Entomostraca

“ Characteristic of the tropical belt are the absence of Crayfishes and the great development of Prawns and River-crabs. In the case of Entomostraca the great majority of the genera are cosmopolitan, especially those which live in small bodies of water liable to dry up, because these forms always have special means of dissemination in the shape of resting eggs which can be transported in a dry state by water-birds and other agencies to great distances; but those genera which inhabit large lakes are more confined in their distribution. ”

“ The genera and species of the Entomostraca are very numerous. Those mentioned above will serve{130} to illustrate the general structure of the order. To distinguish the man ́dibles (mandib ́ula, a jaw) or proper jaws, the foot-jaws, and the branchial legs, the animals must be dissected in water with the mounted needles. The very delicate feathery filaments of the branchiæ may be best observed when these organs are dried on a slide. ”

“ The delicate balance of conditions between organisms, whether it be between individuals of the same species or between widely-separated groups, is an important feature in the question of survival. Professor S. A. Forbes, [37] in a thoughtful study of certain species of entomostraca in Lake Michigan and the surrounding waters, calls attention to the important part played by these minute crustaceans, showing how they furnished almost the entire food for young fishes, larger crustaceans and even insect larvæ. ”

“ 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. ”

“ The Entomostraca, however, are certainly a heterogeneous assemblage, defined only by negative characters, and the name is retained only for the sake of convenience, just as it is often useful to speak of a still more heterogeneous and unnatural assemblage of animals as Invertebrata. The barnacles and their allies, forming the group Cirripedia or Thyrostraca, sometimes treated as a separate sub-class, are distinguished by being sessile in the adult state, the larval antennules serving as organs of attachment, and the antennae being lost. ”

“ The cause was, no doubt, comparative scarcity of entomostracan life, dependent, most likely, upon certain undefined conditions of sea-temperatures, affected, possibly, from the river-basin of Tay and its tributaries. If light can be thrown upon these not-difficult-to-ascertain data, in a few years, at most, much of our uncertainty as to causes and effects of migration of Entomostraca, Sea-fish, and even Salmon and migratory Salmonidæ will be removed. ”

“ If we except the Crayfishes and River-crabs, the Crustacean fauna of running water is exceedingly poor, but in all standing fresh-water, from the smallest pond to the large lakes and inland seas, Crustacea, especially Entomostraca, are abundant and characteristic, and form an important item in the food of fresh-water fishes. In small ponds a vast assemblage of Cladocera is met with ”

“ Gills.—In many of the smaller Entomostraca (Copepoda and most Ostracoda) no special gills are present, and respiration is carried on by the general surface of the body and limbs. When present, the branchiae are generally differentiations of parts of the appendages, most often the epipodites, as in the Phyllopoda. In the Cirripedia, however, they are vascular processes from the inner surface of the mantle or shell-fold, and in some Ostracoda they are outgrowths from the sides of the body. ”

“ The limbs of the post-cephalic series show little differentiation among themselves in many Entomostraca. In the Phyllopoda they are for the most part all alike, though one or two of the anterior pairs may be specialized as sensory (Apus) or grasping (Estheriidae) organs. In the Cirripedia (Thyrostraca) the six pairs of biramous cirriform limbs differ only slightly from each other, and in many Copepoda this is also the case. In other Entomostraca considerable differentiation may take place, but the series is never divided into definite “tagmata” or groups of similarly modified appendages. ”

“ The abdomen is composed of six segments, which typically carry as many pairs of biramous “pleopods,” and the body terminates in a telson. Not counting the paired eyes or the telson, there are present nineteen segments. The excretory organs in the adult open at the bases of the second antennae, and are known as “green glands,” but in the larva maxillary glands may be present homologous to those which persist in the adult Entomostraca. This is the typical arrangement, but sometimes the maxillary glands persist in adult Malacostraca, e.g. Nebalia, Anaspides, and some Isopods. ”

“ By the ventral sucker each species adheres to its own particular host, the tentacles being used as an anterior sucker for "looping" movements. The food, consisting of Entomostraca, Rotifera, and Diatoms, is first swallowed whole by the large pharynx (Fig. 20, ph) , which can be protruded through the ventrally-placed mouth, and is then received into a simple lobed intestine (d) . The skin, especially on the surface of the tentacles, is provided here and there with patches of cilia borne by the cellular epidermis,—the only undoubted case of external cilia occurring in an adult Trematode. ”

“ Many, perhaps the majority, of the Crustacea are omnivorous or carrion-feeders, but many are actively predatory in their habits, and are provided with more or less complex and efficient instruments for capturing their prey, and there are also many plant-eaters. Besides the sedentary Cirripedia, numbers of the smaller forms, especially among the Entomostraca, subsist on floating particles of organic matter swept within reach of the jaws by the movements of the other limbs. ”

“ A large and important group of the higher Crustacea, but confined to the older rocks of Victoria, is the order PHYLLOCARIDA. This seems to form a link between the Entomostraca, including the bivalved Ostracoda and the well-known group of the lobsters, shrimps and crabs. The body of these phyllocarids consists of five segments to the head, eight to the thorax, and from two to eight to the abdomen. The portion usually preserved in this group is the carapace, which covers the head and thorax, and although often in one piece, is sometimes hinged, or « 244 » otherwise articulated along the back. ”

“ Apart from ordinary methods of fishing for them at the sides or in the centre of pools and watercourses, that which more than anything else stands in antithesis to the costly labour of dredging and trawling in submarine abysses is the process, applicable at least to many fresh-water Entomostraca, of dredging on dry land. Many sheets of water at certain seasons completely evaporate, and expose a moistureless floor. ”

“ Further, the marshes and shallower lakes are the favorite breeding-grounds of fishes, which migrate to them in spawning-time, if possible, and it is from the entomostraca found here that most young fishes get their earliest food-supplies—a danger from which the deep-water species are measurably free. ”

“ Those, too, who send or bring the foreign soil should exercise a little thought in the choice of it, since dry earth that has never had any Entomostraca near it at home will not become fertile in them by the mere fact of exportation. ”

“ The more primitive forms (Entomostraca) are anomomeristic, presenting great variety as to number of somites, form of appendages, and tagmatic grouping; the higher forms (Malacostraca) are nomomeristic, showing in front of the telson twenty somites, of which the six hinder carry swimmerets and the five next in front ambulatory limbs. The genital apertures are neither far forward nor far backward in the series of somites, e.g. on the fourteenth post-oral in Apus, on the ninth post-oral in female Astacus and in Cyclops. ”

“ They form, in fact, the substratum of all animal life, constituting the food not only of animals somewhat larger than themselves, but of many much larger, as the various species of the cyprinus. Many valuable gregarious fishes, as the herring, char, and the finer species of trout, live on entomostraca; they in their turn become the food of larger and more voracious fishes. ”

“ During the summer of 1874 he was at work on the genus Utricularia, and he wrote (July 16th) to Sir J.D. Hooker giving some account of the progress of his work:—
"I am rather glad you have not been able to send Utricularia, for the common species has driven F. and me almost mad. The structure is MOST complex. The bladders catch a multitude of Entomostraca, and larvae of insects. The mechanism for capture is excellent. But there is much that we cannot understand. From what I have seen to-day, I strongly suspect that it is necrophagous, i.e. that it cannot digest, but absorbs decaying matter." ”

“ It has been usually held that the indefinite number of segments characteristic of the Entomostraca, and especially the indefinitely large number of segments characteristic of such Phyllopods as Apus, preserves the ancestral condition from which the definite number found in the Malacostraca has been derived; but recently it has been clearly pointed out by Professor Carpenter [3] that the number of segments found in the Malacostraca and Leptostraca corresponds with extraordinary exactitude to the number determined as typical in all the other orders of Arthropoda. ”

“ When they are confined in a small quantity of water and are about to die, a curious phenomenon may be observed in them, a number of oil-like sarcodic globules exuding from the body, and{164} within these, vacuoles may often be seen to form spontaneously.
The Infusoria may be collected in small phials; but it is difficult to keep them, as they form the food of the Entomostraca, the Rotatoria, and the larvæ of insects; so that their enemies are very numerous, and they soon disappear. ”

“ Having no resource but to devour one another, they would quickly have brought life to an end. There must have been smaller types serving as prey for larger ones before these could continue to exist and to multiply: microbes affording food to infusoria, infusoria affording food to such sized creatures as the Entomostraca, these again supplying food to small fishes, such as loch-trout, and these last yielding to larger fishes masses sufficiently great for their needs: each higher grade requiring lower grades of appropriate bulk. ”

“ In the great class of the Crustacea, forms wonderfully distinct from each other, namely, suctorial parasites, cirripedes, entomostraca, and even the malacostraca, appear at first as larvæ under the nauplius-form; and as these larvæ live and feed in the open sea, and are not adapted for any peculiar habits of life, and from other reasons assigned by Fritz Müller, it is probable that at some very remote period an independent adult animal, resembling the Nauplius, existed, and subsequently produced, along several divergent lines of descent, the above-named great Crustacean groups. ”

“ Entomostraca, Medusæ, Ctenophora, Siphonophora, Pteropods, and Radiolarians are also to be found in great abundance on the whaling grounds. The deep-sea dredgings of the Norwegians do not show such an abundant or rich fauna as is met with at similar depths in more southern latitudes. ”

“ Of the last genus the type species B. hodgsoni belongs to Cape Colony, but the specimens described were born and bred and observed in Norway. For the study of fresh-water Entomostraca large possibilities are now opened to the naturalist. A parcel of dried mud, coming for example from Palestine or Queensland, and after an indefinite interval of time put into water in England or elsewhere, may yield him living forms, both new and old, in the most agreeable variety. Some caution should be used against confounding accidentally introduced indigenous species with those reared from the imported eggs. ”

“ This variety is one of the most abundant of all coast crustacea, swimming about and laying on the sands (which they closely resemble in colour) in immense shoals. Sometimes they are also found in deep water, but the margin of the sea is their favourite habitat. It may be added, that large quantities of the smaller palæmonidæ are caught with and sold as shrimps. Shrimps are in spawn all the year through, and cast their shells during the three months of spring.
The Entomostraca of Milne Edwards, or the Lophyropoda of Latreille, have no suctorial mouth or mandibles capable of mastication ”

“ It is not known that male crustaceans fight together for the possession of the females, but it is probably the case; for with most animals when the male is larger than the female, he seems to owe his greater size to his ancestors having fought with other males during many generations. In most of the orders, especially in the highest or the Brachyura, the male is larger than the female; the parasitic genera, however, in which the sexes follow different habits of life, and most of the Entomostraca must be excepted. ”

“ The crustaceous and molluscous animals, which have ten feet, such as the Crab, Cray-fish, &c.
Digitate (L. digitus, a finger) . When a part supports processes like fingers.
Effete. Barren, worn out.
Elytra (Gr. elytron, a sheath) . The sheath or wing covers of coleopterous insects (Beetles) .
Entomostraca (Gr. entoma, insects; ostracon, a shell) . The order of small crustaceans, many of which are enclosed in an integument like a bivalve shell.
Entomology (Gr. entoma, insects; logos, a discourse) . The branch of science treating of insects.
Exuvium, Pl. exuviæ (L. exuo, I cast off) . ”