Part 30

The Invertebrate animals are divided into the following sub-families, classes, and orders by Dr. William Carpenter.

Sub-family 1.--RADIATA, having Four classes.

1. Infusoria. 2. Entozoa.

{Hydroida } 3. Zoophyta. {Asteroida } 4 Orders. {Helianthoida } {Ascidioda }

4. Radiaria. {Acalephæ } 2 Orders. {Echinodermata}

Sub-family 2.--ARTICULATA, having Five classes.

1. Annellata. 2. Cerripeda. 3. Crustacea. {Coleoptera } {Orthoptera } {Neuroptera } {Hymenoptera } 4. Insecta. {Strepsiptera} 9 Orders. {Lepidoptera } {Hemiptera } {Diptera } {Aptera } 5. Arachnida.

Sub-family 3.--MOLLUSCA, having Six classes.

ACEPHALA, or Headless. 1. Tunicata. 2. Brachiopoda. 3. Lamellibranchiata.

ENCEPHALA, Having a Head. 4. Pteropoda. 5. Gasteropoda. 6. Cephalopoda.

The Infusoria, the first class of the Radiata, comprise all those microscopic organisms called "Protozoa;" they consist of a membranous cell with fluid contents, and are very nearly allied to the Protophytes or lowest vegetable existences. It is still a matter of discussion amongst physiologists and microscopists as to which kingdom shall claim certain of them, but, according to Dr. Carpenter, it appears that the vegetable cell-beings have two envelopes, the inner one albuminous and the outer one cellulose or starchy, and the following passage occurs in his treatise on the Microscope (which should be in the hands of all enquiring minds):--

"The animal cell, in its most complete form, is comparable in most parts of its structure to that of the plant, but differs from it in the entire absence of the 'cellulose wall' or of anything that represents it, the cell-contents being enclosed in only a single limitary membrane, the chemical composition of which, being albuminous, indicates its correspondence with the primordial utricle. In its young state it seems always to contain a semi-fluid plasma, which is essentially the same as the protoplasm of the plant, save that it does not include chlorophyll granules, and this may either continue to occupy its cavity (which is the case in cells whose entire energy is directed to growth and multiplication) or may give place, either wholly or in part, to the special product which it may be the function of the cell to prepare. Like the vegetable cell, that of animals very commonly multiplies by duplicative sub-division, it also (especially among Protozoa) may give origin to new cells by the breaking up of its contents into several particles."

Some of these creatures are so exceedingly minute that hundreds of millions may be contained in a drop of water. They have no organs of any kind, but consist of single cells of a soft substance called "sorcode;" there is no distinction of sexes, and generation takes place by sub-division, each cell separating into two, and these again into two more, and so on with marvellous rapidity; this is the usual method of increase, but occasionally an act of conjugation takes place, two cells blending into one at the points of contact, the result of which is a nucleus or cell within the original one. These animated cells are of all forms, having prolongations which appear to be thrown out to absorb any particle of organic matter within reach, which then enters within the soft substance of the animalcule and is digested or dissolved, such are the Amboeba, the Actinophrys, &c. (see fig. 1); a step or two higher from these, the very lowest, infusoria are found to possess the first rudiments of organs in the form of cilia, which consist of minute elongations having a constant vibratory motion, for the purpose of causing a current in the water in which they live and bringing food into their vicinity.

Some very curious and complicated changes take place in the lowest of the animals, very similar to those in the lowest vegetables; and what has been called the life-history of these beings, often embraces a great number of forms before the circle of their metamorphoses comes round to the starting-point. Some of these phases of existence are quite different from those going before and after them, and as the vegetable free-cell at one time is capable of motion, it has long been mistaken for an active living animalcule (the Protococcus pluvialis, &c.), and there can be no doubt but that it will be found that many of the lower forms of animated life described by Ehrenberg and others, and still considered as such, will prove to be merely different stages of the metamorphosis of the same protozoon; or it may be not an animal at all, but a simple vegetable or protophyte.

Dr. Carpenter says, "It is quite certain that the Desmidiaceæ, like the confervoid plants in general, grow at the expense of the inorganic elements which surround them, instead of depending upon other living beings for their subsistence, and that they decompose carbonic acid and give off oxygen under the influence of sunlight. They have the power of generating from these materials the organic compounds which they require for their own development, and these are such as are formed by other undoubted protophytes, as is proved by the application of the appropriate tests." These "Desmidiaceæ" were long considered to belong to the animalculæ, as were the "Diatomaceæ," "Volvox globator" (fig. 2), &c.

Some of the Infusoria, however, have a far higher organisation, such as the "Rotiferæ," and especially the "wheel animalcule" (fig. 3), which at one time was supposed, absurdly enough, to possess a living wheel turning round rapidly on its head! This wheel, however, proved to be nothing else than a vortex of water produced by rotatory cilia. This and many other mistakes were owing to the imperfect construction of the microscopes of that date. These and their fellow animalcules possess several organs, as a stomach, &c. The protozoa at one time were supposed to possess a number of stomachs, and were thence called "poly-gastric infusoria," but it has now been demonstrated that these beings merely absorb or entangle portions of organic matter into their substance, and that there are no cysts or stomachs for their reception. Some of the Infusoria are able to withstand enormous changes of temperature, without losing their vitality; cold far below freezing point, and heat equal to boiling water does not destroy them, and they may be dried in the most complete manner which chemists can devise, and kept in this dry state for years, and yet, on the application of moisture, return to life. There is no doubt but every inch of air contains the germs of some of the Infusoria in a dry state or their ova, and make up a portion of that fine halo of dust which always appears in the air when a gleam of sunshine is let into a dark place.

The second class of the Radiata, the Entozoa, are those beings which inhabit, as parasites, the intestines and other parts of animals. Their history is very obscure, but there seems to be about twenty varieties of these creatures, and a great number of animals have their peculiar Entozoa. The best known in the human subject are the "Ascaris" or thread-worm, the "Lumbricus Teres" or long-worm, and the "Tænia" or tape-worm; this last is jointed, and grows to several yards in length.

The third class, the Zoophyta, includes four orders. The first of these, the Hydroida, comprise the Tubularia, Sertularia (fig. 4), and Sponges (fig. 5); these are minute gelatinous creatures, secreting a sort of tube in which they dwell. These tubes are congregated into masses of different forms and colours, and, with other structures totally different, are erroneously known by the general name of "sea-weed." The Zoophyte which dwells in these tubes is furnished with a set of cilia forming a sort of plume. Most of these Hydroida are phosphorescent, such are those which, scattered about on the surface of the sea, contribute to that luminosity so frequently seen.

The second order of the Zoophyta, Asteroida, are those which have a star-like shape; they are congregated into compound structures, forming what are called sea-fans (Gorgonia, fig. 6). These Gorgonia are of all colours, and very beautiful; the animals or "polypes" are gelatinous, and have a gelatinous mass or stalk with branches, strengthened and supported by a horny centre. This gelatinous mass forms the animal itself, as well as a sort of common integument connecting the whole family of many thousands. The Gorgonia or sea-fans usually preserved, are dry and hard, as the gelatinous covering has so little substance that, when dried, it forms a rough powdery covering only to the horny support. This powdery surface, if washed off with a little water and placed under the microscope, is found to contain and be made up of spiculæ or crystals of carbonate of lime, of various colours and beautiful forms (fig. 7); these, when the animal is alive, help to support its gelatinous frame; the Tubiporæ also belong to this class. Fig. 8 represents the Organ-coral (Tubipora musica); it is of a bright red colour, and, when the polypes are living, is a very beautiful object.

The third order of Zoophytes is the Helianthoida, which include the "Actinia." These Zoophytes are entirely gelatinous, and have beautiful variations of colour. They throw out tentacles like the petals of a flower, and form very beautiful objects; they may be seen in the aquatic vivaria of the Zoological Gardens. To this order of Zoophytes belong the "corals" or Zoanthoid polypes, which produce the coral reefs, extending in some cases hundreds of miles. Dr. Baird gives the following account of them:--

"By far the greater part of the Zoanthoid polypes, as they grow, deposit in the cellular substance of the flesh of their back an immense quantity of calcareous matter which enlarges as the animal increases in size, and, in fact, fills up those portions of the substance of the animal, which by the growth of new parts are no longer wanted for its nourishment, and in this manner they form a hard and strong case, amongst the folds of which they contract themselves so as to be protected from external injury, and by the same means they form for themselves a permanent attachment which prevents their being tossed about by every wave of the element in which they live. The stony substances so formed are called corals, and their mode of formation causes them exactly to represent the animal which secretes them. The upper surface is always furnished with radiating plates, the remains of the calcareous particles which are deposited in the longitudinal folds of the stomach. These, in all probability were the constructors of all the strata of limestone rocks, which form some of our largest mountains--they were once the beds of seas, and were up-heaved by internal forces. The Madrepores (fig. 9) are also of this order, and form the stony matter in the same way. These are of various patterns, and the beautiful brain-stones in the British Museum are of their construction."

The fourth order of Zoophytes are the Ascidioida, including formerly the Flustræ (fig. 10), but of late these have been shown to be closely allied, if not identical with, the Tunicated Mollusca, and have received the name of Polyzoa, they will therefore be noticed under that head.

The fourth class of the Radiata, are the Radiaria, including two orders. The first of these are the Acalephæ; the Medusæ or Jelly-fishes belong to this order; these creatures float about sometimes in great numbers, they are transparent and gelatinous, of a hemispherical form, the flat surface being fringed round with fibres, some of which sting like nettles if brought into contact with the skin.

The second order of the Radiaria include the Echinodermata or Sea-hedgehogs, Star-fishes, &c., these are many of them furnished with a profusion of spines, and hence their name; they are also called Sea-eggs or Sea-urchins (fig. 11), and are mostly of a conical or spherical shape, divided into five segments covered with minute holes, from which project tentaculæ serving as organs of locomotion, touch, &c., and the surface of the body is covered with shelly spines of various forms and thicknesses; the mouth is in the centre, and they feed upon marine productions. The spines, if cut across and ground thin so as to be transparent, show a most complicated and beautiful structure when under the microscope (fig. 12).

The Star-fishes are inhabitants of the sea, and have generally five arms covered beneath with tentaculæ having small suckers at the point, it is by means of these that they move from place to place; the mouth is in the centre. They live upon small marine animals, and have the power of reproducing any of the rays which happen to be broken off; and if cut into two or more pieces, each will produce a fresh animal. There is a species of Star-fish, so brittle, that it is quite impossible to touch it without breaking it, or rather it breaks itself, for it seems to have the power of casting off its arms at pleasure. These forms complete the Radiate series of animals, they have nearly all a somewhat regular radiated form, and some of them are very beautiful, such as Sertularia, which grow in communities and form many beautiful branched objects, much resembling sea-weeds and flowers, hence the name Zoophyte, which means animal flowers.

The Articulata form the second division of the Invertebrata. It has five classes, the first of which is the Annellata; in this order are included the Worms and Leeches (fig. 13), these have no hard covering but are soft throughout, they are made up of a number of soft rings which encircle their bodies from one end to the other, and are capable of being shortened or elongated by the muscles which draw together the rings (these are placed longitudinally), or by those which contract and widen them, placed circularly; this enables them to move along, and in so peculiar a manner that it has received the name of "vermicular," or worm-like. The most familiar example of this class is the Earth-worm (Lumbricus terrestris), this lowly creature lives in soft mud and clay and bores its way into the ground, in doing so it is assisted by a slimy secretion which covers it, its food consists of earth containing organic matter, this it swallows constantly and extracts what is fit for its nourishment ejecting the remainder apparently unaltered; the curious little heaps of soft dirt which are so often seen in wet weather on the garden-paths, &c., are formed in this way. The Leech is another familiar example of the Annellata or Annelidæ, this little useful creature lives in ponds and stagnant waters, and feeds by sucking the fluids of other animals (chiefly frogs and fish) with which it will gorge itself till it is double its original size, it will then remain (often for months) without food till it is all appropriated, the process going on very slowly; a full-grown Leech is perhaps twenty years old, they grow so slowly, (hence it is, that Leeches may be kept for months after being gorged with blood without any food, and they will live and be healthy). They are furnished with two suckers, one at each end, and a mouth with numerous fine teeth placed in a tri-radiate manner, with which they pierce the skin when about to feed. They move in the water with great rapidity by a wavy motion of their bodies, but on solid substances their mode of progression is that of adhering by the sucker in front, then drawing the other one close up and extending the body to its full length, taking hold again by the front sucker and again bringing up the hindermost.

The Cirripeda constitute the second class of the Articulata, which comprise the "Acorn shells" and "Barnacles" (fig. 14); these little creatures were formerly described as Molluscs, but are now considered to be articulate animals, and by some as Crustaceans. Dr. Baird gives the following description of them:--"The Cirripeds are articulated animals contained within a hard covering composed of several pieces and consisting of calcified chitine. The body of the animal is enclosed in a sac lined with the most delicate membrane of chitine, which in one group is prolonged into a peduncle and contains the ova; the body is distinctly articulated and placed with the back downwards."

Dr. Carpenter describes the young of the Cirripedes as not fixed like the adult creatures, but moving about freely, and only becoming fixed in the form of the usual acorn shell after undergoing several strange metamorphoses, during which stages they more resemble the ordinary Crustaceans than they do in their fixed state.