Copepod

“ They usually possess small but well-developed swimming-legs, and by means of these they seek out the female, dying after fertilization is accomplished. They are thus not sessile parasites at all, and have therefore to go through the stages of the free-swimming Copepods much more completely than the females, whose task is to accumulate within themselves from the blood of the fish as much material as possible for the forming of the eggs, and to produce the largest possible number of these. ”

“ And a lot of them have simply succumbed to environmental pressure. Given a generation or two of rational training and a nobler ideal of what befits a human being, and the copepod will yet succour herself.... Denny and I see more of the other kind. The drudges outnumber the copepods, and neither need be.... There's a girl over there I like—the one with the braided hair. Many of the young girls of to-day are rather wonderful. It's going to be interesting to see what they'll do when they're older, and what their daughters will do. ”

“ When once settled down it has little to fear from the attacks of other animals. The size of its shell relative to its body would deter most animals from regarding it as a desirable article of food, and as far as is known at present the Brachiopoda suffer but little from internal parasites, the only case I know being a minute parasitic Copepod belonging to a new and as yet unnamed genus which I found within the mantle cavity of Cistella (Argiope) neapolitana in Naples. ”

“ In those Copepods in which the palps of the mandibles as well as the antennae are biramous and natatory, the first three pairs of appendages retain throughout life, with little modification, the shape and function which they have in the nauplius stage, and must, in all likelihood, be regarded as approximating to those of the primitive Crustacea. In other respects, however, such as the absence of paired eyes and of a shell-fold, as well as in the characters of the post-oral limbs, the Copepoda are undoubtedly specialized. ”

“ A longer step takes us to the Crustacea, represented in the pelagic plankton by queer little shrimplike forms that in countless hosts of individuals play a part in the ocean comparable to that of insects on land. The copepods are the most numerous probably—little things only a fraction of an inch in length, but amazingly abundant, and the principal users of plant food. ”

“ It is probable also, as already mentioned, that the leaf-like appendages of the Phyllopoda are of a primitive type, and attempts have been made to refer their structure to that of the Annelid parapodium. In many respects, however, the Phyllopoda, and especially Apus, have diverged considerably from the primitive Crustacean type. All the cephalic appendages are much reduced, the mandibles have no palps, and the maxillulae are vestigial. In these respects some of the Copepoda have retained characters which we must regard as much more primitive. ”

“ Most Copepoda have only a single eye in the middle of the head, corresponding to the single eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea. In many plankton species, however, this simple eye becomes much enlarged and complicated in various ways. The three parts of which it is normally made up may become separated from each other, and are sometimes increased in number to five, while lenses serving to concentrate the light are often developed by thickening of the overlying cuticle. ”

“ Besides all these more or less temporary visitors, however, there are numerous species, even in the inshore waters, which are adapted to a floating life, and pass their whole existence as members of the plankton. Copepoda of many kinds, some Mysidæ, Amphipods like Hyperia—which is commonly found sheltering under large jellyfish—some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean. ”

“ Parker, [229] in a study of Copepods, finds that the females have a strong positive phototaxis for light of a low [Pg 617] intensity, while males show a weak negative phototaxis. To light of over 100-candle power at a distance of 10 cm. or to direct sunlight the female Copepods are negative, while the reaction of the males does not seem to be altered.
In his work on Carcinus, Bethe obtained retraction of the eye-stalks by suddenly throwing a strong light on the eye by means of a mirror. ”

“ Many of the larger species of Crustacea are used as food by man, the most valuable being the lobster, which is caught in large quantities on both sides of the North Atlantic. Perhaps the most important of all Crustacea, however, with respect to the part which they play in the economy of nature, are the minute pelagic Copepoda, of which incalculable myriads form an important constituent of the “plankton” in all the seas of the globe. It is on the plankton that a great part of the higher animal life of the sea ultimately depends for food. ”

“ 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, [41] who gives analytical tables for distinguishing various genera and species. ”

“ In some cases three separate nerves arise from the front of the brain, one going to each of the three divisions of the eye. In the Copepoda the median eye may undergo considerable elaboration, and refracting lenses and other accessory structures may be developed in connexion with it.
The compound eyes are very similar in the details of their structure (see Arthropoda) to those of insects (Hexapoda) . They consist of a varying number of ommatidia or visual elements, covered by a transparent region of the external cuticle forming the cornea. ”

“ No group of Crustacea exhibits more numerous or more varied examples of parasitism than the Copepoda. Every grade of transition between a free predatory habit of life and the most complete dependence upon a host may be traced in various families of the subclass. Only a few examples can be mentioned here.
The commonest "fish-lice" are the numerous [225] species of the family Caligidæ, many of which, belonging to the genera Caligus (Fig. 72) , Lepeophthirus, etc., are found on marine fishes on our own coasts. ”

“ This luminosity is due to the animals of the plankton, largely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea. A number of pelagic Copepoda have been shown by Giesbrecht to secrete, from special glands on the surface [151] of the body, a substance which becomes luminous on coming in contact with the water. Even specimens which had been dried were found to give out light on being wetted. Some pelagic Ostracods of the family Halocypridæ have been observed to emit clouds of a luminous secretion from a gland in the neighbourhood of the mouth. ”

“ Among the Copepoda taken [230] by the tow-net in British seas, there may sometimes be found species of the family Monstrillidæ (Fig. 74, F) , which are remarkable for having no appendages between the antennules and the first pair of swimming feet. They have no trace of jaws, and only a minute vestige of a mouth-opening; while internally there is no food-canal, so that the animals are incapable of taking nourishment. ”

“ In the examination of about fifteen hundred specimens of herring at Eastport, Maine, and vicinity, in the summer and fall of 1893, Mr. H. F. Moore, of the Fish Commission, found only two kinds of food—copepods or "red seed," which appeared to constitute the sole food of the small herrings, and shrimps the principal food of the larger ones. In many cases the stomachs of the fish were densely gorged with these shrimps, which are extremely abundant in the waters of the vicinity. ”

“ We can nowhere find more graceful and regular forms during the whole of their early youth than those of many of these parasites; we can never see more ungraceful, we might almost say more comical, attitudes than those of the greater part of these creatures when full grown. One might take them for some misshapen excrescence, or some scrap of wasted flesh on the body of their host. A certain number of insects are found which lead this singular kind of life, but this is more especially the case among the crustaceans, particularly the copepod crustaceans. ”

“ In many Entomostraca (Phyllopoda, Cladocera, Ostracoda, Copepoda) they are important, and sometimes the only, organs of locomotion. In some male Phyllopoda they form complex “claspers” for holding the female. They are frequently organs of attachment in parasitic Copepoda, and they may be completely pediform in the Ostracoda. In the Malacostraca they are chiefly sensory, the endopodite forming a long flagellum, while the exopodite may form a lamellar “scale,” probably useful as a balancer in swimming, or may disappear altogether. ”

“ Most free-living forms are minute, and all have thin tests.
[111]
The eyes of copepods are of interest, in that they suggest the paired ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in the simplest and typical form of these organs, three ocelli, each supplied with its own nerve from the brain. Two of these are dorsal and look upward, while the third is ventral. In some forms the dorsal ocelli are doubled, so that five in all are present (cf. some species of Harpes with three ocelli on each mound) . ”

“ Differences in size are frequent; in the higher Decapods the males and in the lower Crustacea the females frequently being larger, the disparity being extreme in some of the parasitic Copepods and Isopods where the males are minute and attached to the females, whilst in the Cirripedes, as Darwin himself discovered, very minute complementary males may live as parasites in the mantle cavity of large hermaphrodite or female forms. ”

“ Thus the males of the Copepods possess on their anterior antennæ an arrangement which enables them to throw a long whiplike structure like a lasso round the head of the female as she rapidly swims away. ”

“ The general condition is somewhat more primitive than in the copepods, for all the appendages are biramous, while in the modern forms the maxillipeds are uniramous and the sixth pair of thoracic appendages are usually modified in the male as copulatory organs. In the copepods the modification is in the direction of reduction, both endopodites and exopodites usually possessing fewer segments than the corresponding branches in the trilobites. ”

“ A very remarkable condition of the blood-system, unique, as far as is yet known among the Arthropoda, is found in a few genera of parasitic Copepoda (Lernanthropus, Mytilicola) . In these there is a closed system of vessels, not communicating with the body-cavity, and containing a coloured fluid. There is no heart. The morphological nature of this system is unknown.
Excretory System.—The most important excretory or renal organs of the Crustacea are two pairs of glands lying at the base of the antennae and of the second maxillae respectively. ”

“ When the back of a dog's brain has been injured on both sides, the dog has a tendency to keep on running forward all the time and while he is unwilling to climb stairs he will willingly go downstairs.
The Behavior of Copepods. When we pour carbonated water or beer or alcohol into an aquarium, certain crustaceans called copepods will at once swim toward the source of light, as tho they "loved" light, and appear so interested in light that they will "forget," to eat their food, if that food is placed away from the source of light. ”

“ None has the power of flight, though certain pelagic Copepoda are said to leap from the surface of the sea like flying-fish. Apart from the numerous parasitic forms, the only Crustacea which have adopted a strictly sedentary habit of life are the Cirripedia, and here, as elsewhere, profound modifications of structure have resulted, leading ultimately to a partial assumption of the radial type of symmetry which is so often associated with a sedentary life. ”

“ Besides adult animals belonging to various groups, such as the Copepoda, which pass the whole of their life swimming near the surface of the sea, there will be numerous larval stages of species which in their adult form live on the sea-bottom. The identification of the species to which the various larvæ belong is a matter of considerable difficulty, and, although the general course of development is now well known for all the chief groups of Crustacea, there are very many even of the common British species in which the larval transformations have not yet been worked out in detail. ”

“ ENTOMOSTRACA. This zoological term, as now restricted, includes the Branchiopoda, Ostracoda and Copepoda. The Ostracoda have the body enclosed in a bivalve shell-covering, and normally unsegmented. The Branchiopoda have a very variable number of body-segments, with or without a shield, simple or bivalved, and some of the postoral appendages normally branchial. The Copepoda have normally a segmented body, not enclosed in a bivalved shell-covering, the segments not exceeding eleven, the limbs not branchial. ”

“ The Copepoda live upon the diatoms and other important microscopic vegetable life at the surface of the sea, and in their turn serve as food for fishes and other larger forms and thus, indirectly, for man himself. ”