CHAPTER III. Crustacea (_continued_): Copepoda

CRUSTACEA (CONTINUED): COPEPODA

=Order II. Copepoda.=

The Copepods are small Crustacea, composed typically of about sixteen segments, in which the biramous type of limb predominates. They are devoid of a carapace. Development proceeds gradually by the addition posteriorly of segments to a Nauplius larval form. Paired compound eyes are absent, except in Branchiura, the adult retaining the simple eye of the Nauplius.

In a typical Copepod, such as Calanus hyperboreus (Fig. 25), we can distinguish the following segments with their appendages: a cephalothorax, carrying a pair of uniramous first antennae (1^{st} Ant.); a pair of biramous second antennae (2^{nd} Ant.); mandibles (Md.) with biting gnathobases and a palp, and a pair of foliaceous first maxillae (Mx.^1). Two pairs of appendages follow, which were looked upon as the two branches of the second maxillae, but it is now certain that they represent two pairs of appendages, which may be called second maxillae (Mx.^2) and maxillipedes (Mxp.) respectively. Behind these are five pairs of biramous swimming feet, the first pair (Th.^1) attached to the cephalothorax, the succeeding four pairs to four distinct thoracic somites. Behind the thorax is a clearly delimited abdomen composed of five segments, the first of which (Abd.^1) carries the genital opening, and the last a caudal furca.

The Copepods exhibit a great variety of structure, and their classification is attended with great difficulties. Claus based his attempt at a natural classification on the character of the mouth and its appendages, dividing the free-living and semi-parasitic forms as Gnathostomata from the true parasites or Siphonostomata. This division, although convenient, breaks down in many places, and it is clear that the parasitic mode of life has been acquired more than once in the history of Copepod evolution, while the free-living groups do not constitute a natural assemblage.

FIG. 25.—Calanus hyperboreus, × 30. Abd^1, First abdominal segment; 1st Ant, 2nd Ant, 1st and 2nd antennae; Md, mandible; Mx^1, Mx^2, 1st and 2nd maxillae; Mxp, maxillipede; Th^1, 1st thoracic appendage. (After Giesbrecht.) ]

Giesbrecht has more recently founded a classification of the free-living pelagic Copepods upon the segmentation of the body and certain secondary sexual characters, and he has hinted that this scheme of classification applies to the semi-parasitic and parasitic forms. Although much detail remains to be worked out and the position of some families is doubtful, Giesbrecht’s scheme is the most satisfactory that has hitherto been suggested, and will be adopted in this chapter.

The peculiarity in structure of the Argulidae, a small group of ectoparasites on fresh water fish, necessitates their separation from the rest of the Copepods (Eucopepoda) as a separate Branch, Branchiura.

BRANCH I. EUCOPEPODA.

=Sub-Order 1. Gymnoplea.=

The division between the front and hind part of the body falls immediately in front of the genital openings and behind the fifth thoracic feet. The latter in the male are modified into an asymmetrical copulatory organ.

TRIBE I. AMPHASCANDRIA.

The first antennae of the male are symmetrical, with highly-developed sensory hairs.

=Fam. Calanidae.=—The Calanidae are exclusively marine Crustacea, and form a common feature of the pelagic plankton in all parts of the world. Some species of the genus Calanus often occur in vast shoals, making the sea appear blood-red, and they furnish a most important article of fish food. These swarms appear to consist chiefly of females, the males being taken rarely, and only at certain seasons of the year. Some of the Calanidae are animals of delicate and curious form, owing to the development of plumed iridescent hairs from various parts of their body, which may often exhibit a marked asymmetry, as in the species figured, Calocalanus plumulosus (Fig. 26), from the Mediterranean.

FIG. 26.—Calocalanus plumulosus, × 15. (After Giesbrecht.) ]

Sars makes a curious observation with regard to the distribution of certain Calanidae. He reports that along the whole route of the “Fram,” species such as Calanus hyperboreus and Euchaeta norwegica were taken at the surface, which, in the Norwegian fjords, only occur at depths of over 100 fathoms. He suggests that the Norwegian individuals, instead of migrating northwards as the warmer climate supervened, have sought boreal conditions of temperature by sinking into the deeper waters.

TRIBE II. HETERARTHRANDRIA.

The first antennae of the male are asymmetrical, one, usually the right, being used as a clasping organ.

The males of the Centropagidae, Candacidae and Pontellidae, besides possessing the asymmetrically modified thoracic limbs of the fifth pair also exhibit a modification of one of the first antennae, which is generally thickened in the middle, and has a peculiar joint in it, or geniculation, which enables it to be flexed and so used as a clasping organ for holding the female.

=Fam. 1.—Centropagidae.=—These Copepods are very common in the pelagic plankton, and some of the species vie with the Calanidae in plumed ornaments, e.g. Augaptilus filigerus, figured by Giesbrecht in his monograph. The use of these ornaments, which are possessed by so many pelagic Copepods, is entirely obscure. Certain of the Centropagidae live in fresh water. Thus Diaptomus is an exclusively fresh-water genus, and forms a most important constituent of lake-plankton; various species of Heterocope occur in the great continental lakes, and certain Eurytemora go up the estuaries of rivers into brackish water.

An excellent work on the fresh-water Copepods of Germany has been written by Schmeil, who gives analytical tables for distinguishing various genera and species. The three fresh-water families are the Centropagidae, Cyclopidae, and Harpacticidae (see p. 62). The Centropagidae may be sharply distinguished from the other fresh-water families by the following characters:—The cephalothorax is distinctly separated from the abdomen; the first antennae are long and composed of 24–25 segments, in the male only a single antenna (generally the right) being geniculated and used as a clasping organ. The fifth pair of limbs are not rudimentary; a heart is present, and only one egg-sac is found in the female. The second antennae are distinctly biramous.

Diaptomus.—The furcal processes are short, at most three times as long as broad; endopodite of the first swimming appendage 2–jointed, endopodites of succeeding legs 3–jointed.

Heterocope.—The furcal processes are short, at most twice as long as broad; endopodites of all swimming legs 1–jointed.

Eurytemora.—The furcal processes are long, at least three and a half times as long as broad; the endopodite of the first pair of legs 1–jointed, those of the other pairs 2–jointed.

FIG. 27.—Dorsal view of Anomalocera pattersoni, ♂, × 20. (After Sars.) ]

It has been known for a long time that some of the marine Copepods are phosphorescent, and, indeed, owing to their numbers in the plankton, contribute very largely to bring about that liquid illumination which will always excite the admiration of seafarers. In northern seas the chief phosphorescent Copepods belong to Metridia, a genus of the Centropagidae; but in the Bay of Naples Giesbrecht states that the phosphorescent species are the following Centropagids: Pleuromma abdominale and P. gracile, Leuckartia flavicornis and Heterochaeta papilligera; Oncaea conifera is also phosphorescent. It is often stated that Sapphirina (p. 69) is phosphorescent, but its wonderful iridescent blue colour is purely due to interference colours, and has nothing to do with phosphorescence. Giesbrecht has observed that the phosphorescence is due to a substance secreted in special skin-glands, which is jerked into the water, and on coming into contact with it emits a phosphorescent glow. This substance can be dried up completely in a desiccated specimen and yet preserve its phosphorescent properties, the essential condition for the actual emission of light being contact with water. Similarly, specimens preserved in glycerine for a long period will phosphoresce when compressed in distilled water. From this last experiment Giesbrecht concludes that the phosphorescence can hardly be due to an oxidation process, but the nature of the chemical reaction remains obscure.

=Fam. 2. Candacidae.=—This family comprises the single genus Candace, with numerous species distributed in the plankton of all seas. Some species, e.g. C. pectinata, Brady, have a practically world-wide distribution, this species being recorded from the Shetlands and from the Philippines.

=Fam. 3. Pontellidae.=—This is a larger family also comprising widely distributed species found in the marine plankton. Anomalocera pattersoni (Fig. 27) is one of the commonest elements in the plankton of the North Sea.

=Sub-Order 2. Podoplea.=

The boundary between the fore and hind part of the body falls in front of the fifth thoracic segment. The appendages of the fifth thoracic pair in the male are never modified as copulatory organs.

TRIBE I. AMPHARTHRANDRIA.

The first antennae in the male differ greatly from those in the female, being often geniculated and acting as prehensile organs.

FIG. 28.—Euterpe acutifrons, ♀, × 70. Abd.1, 1st abdominal segment; Th.5, 5th thoracic segment. (After Giesbrecht.) ]

FIG. 29.—First antenna of Euterpe acutifrons, ♂. (After Giesbrecht.) ]

=Fams. 1–2.= =Cyclopidae= and =Harpacticidae=, and other allied families, are purely free-living forms; they are not usually pelagic in habit, but prefer creeping among algae in the littoral zone or on the sea-bottom, or especially in tidal pools. Some genera are, nevertheless, pelagic; e.g. Oithona among Cyclopidae; Setella, Clytemnestra, and Aegisthus among Harpacticidae.

The sketch (Fig. 28) of Euterpe acutifrons ♀, a species widely distributed in the Mediterranean and northern seas, exhibits the structure of a typical Harpacticid, while Fig. 29 shows the form of the first antenna in the male.

Several fresh-water representatives of these free-living families occur. The genus Cyclops (Cyclopidae) is exclusively fresh-water, while many Harpacticidae go up into brackish waters: for example on the Norfolk Broads, Mr. Robert Gurney has taken Tachidius brevicornis, Müller, and T. littoralis, Poppe; Ophiocamptus brevipes, Sars; Mesochra lilljeborgi, Boeck; Laophonte littorale, T. and A. Scott; L. mohammed, Blanchard and Richard; and Dactylopus tisboides, Claus.

Schmeil gives the following scheme for identifying the fresh-water Cyclopidae and Harpacticidae (see diagnosis of Centropagidae on p. 59):—

=Fam. 1. Cyclopidae.=—The cephalothorax is clearly separated from the abdomen. The first antennae of the female when bent back do not stretch beyond the cephalothorax; in the male both of them are clasping organs. The second antennae are without an exopodite. The fifth pair of limbs are rudimentary, there is no heart, and the female carries two egg-sacs.

Cyclops.—Numerous species, split up according to segmentation of rudimentary fifth pair of legs, number of joints in antennae, etc.

=Fam. 2. Harpacticidae.=—The cephalothorax is not clearly separated from the abdomen. The first antennae are short in both sexes, both being clasping organs in the male. The second antennae have a rudimentary exopodite. The fifth pair of limbs are rudimentary and plate-shaped; a heart is absent, and the egg-sacs of the female may be one or two in number.

1. Ophiocamptus (Moraria).—Body worm-shaped; first antennae of female 7–jointed, rostrum forming a broad plate.

2. Body not worm-shaped; first antennae of female 8–jointed, rostrum short and sharp.

(a) Endopodites of all thoracic limbs 3–jointed. The first antennae in female distinctly bent after the second joint. Nitocra.

(b) Endopodite of at least the fourth limb 2–jointed; first antennae in female not bent. Canthocamptus.

3. Ectinosoma.—Body as in 2, but first antennae are very short, and the maxillipede does not carry a terminal hooked seta as in 1 and 2.

=Fam. 3. Peltiidae.=—This is an interesting family, allied to the Harpacticidae, and includes species with flattened bodies somewhat resembling Isopods, and a similar habit of rolling themselves up into balls. No parasitic forms are known, though Sunaristes paguri on the French and Scottish coasts is said to live commensally with hermit-crabs.

We have now enumerated the chief families of free-living Copepods; the rest are either true parasites or else spend a part of their lives as such. A number of the semi-parasitic and parasitic Copepods can be placed in the tribe Ampharthrandria owing to the characters of their antennae; but it must be remembered that many parasitic forms have given up using the antennae as clasping organs; however, the sexual differences in the antennae, and the fact that many of the species which have lost the prehensile antennae in the male have near relations which preserve it, enable us to proceed with some certainty. The adoption of this classification necessitates our separating many families which superficially may seem to resemble one another, e.g. the semi-parasitic families Lichomolgidae and Ascidicolidae, and the Dichelestiidae from the other fish-parasites; it also necessitates our treating the presence of a sucking mouth as of secondary importance. This characteristic must certainly, however, have been acquired more than once in the history of the Copepods, for instance in the Asterocheridae and in the fish-parasites, while it sometimes happens that genera belonging to a typically Siphonostomatous group possess a gnathostome, or biting mouth, e.g. Ratania among the Asterocheridae. Again, it is impossible even if we use the character of the mouth as a criterion to place together all the true parasites on fishes in one natural group, because the Bomolochidae and Chondracanthidae, which are otherwise closely similar to the rest of the fish-parasites, possess no siphon. It seems plain, therefore, that the parasitic habit has been acquired several times separately by diverging stocks of free-swimming Copepods, and that it has resulted in the formation of convergent structures.

FIG. 30.—Haemocera danae, × 40. =A=, Side view ♀; =B=, ventral view ♂. Ant.1, 1st antenna; e, eye; ov, ovary; ovd, oviduct; St, stomach; Th.1, 1st thoracic appendage; Th.5, 5th thoracic segment; vd, vas deferens. (After Malaquin.) ]

FIG. 31.—Free-swimming Nauplius larva of Haemocera danae; Ant.1, Ant.2, 1st and 2nd antennae; e, remains of eye; Md, mandible. (After Malaquin.) ]

=Fam. 4. Monstrillidae.=—These are closely related to the Harpacticidae. The members of this curious family are parasitic during larval life and actively free-swimming when adult. There are three genera, Monstrilla, Haemocera, and Thaumaleus. The best known type is Haemocera danae (often described as Monstrilla danae). In the adult state (Fig. 30) there are no mouth-parts; the mouth is exceedingly small and leads into a very small stomach, which ends blindly, while the whole body contains reserve food-material in the form of brown oil-drops. The sole appendages on the head are the first antennae; but on the thorax biramous feet are present by means of which the animal can swim with great rapidity. This anomalous organisation receives an explanation from the remarkable development through which the larva passes. The larva is liberated from the parent as a Nauplius with the structure shown in Fig. 31; it does not possess an alimentary canal. It makes its way to a specimen of the Serpulid worm, Salmacina dysteri, into the epidermis of which it penetrates by movements of the antennae, hanging on all the time by means of the hooks on the mandibles. From the epidermis it passes through the muscles into the coelom of the worm, and thence into the blood-vessels, usually coming to rest in the ventral blood-vessel. As the Nauplius migrates, apparently by amoeboid movements of the whole body, it loses all its appendages, the eye degenerates, and the body is reduced to a minute ovoid mass of cells, representing ectoderm and endo-mesoderm, surrounded by a chitinous membrane (Fig. 32, A). Arrived in the ventral blood-vessel it begins to grow, and the first organ formed is a pair of fleshy outgrowths representing the second antennae (Fig. 32, B), which act as a nutrient organ intermediary between host and parasite. The adult organs now begin to be differentiated, as shown in Fig. 32, C, from the undifferentiated cellular elements of the Nauplius, the future adult organism being enclosed in a spiny coat from which it escapes. At this stage it occupies a large part of its host’s body, lying in the distended ventral blood-vessel, and it escapes to the outside world by rupturing the body-wall of the worm, leaving behind it the second antennae, which have performed their function as a kind of placenta. Malaquin, to whom we owe this account, makes the remarkable statement that if two or three Monstrillid Nauplii develop together in the same host they are always males, if only one it may be either male or female. The only parallel to this extraordinary life-history is found in the Rhizocephala (see pp. 96–99).

FIG. 32.—Later stages in the development of Haemocera danae. Abd, Abdomen; Ant.1, Ant.2, 1st and 2nd antennae; ch, chitinous investment; e, eye; Ect, ectoderm; En, endoderm; Mes, mesoderm; Mes & en, mesoderm and endoderm; R, rostrum; St, mouth and stomach; Th, thoracic appendages. (After Malaquin.) ]

FIG. 33.—Side view of Doropygus pulex, ♀, × 106. Abd.1, 1st abdominal segment; Ant.1, 1st antenna; b.p, brood-pouch; Th.1, 1st thoracic appendage; Th.4, 4th thoracic segment. (After Canu.) ]

=Fam. 5. Ascidicolidae.=—Although the members of this family, which live semiparasitically in the branchial sac or the gut of Ascidians, betray their Ampharthrandrian nature by the sexual differences of their first antennae, only two genera, Notodelphys and Agnathaner, possess true prehensile antennae. According as the parasitism is more or less complete, the buccal appendages either retain their masticatory structure or else become reduced to mere organs of fixation. In Notodelphys both sexes can swim actively and retain normal mouth-parts; they live parasitically, or perhaps commensally, in the branchial cavities of Simple or Compound Ascidians, feeding on the particles swept into the respiratory chamber of the host. They leave their host at will in search of a new home, and are frequently taken in the plankton.

Doropygus (Fig. 33), a genus widely distributed in the North Sea and Mediterranean, also inhabiting the branchial sac of Ascidians, is more completely parasitic, and the female cannot swim actively. Forms still more degraded by a parasitic habit are Ascidicola rosea (especially abundant in the stomach of Ascidiella scabra at Concarneau), in which the female has lost its segmentation, the mouth-parts and thoracic legs being purely prehensile, and various species of Enterocola, parasitic in the stomach of Compound Ascidians, in which the female is a mere sac incapable of free motion, while the male preserves its swimming powers and a general Cyclops-form (Fig. 34). We have here the first instance of the remarkable parallelism between the degree of parasitism and the degree of sexual dimorphism, a parallelism which holds with great regularity among the Copepoda, and can be also extended to other classes of parasitic animals.

FIG. 34.—Enterocola fulgens. =A=, Ventral view of ♀, × 35; =B=, side view of ♂, × 106. Abd.1, 1st abdominal segment; Ant.1, Ant.2, 1st and 2nd antennae; c.m, gland-cells; n, ventral nerve-cord; og, oviducal gland; ov, ovary; po, vagina; Th.1, 1st thoracic appendage; Th.4, Th.5, 4th and 5th thoracic segments. (After Canu.) ]

FIG. 35.—Asterocheres violaceus, ♀, with egg-sacs, × 57. (After Giesbrecht.) ]

FIG. 36.—Diagrammatic transverse section through the distal part of the siphon of Rhynchomyzon purpurocinctum (Asterocheridae). Md, mandible. (After Giesbrecht.) ]

=Fam. 6. Asterocheridae.=—These forms retain the power of swimming actively, and are very little modified in outward appearance by their parasitic mode of life (Fig. 35), though they possess a true siphon in which the styliform mandibles work. The siphon is formed by the upper and lower lips, which are produced into a tube with three longitudinal ridges; in the outer grooves are the mandibles, while the inner groove forms the sucking siphon (see transverse section, Fig. 36). In Ratania, however, there is no siphon. The first antennae possess a great number of joints, and may be geniculated in the male (Cancerilla). The members of this family live as ectoparasites on various species of Echinoderms, Sponges, and Ascidians, but they frequently change their hosts, and it appears that one and the same species may indifferently suck the juices of very various animals, and even of Algae. Cancerilla tubulata, however, appears to live only on the Brittle Starfish, Amphiura squamata.

=Fam. 7. Dichelestiidae.=—The males and females are similarly parasitic, and the body in both is highly deformed, the segmentation being suppressed and the thoracic limbs being produced into formless fleshy lobes; they are placed among the Ampharthrandria owing to sexual differences in the form of the first antennae. There is a well-developed siphon in which the mandibular stylets work, except in Lamproglena, parasitic on the gills of Cyprinoid fishes; the succeeding mouth-parts are prehensile.

The majority of the species are parasitic on the gills of various fish (Dichelestium on the Sturgeon, Lernanthropus on Labrax lupus, Serranus scriba, etc.), but Steuer has recently described a Dichelestiid (Mytilicola) from the gut of Mytilus galloprovincialis off Trieste. This animal and Lernanthropus are unique among Crustacea through the possession of a completely closed blood-vascular system which contains a red fluid; the older observers believed this fluid to contain haemoglobin, but Steuer, as the result of careful analysis, denies this. The parasite on the gills of the Lobster, Nicothoe astaci, possibly belongs here.

The inclusion of Nicothoe and the Dichelestiidae among the Ampharthrandria rests on a somewhat slender basis; this basis is afforded by the fact that none of the parasitic Isokerandria have more than seven joints in the first antennae, whereas Nicothoe and some of the Dichelestiidae have more numerous joints. In most of the Dichelestiidae, however, the number of joints is less than seven and practically equal in the two sexes.

TRIBE II. ISOKERANDRIA.

The first antennae are short, similar in the two sexes, and are never used by the male as clasping organs. This function may be subserved by the second maxillae.

FAMS. ONCAEIDAE, CORYCAEIDAE, LICHOMOLGIDAE, ERGASILIDAE, BOMOLOCHIDAE, CHONDRACANTHIDAE, PHILICHTHYIDAE, NEREICOLIDAE, HERSILIIDAE, CALIGIDAE, LERNAEIDAE, LERNAEOPODIDAE, CHONIOSTOMATIDAE.

The families Oncaeidae and Corycaeidae contain pelagic forms of flattened shape and great swimming powers, but the structure of the mouth-parts in the Corycaeidae points to a semi-parasitic habit.

=Fam. 1. Oncaeidae.=—This family, including the genera Oncaea, Pachysoma, etc., does not possess the elaborate eyes of the next family, nor is the sexual dimorphism so marked.

=Fam. 2. Corycaeidae.=—These are distinguished from the Oncaeidae, not only by their greater beauty, but also by the possession of very elaborate eyes, which are furnished with two lenses, one at each end of a fairly long tube. The females of Sapphirina are occasionally found in the branchial cavity of Salps, and their alimentary canal never contains solid particles, but is filled with a fluid substance perhaps derived by suction from their prey. S. opalina may occur in large shoals, when the wonderful iridescent blue colour of the males makes the water sparkle as it were with a sort of diurnal phosphorescence. The animal, however, despite the opinion of the older observers, is not truly phosphorescent. It may be that the ornamental nature of some of the males is correlated with the presence of the curious visual organs, which are on the whole better developed in the females than in the males. As in so many pelagic Copepods, the body and limbs may bear plumed setae of great elaboration and beautiful colour, e.g. Copilia vitrea (Fig. 37).

We now pass on to the rest of the parasitic Copepods, which probably belong to the tribe Isokerandria, and we meet with the same variety of degrees of parasitism as in the Ampharthrandria, often leading to very similar results.

FIG. 37.—Copilia vitrea (Corycaeidae), ♀, × 20. (After Giesbrecht.) ]

In the first seven families mentioned below there is no siphon. The Lichomolgidae and Ergasilidae have not much departed from the free-living forms just considered, retaining their segmentation, though in the Ergasilidae the body may be somewhat distorted (Fig. 39). In both families the thoracic swimming feet are of normal constitution.

FIG. 38.—Lichomolgus agilis, × 10. Abd. 1, 1st abdominal segment; cpth, cephalothorax; Th.1, 1st thoracic segment; Th.5, 5th thoracic appendage. (After Canu.) ]

=Fam. 3. Lichomolgidae.=—These are semi-parasitic in a number of animals living on the sea-bottom, such as Actinians, Echinoderms, Annelids, Molluscs, and Tunicates. Lichomolgus agilis (Fig. 38) occurs in the North Sea, Atlantic, and Mediterranean, on the gills of large species of the Nudibranch, Doris, while L. albeus is found in the peribranchial cavity and cloaca of various Ascidians. Sabelliphilus may infect the gills of Annelids such as Sabella, and is common at Liverpool.

=Fam. 4. Ergasilidae.=—Thersites (Fig. 39) is parasitic on the gills of various fishes, e.g. T. gasterostei, which is common on Gasterosteus aculeatus on the French and North Sea coasts, and may even be found on specimens of the fish that have run up the River Forth into fresh water. The animal possesses claw-like second antennae by which it clings to its host.

FIG. 39.—Thersites gasterostei. =A=, ♀, × 10; =B=, ♂, × 20. Abd. 1 & 2, Fused 1st and 2nd abdominal segments; Ant.1, Ant.2, 1st and 2nd antennae; e.s, egg-sac; Th, thoracic appendages. (After Gerstaecker.) ]

Similarly characterised by the absence of a siphon are three other families of fish-parasites, the Bomolochidae, Chondracanthidae, and Philichthyidae.

=Fam. 5. Bomolochidae.=—Bomolochus (Fig. 40), parasitic on the skin of the Sole (Solea) and in the nostrils of Cod (Gadus), is held to be related to the Ergasilidae. The first thoracic limb is remarkably modified. Were it not for the absence of a siphon, it would be hard to separate this family from the Caligidae.

FIG. 40.—Bomolochus, sp. (Bomolochidae), × 8. Abd. 1, 1st abdominal segment; Ant.1, Ant.2, 1st and 2nd antennae; Mx.1, Mx.2, 1st and 2nd maxillae; Mxp, maxillipede; Th.1, 1st thoracic appendage. (After Gerstaecker.) ]

FIG. 41.—Chondracanthus zei, ♀, × 4. ]

FIG. 42.—Dwarf male of Lernentoma cornuta (Chondracanthidae), × 10. Ant.1, Ant.2, 1st and 2nd antennae; Th.1, 1st thoracic segment. (After Gerstaecker.) ]

=Fam. 6. Chondracanthidae.=—These Copepods infest the gills and even the mouth of various marine fish, such as the Gurnard, Plaice, Skate, Sole, and many others. The sexual dimorphism is very marked, the female being large, indistinctly segmented, and with irregular paired processes protruding from the sides of the body, giving the animal a monstrous form (Fig. 41); while the male (Fig. 42) is very small, has a completely segmented thorax, and lives clinging on to the female by its prehensile second antennae—Chondracanthus, Lernentoma.

=Fam. 7. Philichthyidae.=—These parasites, which are hardly known to occur in British waters, are mucus-feeders and infest the skin of Teleosts, e.g. the Sole; often taking up a position in the lateral line or in a slime canal. They show a similar sexual dimorphism to the foregoing family, the adult female being extraordinarily drawn out into finger-like processes (e.g. Philichthys) or else long, slender, and Nematode-like, with much reduced appendages (Lernaeascus), while the male retains a more normal structure. As in all the foregoing forms there is no siphon.

We now return to two semi-parasitic families, =Fam. 8, Nereicolidae=, and =Fam. 9, Hersiliidae=, in which there is certainly no well-developed siphon, but the upper and under lips protrude, forming a hollow between them in which the mouth-parts work. Both families are ectoparasites which frequently leave their hosts, and they retain their segmentation and powers of swimming. Perhaps the best-known form is the Hersiliid, Giardella callianassae, which lives in the adult state in the galleries excavated in the sand by Callianassa subterranea, gaining its nourishment as an ectoparasite on the Decapod. The larvae are pelagic, and are said by Thomson to occur in Liverpool Bay.

List describes Gastrodelphys, a parasite of doubtful position, from the gills of tubicolous worms, such as Myxicola and Sabella, which possesses a perfectly siphonostomatous mouth.

The remaining families to be dealt with are those containing all the fish-parasites which possess a true siphonostome, as well as the siphonostomatous family Choniostomatidae, which is parasitic on other Crustacea. In all these forms the mouth is prolonged into a tube in which the styliform mandibles work.

=Fam. 10. Caligidae.=—Ectoparasites on fish, lodging most frequently in the gill-chamber. In most of the genera the segmentation and power of swimming are retained in both sexes, the sexual dimorphism not being very well marked, though the males are smaller than the females, and were in some cases originally described as belonging to a special genus Nogagus. The females carry two long egg-sacs; the general structure may be made out from the ventral view of Caligus nanus (Fig. 43).

Some of the Caligidae are distinguished by the terga of the thoracic segments being expanded to form large chitinous elytra, e.g. Cecrops, found parasitic on the gills of the Tunny and on the Sun-fish (Orthagoriscus mola). Caligus rapax is parasitic on the skin and in the gills of Sea-Trout, Pollan, etc.; and C. lacustris is common in fresh-water lakes and streams on Pike and Carp.

FIG. 43.—Caligus nanus, × 10. Abd.1, 1st abdominal segment; Ant.1, Ant.2, 1st and 2nd antennae; Mx.1, Mx.2, 1st and 2nd maxillae; Mxp, maxillipede; s, siphon; Th.1, Th.5 1st and 5th thoracic appendages. (After Gerstaecker.) ]

FIG. 44.—Lernaea branchialis from the Haddock, ♀, × 1. Ceph, cephalothorax; e.s, egg-sacs. (After Scott.) ]

=Fam. 11. Lernaeidae.=—These parasites burrow with their heads deep into the skin, or even into the blood-vessels or body-cavity, of various marine fish. The body of the adult female Lernaea is extraordinarily deformed, consisting of a mere shapeless sac with irregular branched processes on the head, and two egg-sacs attached behind (Fig. 44). Pennella sagitta bores so deeply into the flesh of its host, Chironectes marmoratus, that only the egg-sacs and some remarkable branchial processes attached to its abdomen protrude outside the host to the exterior. Peroderma cylindricum bores similarly into the flesh of the Sardine, and where it is common, inflicts considerable damage. The males of these curious animals are of more normal structure (Fig. 45). Claus states that fertilisation takes place when both sexes are free-swimming, and of a more or less similar structure, and that subsequently the female becomes fixed to her host and degenerates into the shapeless mass shown in Fig. 44.

FIG. 45.—Lernaea branchialis, ♂, × 10. Ant.1, Ant.2, 1st and 2nd antennae; Br, brain; e, eye; g, stomach; t, testis; vd, vas deferens; ves. sem, vesicula seminalis. (After Claus.) ]

FIG. 46.—Achtheres percarum. =A=, ♀, × 4; =B=, ♂, × 4. Ant.2, 2nd antenna; g, stomach; Mx.2, 2nd maxilla; Mxp, maxillipede; ov, ovary; ovd, oviduct. (After Gerstaecker.) ]

=Fam. 12. Lernaeopodidae.=—This family may be illustrated by the common gill-parasite of Perch and Trout, known as Achtheres percarum. The female (Fig. 46), which is much larger than the male, and is not clearly segmented, is attached to the host by means of the maxillipedes, which are fused distally into a pad armed with chitinous hooks. In the male the maxillipedes are prehensile, but are not so fused. Besides Achtheres there are other fresh-water forms, e.g. Lernaeopoda salmonea on Salmon, and a number of marine genera. It appears that the larvae fix themselves to their hosts by means of a long glandular thread, which proceeds from the middle of the forehead.

FIG. 47.—Ventral view of Stenochotheres egregius (Choniostomatidae), ♂. A, A′, 1st and 2nd antennae; M, mouth; Mx, 2nd maxilla; T, 1st thoracic leg. (After Hansen.) ]

=Fam. 13. Choniostomatidae.=—The members of this family are all parasitic on other Crustacea. The majority live parasitically in the marsupial pouches of female Amphipods, Isopods, Mysidae, and Cumacea, e.g. Sphaeronella and Stenochotheres in the marsupia of Gammarids; but Choniostoma occurs in the branchial cavity of Hippolyte, Homoeoscelis is common in the branchial cavity of Diastylis and Iphinoe, and Aspidoecia on the outside of the body of the Mysid Erythrops. The males and females live together in the same marsupium, but the adult males retain the power of roving about, and do not feed so much as the females, though their mouth-parts are similarly constructed (Fig. 47). Representatives occur all over the world, but the majority of species known at present are from the North Sea, the most abundant being Stenochotheres egregius, parasitic on the Gammarid Metopa bruzelii, Goës.

The male bears a median glandular thread on the forehead by which it attaches itself to the females or to the host. Hansen considers that the family is most closely allied to the Lernaeopodidae.

FIG. 48.—Argulus foliaceus, young ♂, × 15. a^1, a^2, First and second antennae; ab, abdomen; E, compound eye; l, liver; m, mandibles and first maxillae; mx, second maxilla (the median eye is seen between the two second maxillae); mxp, maxillipede; s.g, shell-gland; sp, spine; t, testis; 1, 4, first and fourth swimming appendages. (After Claus.) ]

BRANCH II. BRANCHIURA.

=Fam. Argulidae.=—We have yet to mention this group of fish-parasites, related to the Copepoda, but occupying an isolated position. They are ectoparasites upon various species of fish, Argulus foliaceus being common in the fresh waters of Europe, infesting the branchial chamber or the skin of fresh-water fish, but being frequently taken swimming freely in the water. Both males and females can swim with great agility, and they leave their hosts regularly at the breeding season in spring and autumn; fertilisation is internal, and the female deposits the eggs on stones and other objects. After leaving its host, an Argulus, if it cannot find a fish of the same species, can live on almost any other species, and may even attack Frog tadpoles; while the kinds that infest migratory fish can change with their hosts from salt to fresh water, or the reverse. America appears to be the home of the Argulidae.

The structure of an Argulid is exhibited in Fig. 48. In front of the siphon, within which the styliform mandibles and first maxillae work, there is a poison-spine (sp); the appendages which correspond to the second maxillae (mx) are modified into sucking discs, but in the genus Dolops they terminate in normal claws. The next pair of appendages, usually spoken of as maxillipedes (mxp), are clasping organs, and behind follow four pairs of thoracic swimming feet (1–4). The body is foliaceous, and they always apply themselves to their hosts with the long axis pointing forwards and parallel to that of the host, while on various parts of the under surface of the body are spines pointing backward which prevent the parasite being brushed off by the passage of the host through the water. These animals, alone among the Copepoda, possess compound eyes.

A short sketch has now been given of the variations in Copepod organisation, but we cannot leave the subject without pointing out the rich field which still remains for the morphologist, especially in determining the true relationships of the parasitic families.