (Fig. 12) comes from Greenland; the upper hemispherical part of the test is divided into 8 plates; the branchial and atrial orifices are situated in the joints between the plates.

Fig. 12.

Chelyosoma macleayanum, slightly enlarged. a, branchial; b, atrial orifice. ]

The fine specimen of Phallusia mammillata from Naples consists of several individuals partly fused together; the branchial and atrial orifices are wide open, and the mantle can be seen through the thick knobby translucent test.

In Rhodosoma the test is modified so as to form stiff plates recalling the valves of a bivalve shell. One plate is attached to the rocks, the other closing against the first like a lid; the anterior end of the animal with its branchial and atrial orifices is visible only when the lid is open. The Mediterranean species R. callense (Fig. 13) grows attached to the rocks. The little exhibited specimen is on a fragment of shell in front of a black patch. The figure shows specimens with the lid open and closed.

Fig. 13.

Rhodosoma callense, × 10. A, “valve” open; B, shut. a, branchial; b, atrial orifice. (After Lacaze Duthiers.) ]

Family Clavelinidæ. The body is attached to a creeping stolon or mass of stolons, from which new individuals arise by budding. The other three families of Simple Ascidians included solitary forms, but the Clavelinidæ are social, and form colonies wherein each individual has its own test.

Clavelina lepadiformis (Fig. 14) forms graceful crystal vases about an inch in height. The figure shows one individual, but usually the processes at the base extend out as stolons whence other individuals arise.

Diazona violacea, from Cornwall, forms beautiful purple disk-shaped colonies in which the ascidiozooids arise from a basal mass of stolons. Sometimes the ascidiozooids die down, leaving only a smooth violet pad, which in due time produces a new crop of ascidiozooids.

Fig. 14.

Clavelina lepadiformis; diagrammatic, showing the anatomy. The oval bodies are the eggs; at lower end lies the tubular heart; the root-like processes at the base grow into stolons, whence other ascidiozooids arise. ]

Fig. 15.

Perophora listeri; A, slightly, B, further magnified. Ascidiozooids in right, left, and lateral aspects.

a, branchial; b, atrial orifice. ]

The remarkable Rhopalæa neapolitana, from Naples, may be roughly compared to an hour-glass with a very long constriction. The test is smooth in the upper part, but knobby and encrusted with foreign bodies below. The upper or thoracic end contains the branchial sac, and the lower or abdominal portion the stomach, heart, and reproductive organs, the gullet and intestine traversing the whole length of the narrow central region. Although from its general structure Rhopalæa is a Clavelinid, it is not certainly known to produce buds.

Perophora listeri (Fig. 15) occurs in the form of little jelly-like transparent blobs rising by short stalks from a silvery thread-like stolon. Owing to their small size and transparency, it is possible to examine specimens alive under the microscope, the currents passing through the stigmata in the walls of the branchial sac, and the beating of the heart being distinctly visible. The rapid motion of the cilia surrounding stigmata gives the appearance of dark wheels all rotating in the same direction. The heart beats so as to drive the blood current so many times in one direction, and then after a short pause, in the reverse direction.

The exhibited specimen growing on an oyster shell, is from Plymouth.

Sub-order 2.—ASCIDIÆ COMPOSITÆ.

The Compound Ascidians are fixed forms, which give rise to colonies by budding, the individuals being immersed in a common mass and not possessing separate tests.

Although reduced to an extremely small size each individual or ascidiozooid of a colony possesses the same organs as a large Simple Ascidian, excepting that the former does not possess a separate test. Frequently the individuals of a colony are grouped into systems, in which the atrial orifices open into a common cloaca. The little ascidiozooids vary greatly in shape in the different families. In the Polyclinidæ, for instance, they are long, the organs being so to speak, drawn out, and being arranged in three regions, the thoracic, abdominal and post-abdominal, the first region containing the branchial sac, the second the stomach, and the third the heart and reproductive organs. In the Distomidæ, the body exhibits two regions, thoracic and abdominal, the heart and reproductive organs lying alongside of the stomach. The Botryllidæ comprise only one region, the stomach and the other organs being situated by the side of the branchial sac.

The Compound Ascidians include seven families which are characterised chiefly by the method of bud formation, and by the arrangement of the organs into one, two, or three regions.

It is only possible, from limits of space, to refer to a few interesting forms.

The species of Botryllus are those most commonly met with. They form richly coloured gelatinous incrustations on rocks and seaweeds. B. violaceus (Figs. 16, 17, and 18 D) is blue with white lines; B. smaragdus, green; B. marionis, brown with white and carmine; B. castaneus, purple, and so on. The individuals are arranged in circular systems with the branchial orifices round the circumference and the atrial orifices opening into a common central cavity (Fig. 17), the whole colony being composed of groups of systems.

Fig. 16.

Botryllus violaceus on seaweed. (After H. Milne-Edwards.) ]

The exhibited specimen of B. violaceus was grown in the tanks of the Biological Station at Plymouth. The red specimen of B. aurolineatus, from Naples, shows well the branchial and cloacal orifices. In Botrylloides, the individuals form elliptical or elongated systems.

Colella thomsoni was obtained near the Philippines at a depth of 10 fathoms. The specimen, which is about 7 inches in length, resembles an elongated head of clover on a thickened stalk. The individuals which compose the head are arranged in spiral lines, the atrial orifice of each ascidiozooid opening separately and not into a common cloaca.

Fig. 17.

A. Botryllus violaceus, magnified, showing two systems of 6 and 7 ascidiozooids. B. One ascidiozooid extracted.

a, branchial; b, atrial orifices; c, branchial sac; d, stomach.

(After H. Milne-Edwards.) ]

Colella quoyi (Fig. 18 A), from 25 fathoms off Kerguelen Island, forms a rounded head on a short peduncle, the total height being one inch. The ascidiozooids are arranged in vertical lines in the “head,” each line consisting of a double zigzag series.

Fig. 18.

Colonies of Ascidiæ compositæ, natural size. A. Colella quoyi. B. Leptoclinum neglectum. C. Pharyngodictyon mirabile. D. Botryllus.

(After Herdman, Challenger Report and Encyclopædia Britannica.) ]

Julinia ignota, from the Antarctic regions, forms long narrow colonies, which attain a length of nearly three feet. One end is attached, the rest of the colony apparently lying along the sea-bottom.

Amaroucium roseum from Naples forms translucent gelatinous masses; a slice is exhibited, showing the long slender ascidiozooids immersed in the mass.

Fig. 19.

Pyrosoma elegans, natural size. A. Side view of entire colony. B. End view of open extremity.

(Herdman: Tunicata, Encyclopædia Britannica.) ]

Pharyngodictyon mirabile (Fig. 18 C), from 1600 fathoms in the Southern Indian Ocean, resembles a small mushroom, and is about one inch in height. This species is one of the few deep-sea Compound Ascidians.

Leptoclinum albidum is a common and widely distributed species; it occurs in the form of thin white crusts. The glistening white appearance is due to the common test being densely crowded with minute stellate spicules of carbonate of lime.

The specimen of Leptoclinum neglectum (Fig. 18 B) encrusts a fragment of sponge.

Goodsiria pedunculata from the Straits of Magellan, forms a rounded cartilaginous mass attached by a short peduncle; sometimes several masses are attached to each other. Each of the small dark oval areas on the surface corresponds to the branchial and atrial orifices of one ascidiozooid.

Sub-order 3.—ASCIDÆ SALPIFORMES.

The Salpiform Ascidians comprise only one genus, Pyrosoma, which occurs in the form of free-swimming colonies shaped like hollow cylinders closed and rounded at one end and open and truncate at the other (Fig. 19). The wall of the cylinder is formed of a single layer of ascidiozooids (Fig. 20), so arranged that all the atrial orifices open into the interior of the cylinder, and all the branchial orifices on the exterior, the two kinds of orifices being at opposite ends of the body, and not close together, as in most simple and compound Ascidians.

Specimens vary in size from a few inches to upwards of four feet in length, and, as the name of the genus implies, they are brilliantly phosphorescent. Sometimes they occur in innumerable multitudes, giving rise to a zone of greenish light extending for miles. Professor Moseley records that during the voyage of the Challenger in the North Atlantic a huge specimen of Pyrosoma spinosum, four feet in length, was captured. On tracing his name on its body, the word came out in letters of fire.

Fig. 20.

Section through wall of Pyrosoma, magnified, showing a single layer of ascidiozooids.

br, branchial; at, atrial orifice; tp, process of the test; br s, branchial sac.

(Herdman: Tunicata, Encyclopædia Britannica.) ]

In Pyrosoma elegans (exhibited), from Naples, the ascidiozooids are arranged in verticils, and the mouth of the cylinder is surrounded by a movable diaphragm; the outer end of each ascidiozooid is provided with a membranous spine. Six species of Pyrosoma are known. Pyrosoma atlanticum is found in the tropical Atlantic and Antarctic; P. giganteum in the Atlantic, Pacific, and Antarctic; and P. spinosum in the South Atlantic.

Order II.—THALIACEA.

The Thaliacea are free-swimming Tunicates, which exhibit alternation of generations in their life history. There are three families, Salpidæ, Octacnemidæ, and Doliolidæ.

Salpidæ.—The Salpas are transparent barrel-shaped organisms which occur in abundance at the ocean surface. They are so transparent that they are rarely seen, except in calm weather from the side of small boats; yet they frequently swarm in countless multitudes. From five to ten bands of muscles partially or entirely surround the body, like hoops. The branchial and atrial openings are at or near the opposite ends of the body. The branchial sac has almost disappeared, the dorsal lamina and ventral gutter (or endostyle) alone remaining, the interval between the two on each side representing an enormous stigma; the dorsal lamina, or “gill” is the transversely striated band passing obliquely across the body and forming the only barrier between the branchial and atrial cavities. Water enters at the mouth, and, by the contraction of the muscle-hoops, is driven out through the atrial aperture at the opposite end, which is then closed by a sphincter muscle. The elastic walls of the body expand, and water again enters through the mouth, the valve-like lips of which prevent its being driven out that way. The Salpa swims along in jerks, and along with each gulp of water takes in Radiolaria, Foraminifera, etc., which are retained by the mucus of the endostyle and carried to the gullet. The Salpa, in fact, lives, as Professor Brooks observes, in a “living broth,” so abundant is the food supply.

The intestines usually form an oval mass termed the “nucleus,” which is a conspicuous object at the posterior end.

Fig. 21.

Posterior part of solitary form of Salpa democratica-mucronata, showing a chain of embryos nearly ready to be set free.

gem, young chain of Salpæ; st, stolon; t, test; visc, visceral mass. ]

The solitary Salpa above described is asexual. In the ventral region of its body it forms a stolon which becomes segmented into a series of buds (Fig. 21). As the stolon grows the end series of buds breaks off in the form of a chain and swims away, other chains being detached in succession. A chain is formed of individuals arranged in two rows, the individuals in each row being alternate (not opposite).

Each individual of a chain differs from the solitary individual in shape, arrangement of muscle bands, etc., but especially in having reproductive organs. The chain Salpid is hermaphrodite; the embryo develops into a solitary asexual Salpa which produces the chains by budding. The wonderful life history of Salpa was discovered by the poet Chamisso during a voyage round the world in 1819. He observes: “A Salpa mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother.” Here we have an example of “alternation of generations,” a sexual generation (chain form) giving rise to an asexual generation (solitary form), which latter produces the sexual generation.

Most of the species of Salpa have double names owing to the chain and solitary forms having been regarded as distinct species before they were known to be phases in the life history of one and the same species. Salpa runcinata-fusiformis, solitary form (Fig. 22 B), is barrel-shaped, truncated at each end, with terminal orifices, and with nine muscle-bands on the dorsal surface, some of which converge towards each other. An individual of a chain (Fig. 22 A) is fusiform, with six muscle-bands, and with the orifices not terminal, but at each end of the dorsal surface.

The solitary form of S. africana-maxima is barrel-shaped, with truncated ends and terminal orifices, and with nine broad parallel muscle-bands. The chain form is conical at one end, with six bands, and with orifices on the dorsal surface. The exhibited specimen of the chain form, which is in an early stage of growth, contains 202 individuals. The solitary and chain individuals of Salpa costata-tilesii attain a length of six to eight inches. The solitary form has eighteen muscle-bands and two large spines at the posterior end. The individual of the chain has five muscle-bands. A chain of three individuals is exhibited.

Salpa pinnata produces a circular chain; the exhibited specimen of the solitary form shows a small chain about to be detached; a circular chain of six individuals is also exhibited. Species of Salpa abound in all seas, but specimens from Naples have alone been exhibited on account of their good preservation.

Family Octacnemidæ includes O. bythius, a deep-sea Salpid, in which the body forms a flattened disk produced into eight radiating lobes.

Fig. 22.

Salpa runcinata-fusiformis. A. Chain form. B. Solitary form. 1–9, muscle bands; em, embryo; m, mantle; visc, visceral mass or nucleus.

(Herdman: Tunicata, Encyclopædia Britannica.) ]

Fig. 23.

Doliolum denticulatum, sexual generation, from the left side. m^1-m^8 muscle bands; at, atrial; br, branchial apertures; br s, branchial sac; sg, stigmata; st, stomach; ng, nerve ganglion; so, sense organs.

(After Herdman, Encyclopædia Britannica.) ]

Family Doliolidæ. The body is cask-shaped and surrounded by circular hoops. The branchial and atrial orifices are at the opposite ends. The branchial sac is pierced by two oblique bands of stigmata (Fig. 23 sg). The life history is very complicated. The egg develops into a tailed larva, which develops into a “nurse”; the latter is asexual, and produces three kinds of buds on a stolon, viz. (1) nutritive buds which provide the “nurse” with food, (2) foster forms which are set free as cask-shaped bodies with eight broad muscle-bands, and (3) sexual forms which are attached for a time to the foster forms, but which later become free and give rise to the egg.

Order III.—LARVACEA.

Fig. 24.

Oikopleura cophocerca in its “house” (after Fol); seen from right side, × 6. Arrows indicate course of the water; x, lateral reticulated parts of the “house.” ]

The Larvacea are very minute Tunicata which live at the surface and swim by means of a tail-like appendage, resembling in this and certain other respects the tadpole larva of other Tunicata. They are able to form a temporary test or “house” many times larger than the body (Fig. 24). The organism itself, which is almost lost in its large test, is the little hammer-shaped body in the centre of the figure; the streaked areas bound a space in which the tail lashes vigorously. The animal can leave its test and secrete another in a few hours.

The tail is attached to the under or ventral surface of the tiny little barrel-shaped body, and usually points forwards; a skeletal rod, the urochord, runs along its length. The branchial sac has two ciliated openings or gill-clefts leading directly to the exterior, and not opening like the stigmata of the other orders into an atrial cavity.

The order contains one family, the Appendiculariidæ, and four genera, and is represented in all seas.

Oikopleura cophocerca, one of the largest forms, is about half an inch in length. The exhibited specimens came from St. Andrews, Fife. Professor McIntosh reports that occasionally specimens of this species occur in immense quantities, the tow-nets being filled with them.

THE STARFISH GALLERY.

In the STARFISH GALLERY is exhibited a series of the animals belonging to the class Echinoderma; of these the Starfishes are the best known, while others are the Sea-Lilies, Sea-Urchins, and Sea-Cucumbers or Sea-Slugs.

A small collection of various kinds of Worms is also exhibited in this Gallery (Wall-cases I.–III.).

ECHINODERMA.

Six table-cases contain the dried Echinoderms arranged in systematic order. The seventh is devoted to preparations, models, and figures illustrative of the structure and life history of various members of the group.

An inspection of that Case and the accompanying woodcuts will make clear the distinctive characters of the Echinoderma. Unlike that of a Crayfish or a Mussel, the body does not appear to be divided into two equal or symmetrical halves, though it really is; this is due to the possession of a number of rays, of which there are ordinarily five. The skin is strengthened by the deposition in it of carbonate of lime, which may be in the form of continuous plates or bars, or of separate scattered spicules. A series of tube-feet or suckers (podia) are generally developed along each ray, and these are supplied by a system of water-vessels peculiar to Starfish and their allies. These rays are often called “ambulacra.”

Fig. 1.

A. Anchor and plate of Synapta. B, C. Tables of Holothuria impatiens; and D. Holothuria atra: from various aspects. E. Spicule from sucker of Stichopus variegatus, magnified about 200 times. ]

Fig. 2.

Diagram of Water-vessels.

c.c. Circular canal, with p.v., its Polian vesicles; from it a radial canal (v.c.) is given off along the lower surface of each arm; this supplies, by side branches, the suckers, s; connected with each sucker is a contractile swelling or ampulla (a). The circular canal is in connection with the exterior by s.c., the stone-canal, and opens to it by the madreporite (m). ]

Fig. 3.

Figure of a Starfish (Asterias rubens).

In the ray marked I. the skin has been removed from the upper surface, and the ambulacral ossicles (ao) and the podia (s) are seen in situ; the blind outgrowths (c) from the central stomach (sp) have been dissected out. In II. the gonads (g) are exposed; and in the centre above the stomach the rectal glands (rg) are to be seen. The anus (a) is seen to be subcentral in position. ]

In the body of the Starfish (Fig. 3) the arms are seen to be continuous with the disk and to contain portions or prolongations of the chief organs. The middle of the arm is occupied by two rows of hard pieces (ambulacral ossicles), the fellows of which make an open angle with each other, and so form an open ambulacral groove; along this we find the suckers, the water-canal that supplies them, the blood-vessel of the arm, and a nerve-cord. At the centre of the disk is the mouth. The ossicles at the sides of the arms bear spines, which vary in different species; the surface of the back is supported by a network of hard pieces, and through the intervening spaces there project membranous pouches, which are respiratory in function. The modified plate on the upper surface opens into a tube by means of which the water-vessels communicate with the exterior; this plate is known as the madreporite (Fig. 2, m).

The organs for masticating the food are most highly developed in the regular Echinoids, where the complex apparatus known as the “Lantern of Aristotle” is found (Case 38) to consist of five sets of pieces; the tooth is strong and bevelled at its free end; it is supported by triangular jaws on either side, a pair uniting and having the form of an inverted pyramid; these alveoli are connected with their neighbours by oblong pieces (falces); above these there are elongated bars, which are hinged on to the inner end of the falces and have their outer ends free. The whole lantern is connected to the test by muscles which pass from its sides to the auricles or upstanding pillars which lie round the mouth; and, owing to this muscular apparatus, the teeth are capable of complicated and various movements.

In the Ophiuroids the edges of the mouth-slits are provided with short spinous processes, varying a good deal in arrangement, but never having, apparently, any other function than that of a filtering-apparatus; in the Starfishes the plates round the mouth have a supporting function only; in Crinoids and Holothurians the mouth is unarmed; the latter are often remarkable for a deposit of calcareous plates in the walls of the gullet, and in the former the grooves on the arms are the lines along which food comes to the mouth.

Echinoids live on seaweeds and the animals that are found on them; such as have no teeth, like Spatangus (Case 32), use their spout-like mouth to take up the sand and débris on which they move, and from which they extract some nutriment. Ophiuroids live on the smaller foraminifera; Asteroids on dead fishes (as line-fishermen well know), oysters, and other molluscs, and even on specimens of their own particular species; Holothurians on shell or coral débris and the minute organisms it contains; and Crinoids on small tests of foraminifera and on the adults of small and larvæ of larger crustacea.

In a number of Echinoids and Asteroids some of the spines are specially modified to act as seizing-organs—the free end being divided into two, three, or rarely four pieces, which are moved on one another by special muscles. These minute organs were regarded by earlier observers as parasites, and were named pedicellariæ; they may be movable, when they have a stalk, or the stalk may be absent and the valves sessile. Considerable difficulty attaches to the determination of the use that these organs may be to their possessors; but there is reason to suppose that they may act as cleansing-organs by removing minute particles of dirt, and as temporary organs of fixation, while M. Prouho has observed their use as organs of defence.

Echinoderms move but little; the unstalked Crinoids, if they cannot find stones or worm-tubes around which to attach themselves, swim by beating the water with their delicate arms, five being raised and five depressed alternately. The Echinoid or Asteroid is able to move by the aid of its podia or so-called ambulacral feet, which become erected by being filled with water, and are then contracted; by means of this contraction movement is effected; a similar kind of locomotion obtains with the pedate Holothurians; in the Ophiuroids the flexible arms either serve as the organs of movement, or act as an apparatus whereby the creature becomes coiled round the branches of corals (see Case 20).

Echinoderms are often of exceedingly bright colours, as is shown by the pictures on the wall, and are very conspicuous objects; this may, apparently, be associated with disagreeable tastes or odours; sometimes they cover themselves over with seaweed, and so hide their brilliancy; the spines of some forms are exceedingly painful to the touch, and the stout plates of some of the Goniasters must form admirable organs of protection. The power of restoring lost or injured parts is one of the most remarkable points in the Echinoderm organization (see Case 6).

Echinoderms are of great geological age, and were very abundant in earlier periods of the world’s history. Two groups (the Blastoids and Cystids) have completely disappeared, and the Stalked Crinoids (Lily-Encrinites) are far less common than they used to be. Echinoderms are now found in all seas, and extend to great depths of ocean; many of the species have exceedingly wide areas of distribution, and most are characterized by their gregarious habits, a large number of specimens of a single species being generally obtained by the dredge. They are most abundant in the tropical seas.

Most Echinoderms lay their eggs in the water, where the larvæ are developed and swim about freely; but in a few (Hemiaster, Ophiacantha vivipara, and others) the young do not pass through any metamorphosis, for the eggs are placed in special pouches of the body of the parent, in which they are hatched. The free-swimming larvæ of the other Echinoderms pass through a series of remarkable changes (Figs. 4 and 5); these are illustrated by the twelve models of various forms of larvæ exhibited in Case 36; in Case 35 is a set of models showing in detail the changes undergone by a single species (Asterina gibbosa). A portion only of the body of the larva is converted into the substance of the perfect animal; the rest is either absorbed by the growing animal, or shrivels up and disappears.

Below the twelve models in Case 36 may be seen a representation of three stages in the history of the Feather-star (Antedon bifida). The larvæ of this Echinoderm are not free, but are attached by a stalk (Fig. 6); in the common Feather-star and other Comatulidæ the stalk is found during larval stages only; in others, such as Pentacrinus, it persists throughout life.

The presence or absence of this stalk has been taken as the first character of importance in the classification of Echinoderma which may be divided into two groups:—

A. PELMATOZOA, or Echinoderms provided with a stalk throughout life or in the larval stages only. To this group belong the Crinoidea, and the extinct Blastoidea, and Cystidea.

Fig. 6.

Pentacrinoid stage of Antedon rosacea.

a, arms; b, basals; r, radials; s, stalk. ]

B. ECHINOZOA, or Echinoderms without stalks at any time of their existence. To this group belong the Asteroidea, Ophiuroidea, Echinoidea, and Holothurioidea.

CRINOIDEA.—This Order may be described as stalked, globular, or cup-shaped Echinoderms, in which the oral surface of the calyx or disk looks upwards, and in which five jointed and generally branched rays arise from the central disk. Their joints have jointed pinnules at their sides, and the sucking-feet have the form of tentacles.

The stalked representatives of this Order are placed on tables and brackets near the south door, and are worthy of being particularly noticed for their fine preservation, size, and beauty. The largest specimen of Pentacrinus decorus was taken on a telegraph-wire, to the covering of which the stalk of the Crinoid is still attached. Metacrinus is a more lately discovered genus, which appears to be confined to the eastern seas.

A few dried unstalked Crinoids are shown in Table-case 1; these show the leading modifications of structure in the two great genera Antedon and Actinometra.

Fig. 7.

Comet form of Linckia. ]

ASTEROIDEA.—This Order comprises Echinoderms with a depressed body of pentagonal or star-like shape, to the ventral surface of which the ambulacral feet are confined. The rays are more or less elongate movable arms, with skeletal structures, which consist of transversely arranged, paired, calcareous plates, articulated with each other like vertebræ, the series extending from the mouth to the end of the arms. The groove in which the ambulacral feet are arranged is uncovered.

Typical specimens of this Order are exhibited in Cases 2 & 3, in which the great variety of form in the genus Asterias and beautiful examples of Acanthaster are shown. Cases 6 & 7 contain specimens illustrating the curious habit of self-mutilation possessed by so many Echinoderms; among Starfishes, and notably in the genus Linckia, the single arms separated from the disk are able to develop a fresh disk and arms, and so to multiply the species. Cases 9–11 contain fine series of Oreaster.

OPHIUROIDEA, or “Brittle-stars.”—These Echinoderms appear to resemble the ordinary Starfish; but they differ in having the organs of digestion, respiration, and reproduction confined to the disk, the arms having merely the function of locomotor organs. The arms therefore are more slender and cylindrical in form, and are sharply distinct from the disk; the separate joints consist of two central ossicles, which leave only a narrow canal between them, and these are covered above, below, and at the sides by specially developed investing plates; the lateral plates bear spines, which are always comparatively short and delicate, as compared with the spines found at the sides of the arm in starfishes.

The principal types of this Order are exhibited in Cases 17–22; the most exquisite of them are the forms whose arms are divided and subdivided till they end at last in the finest threads, as in Astrophyton, the so-called Basket-fish or Gorgon’s heads.

ECHINOIDEA, or “Sea-Urchins,” are Echinoderms in which the rays are not free, as in the Starfishes or Brittle-stars, but unite to form a compact, spherical, heart- or disk-shaped test; this test is covered with spines, which may attain to a great length, as is shown in the fine example of Diadema saxatile from the Andaman Islands; some of the tests are flexible and very fragile. Owing to the quantity of specimens that are sometimes dredged at one spot, the naturalist has been able to gain a better idea of the range of variation in the species of Echinoderms than in some other divisions of the Animal Kingdom; an instructive series, showing the variations of Echinometra lucunter, is shown in Case 28.

The genus Hemiaster offers an example of an Echinoderm in which the eggs are laid in special pouches; the hinder ambulacra are deepened to form pits, which are guarded by specially elongated spines (see Case 34); in these pits the young pass through all the stages of their development.

The minute structure of the spines of Sea-Urchins is illustrated by a series of figures on the wall.

The HOLOTHURIOIDEA, or Sea-Cucumbers, form the last order of Echinoderms. Their body, as indicated by their English name, is elongate, subcylindrical, with a more or less flexible integument, according to the extent of the reduction of the calcareous skeleton; the mouth is at one end of the body and surrounded by tentacles, the vent at the opposite end.

As these animals cannot be shown in a dried state, some of them, preserved in spirit, are placed in Wall-Case IV. According as they have or have not the sucking-feet of the Echinoderma, they are ordinarily divided into the Pedata and the Apoda; the latter are represented by Synapta, which may attain to a great length, and by Chiridota; the Pedata are illustrated by the genera Cucumaria, Psolus, and Holothuria. Deep-sea investigations have revealed the existence of another group of specially modified Holothurians—the Elasipoda; these are remarkable for their well-marked bilateral symmetry and the distinctness between the dorsal and ventral portions of the body; the prominent processes on the dorsal surface are not contractile.

An exhibition of some interest is to be found in a Table-Case against the wall, in which there are various specimens of the edible Holothurians—trepang or bêche-de-mer; these were all bought in the market at Canton, and may be taken to be typical of the kinds offered for sale in various eastern countries.

WORMS.

By the name “Worms,” people commonly indicate a number of different forms whose relations with one another are by no means so close as those of a Holothurian and a Crinoid, or a Mussel and an Octopus. There are not, indeed, any common characters by the possession of which the worm-like animals can at once be distinguished from other animals. We take the divisions, examples of which are here represented, either by drawings, models, or specimens preserved in spirit separately.

The groups referred to may be enumerated as follows:—

Platyhelmia.│Turbellaria. „ │Trematoda. „ │Cestoda. Nemertinea. │ Nematoidea. │ Chætopoda. │

PLATYHELMIA, or Flat-Worms.—These form the lowest and simplest division of the group.

The parasitic Platyhelmia—the Tapeworms (Cestoda) and the Flukes (Trematoda)—occupy Case I.; the life history of the common Tapeworm (Tænia solium) is shown by the aid of models and figures. A model of the anterior end of the common Tapeworm shows the four suckers and the crown of hooks; the unjointed neck is followed by the joints (proglottids), which increase in size the farther they are from the neck. Several entire specimens of Tænia follow, showing the size of the whole worm and the form of its joints. The structure of the body is shown in the models of two joints. The growth and development of the Tapeworm is dependent on a migration or a change of the hosts which it inhabits in the various stages of its life; and although the different kinds of Tapeworm differ from each other somewhat in certain details of their migration and development, their life history exhibits, on the whole, the same events which we find in Tænia solium, a common Tapeworm of man in Northern Europe. This worm is matured in the intestines of man; its final joints consist merely of fertilized ova which have already passed through the earlier stages of development; when the joints are detached and discharged, their contents escape in the form of embryos contained in a thick chitinous shell. If these are now swallowed by a pig, the shell is digested by the gastric juices of the new host, and a rounded embryo, which is provided with three pairs of hooks, is set free; by means of these hooks the guest makes its way through the wall of the stomach or intestine, and finally settles down in the muscles of its host. The embryo now loses its hooks, and gradually acquires a bladder-like form, the central cavity of which is filled with fluid. This bladder-worm (Cysticercus) has its outer wall pushed inwards at the anterior end, and on this hooks and suckers become developed. We have now a narrow head and neck with an attached bladder, the head being at this time hollow. If during the long time that these bladder-worms remain alive, the pig is killed for food, its flesh is found to be “measly”; if it is afterwards insufficiently cooked and eaten, the worms are conveyed into the human stomach. Here the bladder-like termination becomes absorbed, and, the neck beginning to grow, we have the commencement of the form from which we started, and the completion of that “vicious circle” which is so curious a characteristic of many forms of parasitic life.

Fig. 8.

Tænia solium: showing the head (h) with its suckers (s′) and crown of hooks (s), the unjointed neck (n), and a few of the succeeding joints (j). ]

In other Tapeworms the cyst may be more complicated than that in the pig, as, for example, the form found in the sheep’s brain or the liver of the horse.

Of the other Cestode parasites mention should specially be made of those of Fishes; the vulgar notion that the parasites of these animals are dangerous to man has been shown to be entirely erroneous.

Fig. 9.

Limnæa truncatula. ]

The Flukes infest animals of all kinds; that which is most dangerous to sheep, and the cause of much pecuniary loss (Distoma hepaticum), is selected here as a type; its structure is shown by a large model, and its life history by a series of diagrams (Figs. 10–13). Here, again, we have a creature which infests two hosts. If the larvæ which escape from the sheep fall on wet ground in or near a pool, they make their way to a small pond-snail (Limnæa truncatula, Fig. 9), into the lung-chamber of which they bore their way. On leaving them the larva may be, and is, too frequently, eaten by a sheep, and makes its way into the liver of that animal, where it causes the disease known as the “liver rot.”

The damage done by the liver-fluke may be imagined from the fact that in the winter of 1879–80 no less than three millions of sheep died of rot in the United Kingdom; this heavy loss is no doubt largely due to the immense number of eggs to which a single fluke may give rise. It has been estimated that every fluke may produce, during its life, several thousands of eggs; and in one case Prof. A. P. Thomas found as many as 7,400,000 eggs in the gall-bladder of a sheep which was suffering from rot, and which, at that time, had in its liver about 200 flukes.

The non-parasitic Flat-worms are shown, magnified, in the upper parts of Cases I. & II. The Turbellaria proper, without any or with a simple or a branched intestine, but without a vent, are represented by Convoluta and Thysanozoon: the general structure is shown by a diagram in Case II., which is here reproduced (Fig. 14). Planaria, Thysanozoon, and Bipalium serve to illustrate the forms of members of this group.

The Nemertine Worms (Nemertinea), with a straight intestine, with a vent, and with a proboscis, may attain to a very considerable length; Carinella and Lineus are represented by large figures, and various species are shown in spirit. These forms, which used to be very unsatisfactory to exhibit, on account of the great difficulty of preserving them complete and uninjured, are now, with improved methods, very satisfactorily shown, as the specimens purchased from the Marine Biological Laboratory at Plymouth prove.

Stages in the life history of the Fluke.

Fig. 10. Egg of Fluke, showing the operculum and the contained yolk-spheres. Magnified 340 diams.

Fig. 11. An embryo forcing its way by its boring-papilla (p) into the wall of the lung of a Snail (e.p.) Magnified about 340 diams.

Fig. 12. A young Rédia (natural size, ½ millimetre or ¹⁄₅₀ inch): pl., pharynx; g, contained germs; p, characteristic posterior processes of the Rédia.

Fig. 13. Free-swimming Cercaria, before the commencement of the formation of the cyst. Magnified 100 diams. ]

Fig. 14. Diagram of the structure of a Turbellarian: ng, nerve- (cerebral) ganglia; nb, nerve-branches; yg, yolk-glands; t, testis; o, ova; ov, ovary; c, cirrus; m, mouth; ph, pharynx.

Fig. 15. Diagram of a Nemertine: b, brain; m, mouth; n, renal organs; id, diverticula of intestine; g, gonads; sn, side nerve-trunk; pr, proboscis in its dorsal sheath.

Fig. 16. Diagram of the structure of a Nematoid; m, mouth; ph, pharynx; a, anus; o, orifice of genital tube. ]

NEMATODES (Thread-Worms or Round-Worms).—These are for the most part parasitic, and infest plants as well as animals; the common Round-Worms living parasitically in man (Ascaris, Stronaylus, Trichocephalus) belong to this Order. Sometimes they are parasitic in their early stages and later live a free life—such are Gordius and Mermis. A specimen of a Mantid is exhibited from which half the body of the infesting Gordius has already protruded (Fig. 17). One of the most remarkable Gordii is the great elongated G. fulgur, or “Lightning Snake,” from Celebes. Another very large Nematode is the so-called Guinea-worm, or Dracunculus medinensis, which is found beneath the skin of the leg; it is very possible that this worm was the cause of the illness which afflicted the Israelites in their journey through the desert from Egypt to the Promised Land.

Fig. 17.

Gordius escaping from a Mantid. ]

Fig. 18.

Figure of Trichina spiralis, showing the worms encysted in muscle. ]

Of all Nematodes the most dangerous to man is the small worm which is known as Trichina spiralis (Fig. 18); a series of models are shown which give a good idea of the structure of the female and the smaller male. The young make their way through the walls of the stomach of their host, and encyst themselves among its muscles: a piece of a sternothyroid muscle is shown, taken from a man in whose body it was calculated there were forty millions of encysted Trichinæ.

Other Nematodes infesting man, such as Filaria sanguinis hominis, are too small for exhibition.

Plants are not free from the attacks of Nematodes, and examples are shown, accompanied by an illustrating figure, of the Ear-cockle gall of wheat; this gall is due to the injuries inflicted by a minute Thread-worm—Tylenchus tritici. Wheat is, of course, by no means the only cultivated plant that is attacked by these minute worms; the history of most has, however, still to be made out.

Holding a somewhat uncertain position in relation to the Round-worms are the parasitic Acanthocephali (Thorn-headed Worms) and the free-swimming Chætognatha, or Bristle-jawed Worms; examples of both of these groups are shown, together with diagrams illustrative of their general structure.

ANNULATA or Chætopoda.—So-called because consisting of a series of rings, and being provided with chætæ or bristles; they are to be associated with the Arthropoda, under the one head “Appendiculata,” a better name than “Articulata,” since Cuvier did not include worms in his group. The creatures that are most familiarly called worms are to be found in Case III.; here are a few examples of the numerous kinds of worms that are found living freely in the sea, of earth and freshwater Worms, and of Leeches. All these worms are distinctly characterized by the fact that they consist of a number of definite rings (somites), whence they have been called Annulata. The marine Worm and the Earthworm differ from the Leech in that these rings are provided with chætæ or bristles, of which there are a number in each bundle in the marine, and a few only in the terrestrial or freshwater form: hence the marine Worms are called Polychæta and the latter Oligochæta.

The former are divisible into two great groups. There are those that are free-swimming and are able to forage for themselves, such as the lovely Sea-mouse (Aphrodite aculeata), the large Eunice gigantea, the common Nereis pelagica, or the exquisitely coloured Chloeia flava. Others live a more retired life, dwelling in tubes, which they fashion for themselves; they lead either a solitary or a social life. Here we have examples of Sabella, Sabellaria, Serpula; a number of forms of worm-tubes, showing their great variety and beauty (see especially the delicate Filograna), are to be seen in the small Table-cases placed against the north wall of the Gallery. Attention should be especially directed to Mr. A. T. Watson’s beautiful preparations of Terebella littoralis. We give a figure (Fig. 19) after a drawing by that gentleman of the home of Panthalis oerstedi, the tube-forming habits of which have been carefully observed by him.

Fig. 19.

Home of Panthalis oerstedi. ]

Fig. 20.

Section across the body of an earth-worm to show the disposition of the more important organs; the body wall (w) consists of dermis, circular, and longitudinal muscles; the body cavity is divided by membranes (c) into a series of chambers, in each of which opens the mouth of a coiled nephridium (n). The axis of the cavity is occupied by the intestine (i); above and below it is a longer blood-vessel (v), and below it is also the central nerve-cord (nc). ]

The Oligochæta are represented by the common Earthworm, the influence of which in the formation of mould and in the general ploughing of the soil was carefully investigated by Mr. Darwin; and by the little Tubifex rivulorum (Bloodworm), which owes both its red colour and its ability to dwell in mud, which is so poor in oxygen as to be unfit for respiration, to the same chemical compound as that which gives the red colour to our blood and carries the oxygen of respiration all over the body.

Acanthobdella: e, eyes; ch, chætæ; s, sucker. ]

The Hirudinea, or Leeches, are often said to be distinguished from the Chætopoda by the absence of bristles, but, as a fact, Acanthobdella (Figs. 21 and 22) has very well marked bristles. They always have a sucker at the hinder end of the body by which they are attached to their prey; they are found in fresh water (Piscicola), on sea-fishes (as Pontobdella), or in moist places, as the Leech (Hirudo). The last-named has three jaws, armed with as many as ninety denticles. Trochetia subviridis (Land-Leech) is a species which is found rarely and sporadically in England.

The Myzostomaria form a division of Polychæta all the members of which live parasitically on Crinoids, and otherwise present great differences in their habits. Some move about freely on the Crinoids they infest, others are more sluggish and rarely move, others produce galls or cysts on their host, and yet others are internal parasites, and live in the alimentary canal. It is of interest to note that there are corresponding degrees of difference between the young and old specimens of the different groups of species.

The general organisation of Myzostomaria is shown in the accompanying figure (Fig. 23) in which the dorsal wall of the body is supposed to be transparent so as to allow of the chief internal organs being seen.

Fig. 23.

Diagram of Myzostomum to show the general form of the body and the marginal extensile cirri (c); within these and on the ventral surface are four pairs of suckers, and more internally five pairs of appendages each bearing two hooks; the proboscis (p), the digestive tract and its ramifications, and the reproductive organs are outlined as if seen through a transparent wall; a, anus. ]

The last group of Worms here represented is that of the Gephyrea; with the advance of our knowledge it is probable that they will be found to be more intimately allied to the Annulata than is now generally supposed; it will be seen indeed that Echiurus has bristles at its hinder end; Sipunculus is the best known representative of the unarmed Gephyrea; Bonellia is interesting both from the fact that it owes its green colour to a matter closely resembling the chlorophyll of green plants, and from the possession by the female of a proboscis, which is protruded from the hole in the rock occupied by the worm: the male is very much smaller than the female, and is not nearly so well developed. Owing to the mode of lighting the Gallery, the visitor may have to shift his position several times before gaining a good view of the whole length of the proboscis.

INDEX.

Acanthaster, 114

Acanthobdella, 124

Acanthocephali, 122

Acmæidæ, 9

Actinometra, 113

Adeonidæ, 66

Ætheriidæ, 39

Alcyonidium, 67

Amaroucium, 99

Amathia, 67, 68

Amphineura, 7–9

Ampullariidæ, 14

Ancylus, 27

Annulata, 122

Anodonta, 39

Antedon, 111–113

Aphrodite, 122

Aplacophora, 9

Aplysiidæ, 23

Apoda, 115

Appendiculariidæ, 105

“Apple-Snails”, 14

Architeuthis, 48

Argonauta, 46

Ascaris, 120

Ascidia, 83, 93

Ascidiacea, 90

Ascidiæ Compositæ, 96

Ascidiæ Salpiformes, 99

Ascidiæ Simplices, 90

Ascopodaria, 73

Astartidæ, 38

Asterias, 108, 114

Asterina, 111

Asteroidea, 112, 113

Astrophyton, 114

Atlantidæ, 22

Auger-shells, 21

Auriculidæ, 26

Aviculidæ, 33

Bartlettia, 39

Basket-fish, 114

Basommatophora, 26

Bêche-de-mer, 115

Bipalium, 118

Bird’s head Coralline, 58

Bladder-worm, 117

Blastoidea, 112

Bloodworm, 124

Boat-shells, 20

Boltenia, 91, 92

Bonellia, 126

Borer, 43

Botryllidæ, 96, 97

Botrylloides, 97

Botryllus, 97, 98

Bowerbankia, 67

Brachiopoda, 74–82

Brechites, 44

Bristle-jawed worms, 122

Brittle Stars, 114

Bubble-shells, 23

Buccinidæ, 18, 19

Bugula, 58, 59

Bullidæ, 23

Caberia, 62

Calamaries, 4

Calyptræidæ, 14

Cardiidæ, 40

Carditidæ, 38

Carinariidæ, 22

Carinella, 118

Carrier-shells, 17

Cassididæ, 18, 19

Catenicellidæ, 66

Cavolina, 23

Cellularina, 58

Cephalopoda, 45–50

Cerithiidæ, 17

Cestoda, 116

Cestode parasites, 116

Chætoderma, 9

Chætognatha, 122

Chætopoda, 116, 122

Chama, 41

Chelyosoma, 93

Chilostomata, 57

Chirodota, 115

Chiroteuthis, 47

Chitonidæ, 7, 8

Chloeia, 122

Clams, 41

Clavagellidæ, 44

Clavelina, 94, 95

Clavelinidæ, 94

Clione, 24

Coat-of-mail shells, 7

Cockle, 40

Colella, 97, 98

Comatulidæ, 111

Conchologists, 17

Conidæ, 21

Convoluta, 118

Coralliophilidæ, 20

Corbula, 42

Cowries, 14

Crania, 74, 80

Creeping Coralline, 60

Crepidula, 14

Crinoidea, 112

Crinoids, 109

Crisia, 69

Crisiidæ 69

Cristatella, 71, 72

Cryptochiton, 8

Cryptoplax, 8

Ctenostomata, 57, 66

Cucumaria, 115

Culeolus, 92

Cup-and-saucer Limpets, 14

Currant Squirter, 92

Cuspidariidæ, 45

Cuttlefish, 48

Cyclophoridæ, 13

Cyclostomata, 57, 68

Cynthiidæ, 91

Cypræidæ, 14

Cysticercus, 117

Cystidea, 112

Dentaliidæ, 28

Desert-snail, 6

Diadema, 114

Diazona, 94

Dipsas, 36

Discinidæ, 80

Distoma, 118

Distomidæ, 96

Doliidæ, 18

Doliolidæ, 100, 103

Doliolum, 90, 103

Dracunculus, 121

Dyscolia, 82

Ear-cockle gall, 122

Ear-shells, 12

Earthworms, 124

Echinoderma, 106–115

Echinoidea, 112, 114

Echinometra, 114

Echinozoa, 112

Echiurus, 126

Ectoprocta, 57

Elasipoda, 115

Electra, 62, 63

Elephant-tooth shell, 1

Entoprocta, 57, 73

Escharina, 64

Eulamellibranchia, 38

Eunice, 122

Euthyneura, 23

False Limpets, 9

Fan-Mussel, 34

Fan-Shells, 38

Fasciolariidæ, 18

Feather-star, 111

Filaria, 122

Filibranchia, 32

Filograna, 123

Fissurellidæ, 11

Flat-worms, 116

Flukes, 118, 119

Flustra, 54, 62

Flustrina, 62

Fountain-shell, 17

Fredericella, 71

Freshwater Limpets, 27

Freshwater Mussels, 38

Freshwater Oyster, 39

Freshwater Polyzoa, 70

Freshwater Snails, 7, 26

Freshwater Worms, 122

Gapers, 42

Gastropoda, 9–28

Gephyrea, 126

Giant Clam, 41

Glassy Nautilus, 22

“Glory-of-the-Sea” Cone, 21

Glottidia, 78

Goodsiria, 99

Gordius, 120, 121

Gorgon’s head, 114

Guinea-worm, 121

Gymnolæmata, 57

Gymnosomata, 24

Haliotidæ, 12

Hammer Oyster, 33

Harpidæ, 20

Harp-shells, 20

Helicidæ, 27, 28

Helmet-shells, 18

Hemiaster, 110, 114

Heteropoda, 22

Hippuritidæ, 41

Hirudinea, 124

Hirudo, 124

Holothuria, 107, 115

Holothurioidea, 112, 115

Ianthinidæ, 16

Idmonea, 69

Julinia, 98

Keyhole Limpets, 11

Kinetoskias, 59–61

Kuphus, 43

Lamellibranchia, 29–45

Land-Snails, 26

Larvacea, 104

Leeches, 124

Lepralia, 64

Leptoclinum, 98, 99

Lichenopora, 69

“Lightning-Snake”, 121

Lily-Encrinites, 110

Limidæ, 37

Limnæa truncatula, 118

Limnæidæ, 26

Limpet, 9

Limpet Snails, 26

Linckia, 113, 114

Lineus, 118

Lingula, 74, 75, 77–79

Lithodomus, 33

Littorinidæ, 14

Liver-fluke, 118

Loligo, 46

Lophopus, 71, 72

Lotoriidæ, 18

Loxosoma, 73

Lucinidæ, 38

Lunulites, 64

Mactridæ, 40

Magellania, 75, 77, 82

Magilus, 20

Malleus, 33

Margaritana, 39

Marine worms, 122

Melaniidæ, 17

Meleagrina, 33–35

Melons, 20

Membranipora, 63

Mermis, 120

Metacrinus, 113

Mineralogists, 17

Mitridæ, 18

Molgula, 91

Molgulidæ, 90

Mollusca, 1–53

Money-Cowry, 15

Moss Animals, 56

Mucronella, 65

Mülleria, 39, 40

Muricidæ, 20

Mussels, 33

Mussels (freshwater), 38

Myidæ, 42

Mytilidæ, 33

Myzostomaria, 125

Naked-gilled Molluscs, 24

Natica, 15

Nautilus, 45, 49

Nematoidea, 116, 120

Nemertinea, 116, 118, 120

Neomenia, 9

Nereis, 122

Neritidæ, 13

Non-parasitic Worms, 118

Nucleobranchiata, 22

Nuculidæ, 31

Nudibranchia, 23, 24

Octacnemidæ, 100

Octopus, 46, 47

Oikopleura, 104

Oligochæta, 122, 124

Olividæ, 20

Ophiacantha, 111

Ophiuroidea, 112, 114

Opisthobranchia, 23

Orange Cowry, 15

Oreaster, 114

Ormers, 12

Ostreidæ, 36

Ovulidæ, 15

Oyster, 36

Panthalis, 123

Paper-Nautilus, 46

Patella, 9

Pearl-Oyster, 33

Pearly Nautilus, 50

Pectinibranchia, 13

Pectinidæ, 38

Pedata, 115

Pedicellinidæ, 73

Pelmatozoa, 111

Pentacrinus, 111, 113

Periwinkle, 14

Perophora, 95, 96

Phallusia, 93

Pharyngodictyon, 98, 99

Pholas, 43

Phylactolæmata, 70

Piddocks, 43

Pinna, 33, 34

Piscicola, 124

Planaria, 118

Platyhelmia, 116

Pleurotomaria, 11

Pleurotomatidæ, 20

Plumatella, 71, 72

Polychæta, 122

Polyclinidæ, 96

Polyplacophora, 7

Polyzoa, 54–73

Pond-Mussel, 5

Pond-Snails, 27

Pontobdella, 124

Poromyidæ, 45

Poulp, 7

Proneomenia, 7

Protobranchia, 31

Pseudolamellibranchia, 33

Psolus, 115

Pterocera, 18

Pteropoda, 23

Pterotracheidæ, 22

Pulmonata, 25–28

Purpura, 20

Pyrosoma, 90, 99

Radiolitidæ, 41

Razor-shells, 42

Retepora, 65, 66

Rhodosoma, 93, 94

Rhopalæa, 96

Rhynchonella, 82

River-Snails, 13

Rock-shells, 20

Round Worms, 118

Sabella, 122

Sabellaria, 122

Saddle-Oysters, 32

Salpa, 90, 100–103

Salpidæ, 100

Scala, 17

Scallops, 38

Scaphopoda, 28

Schizoporella, 65

Scorpion-shells, 18

Screw-shells, 17

Scrobiculariidæ, 40

Scrupocellaria, 60, 62

Scutibranchia, 9

Sea-Butterflies, 23

Sea-Cucumbers, 106, 115

Sea-Hare, 23, 24

Sea-Lilies, 106

Sea-Mat, 54

Sea-Mouse, 122

Sea-Slugs, 106

Sea-Urchins, 106, 114

Sea-Woodlice, 7

Selenariidae, 64

Semele, 40

Sepia, 46–48

Sepiola, 46

Septibranchia, 45

Serpula, 122

Ship-worm, 43

Silk Coralline, 68

Siphonariidæ, 26

Sipunculus, 126

Slipper-Limpets, 14

Slit-Limpets, 11

Slugs, 28

Snails, 27

Solenidæ, 42

Solenomyidæ, 31

Spatangus, 109

Spirula, 49

Spondylidæ, 37

Squid, 46

Stalked Crinoids, 110, 112

Starfishes, 108

Stichopus, 107

Streptoneura, 9

Strombidæ, 17

Strongylus, 120

Styelopsis, 92

Stylommatophora, 27

Synapta, 107, 115

Tænia, 116, 117

Tapeworm, 116

Tectibranchia, 23

Tellina, 40

Terebella, 123

Terebratula, 74, 82

Terebratulina, 82

Terebridæ, 21

Teredo, 43, 44

Testacella, 28

Tethys, 24

Thaliacea, 100

Thecalia, 38

Thecidium, 81

Thecosomata, 23

Thorn-headed Worms, 122

Thorny Oysters, 37

Thread-worms, 118

Thysanozoon, 118

Tooth-shells, 28

Top-shells, 12

Trematoda, 116

Trepang, 115

Trichina, 121

Trichocephalus, 120

Tridacnidæ, 41

Trochetia, 125

Trochidæ, 12

Trumpet-shells, 18

Tubifex, 124

Tubulipora, 69, 70

Tunicata, 83–105

Tun-shells, 18

Turbellaria, 116, 118, 120

Turbinidæ, 12

Turritellidæ, 17

Tylenchus, 122

Umbraculidæ, 23, 24

Umbraculum, 24, 25

Umbrella-shells, 23

Unionidæ, 38, 39

Veneridæ, 40

Venus-shells, 30

Vermetidæ, 17

Vesicularia, 67

Violet Snails, 16

Viviparidæ, 20

“Volutes”, 20

Volutidæ, 20

Water-Clams, 37

Watering pot-shells, 44

Water Spondyli, 37

Weaver’s-shuttle, 15

Whelk, 18

Window-shells, 32

Wing-shells, 33

Winkles, 14

Worms, 116–126

Worm-shells, 17

Worm-tubes, 123

Xenophoridæ, 17

LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, STAMFORD STREET AND CHARING CROSS.

-----

Footnote 1:

A framed series of photographs, illustrating different kinds of radulæ, is placed on the east wall of the gallery.

Footnote 2:

From Woodward’s ‘Manual of the Mollusca,’ published by Lockwood & Son.

Footnote 3:

From ‘The Cambridge Natural History,’ Messrs. Macmillan & Co.

Footnote 4:

From the Greek: pteron, wing, and pous, foot.

Footnote 5:

From the Greek: scaphe, a small boat, and pous, a foot—the foot of some Scaphopods being somewhat pointed like the prow of a vessel.

Footnote 6:

The term Lamellibranchia is used instead of Pelecypoda in deference to the wish of Professor Lankester.—E. A. S.

Footnote 7:

From the ‘Encyclopædia Britannica.’ Messrs. A. & C. Black.

Footnote 8:

From ‘The Cambridge Natural History.’ Messrs. Macmillan & Co.

Footnote 9:

From the Greek: kephale, head, and pous, foot.

Footnote 10:

Zoon, animal; oikos, house.

Footnote 11:

Lophos, plume; pherein, to bear.

Footnote 12:

Polus, many; zoon, animal.

Footnote 13:

Bryon, moss.

Footnote 14:

Ektos, outside; proktos, vent.

Footnote 15:

Cheilos, lip; stoma, mouth.

Footnote 16:

Gumnos, naked; laimos, throat.

Footnote 17:

Ktenos, of a comb.

Footnote 18:

Phulassein, to guard; laimos, throat.

Footnote 19:

Kuklos, circle.

Footnote 20:

Entos, inside; proktos, vent.

Footnote 21:

Vibraculum, a bristle.

Footnote 22:

Pyrosoma—pur, fire; soma, body.

Footnote 23:

It should be mentioned that one high authority, Prof. W. K. Brooks, does not regard the life history of Salpa as an example of alternation of generations, but considers the solitary Salpa to be, not asexual, but a female which produces a chain of males; but it is impossible to enter into a difficult question of controversy here.

Footnote 24:

From the Greek pelma = a stalk.

Footnote 25:

The Asteroidea or Ophiuroidea may be counted under the name Stelliformia.

------------------------------------------------------------------------

TRANSCRIBER’S NOTES

1. Typos fixed; non-standard spelling and dialect retained. 2. Used numbers for footnotes, placing them all at the end of the last chapter. 3. Enclosed italics font in underscores. 4. Enclosed bold font in =equals=. 5. The caret (^) serves as a superscript indicator, applicable to individual characters (like 2^d) and even entire phrases (like 1^{st}).