Crustacea

“ Nowhere, perhaps, is the struggle for existence more keen, and it is not without justice that some naturalists have regarded the shallow waters of the sea as "one of the great battle-fields of life," where, in the long course of evolution, the main branches of the animal kingdom have had their origin.
Conspicuous among the animals of this region are Crustacea of all sorts and sizes. To identify all the species that may be obtained in a single haul of the dredge in British seas would sometimes be a hard task even for the most expert student of the group. ”

“ Be this as it may, there are many deep-sea Crustacea which have well-developed and darkly-pigmented eyes. Some of these are swimming forms, which may at times migrate into the upper strata of water to which some rays of light penetrate; but there are some cases of Crabs and other bottom-living species that have well-developed eyes, although they live at great depths. ”

“ Fish are only found in it as eggs, or larvae. Crustacea of all kinds are present, and form one of its most important constituents. Crabs and lobsters spend their larval, free-swimming career among the plankton, until they reach the adult stage and settle down to the bottom. Various minute crustacea, known as “Copepoda” (lit., oar-footed) spend the whole of their lives drifting about in the surface water. They occur in incredibly large numbers, and are the most abundant of all forms of marine life. ”

“ The Crustacean fauna of fresh water is much less rich and varied than that of the sea. Although the number of individuals in a pond or lake may be enormous, they will be found to belong to a comparatively small number of species. All the subclasses of Crustacea with the exception of the Cirripedia have representatives in fresh water, but in most of them only a very few of the families and genera comprise truly fresh-water species. In spite of the comparative poverty of the fauna, however, it is of very great interest, more especially with regard to the problems of geographical distribution ”

“ Although the Crustacea as a whole are predominantly active animals, many examples have already been mentioned of species which are more or less sluggish and sedentary in their habits. The extreme degree of passivity is reached by the Barnacles (Cirripedia) , which differ from all other Crustacea (except some parasites) in being fixed to one spot, and quite without the power of locomotion in the adult state. Most of the Barnacles met with on the shore or in shallow water belong to the division of the Sessile Barnacles or Acorn-shells (Operculata) . ”

“ CRUSTACEA—GENERAL ORGANISATION The Crustacea are almost exclusively aquatic animals, and they play a part in the waters of the world closely parallel to that which insects play on land. The majority are free-living, and gain their sustenance either as vegetable-feeders or by preying upon other animals, but a great number are scavengers, picking clean the carcasses and refuse that litter the ocean, just as maggots and other insects rid the land of its dead cumber. ”

“ Reptilia and Pisces have not been neglected, and we are glad to see the Crustacea more prominent on our literary menu. The Stalk-eyed Crustacea of Great Yarmouth, and the Malacostracous Crustacea of a section of Australia have been detailed; while a note on "The Struggle for Existence among Hermit Crabs" shows the vast interest attaching to observations on the lives of these creatures. ”

“ Everyone has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible Crabs, Lobsters, Prawns, and Shrimps, are at least superficially familiar, either as brought to the table or as displayed in the fishmonger's, and the most unobservant of seaside visitors must have had his attention attracted by living specimens of some of the more obtrusive species, such as the common Shore Crab. ”

“ There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping themselves afloat. ”

“ The great majority of Crustacea are hatched from the egg in a form very different from that which they finally assume, and reach the adult state only after passing through a series of transformations quite as remarkable as those which a caterpillar undergoes in becoming a butterfly, or a tadpole in becoming a frog. ”

“ For the capture of species living in the deep water of large lakes, a special form of dredge has been devised with runners to prevent it from sinking into the soft mud, while the mouth of the net is raised a few inches above the bottom.
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For preserving Crustacea the best medium to use is 70 per cent. alcohol. Strong spirit diluted with a little less than one-third its bulk of water gives about the required strength. If too strong spirit is used, the specimens tend to be hard and brittle, and delicate organisms become shrivelled. ”

“ It follows that only those forms which possess special adaptations for dispersal are able to flourish in fresh water. In many cases, as will be described below, the eggs of the smaller Crustacea can survive being dried up, and in this state they may be blown about by wind or carried to great distances in mud, adhering to the feet of migratory wading birds. ”

“ They have a distinct head, furnished with antennæ, eyes, and mouth; and their bodies are{xviii} covered with a crust or shell, divided into segments by transverse incisions, the segments being united by a strong membrane. Once a year the larger species of these animals moult, throwing off their old crust or shell, and forming a new one, the animal remaining in a naked and greatly weakened state during the intermediate time. Many of the Crustacea swim with great ease, but on land their motions are generally cramped and awkward; and they are confined to crawling, or leaping by means of the tail. ”

“ In ordinary Crustacea the carapace, like the rest of the chitinised and calcified integument, is shed off in successive moults, and is restored again as a whole. But in Estheria (and one or two other small crustacea) the moult is incomplete: the old carapace is retained, and the new, growing up underneath it, adheres to it like a lining, and projects beyond its edge: so that in course of time the margins of successive old carapaces appear as “lines of growth” upon the surface of the shell. ”

“ In the inshore waters the plankton consists not only of organisms that pass the whole of their life at or near the surface, but also, and very largely, of the free-swimming larvæ of bottom-living species, and of others that make occasional and temporary excursions to the surface. For example, if the tow-net be used a short distance from land—say in some sheltered bay on our own coasts—the catch will often be found to consist largely of larval Crustacea. ”

“ The Crustacea, so called from the hard crust or shell which envelops them, form, with all their orders and genera, a very numerous family. They are of interest to us as containing among their number some of the oldest forms of life. Away down in the lower Silurian rocks, among the most ancient fossiliferous strata, we find the crustacean with its armour of plates and its prominent sessile eyes set round with lenses, still visible on the stone. Thus, on the first page of the stony records of our planet's history are these primeval organisms engraved. ”

“ In the case of deep-sea Crustacea, we are frequently confronted with a difficulty which does not occur in the case of some other groups of animals—Corals or Echinoderms, for example—the difficulty, namely, of deciding whether the animals really lived on or near the bottom, or were captured by the open mouth of the trawl on its way to the surface. ”

“ The preservation of the soft parts of animals in the fossil state depends upon favourable conditions of rare occurrence; and, in the case of the Crustacea, it is not often that one can hope to meet with such small hard parts as the abdominal members, in a good state of preservation. But without recourse to the branchial apparatus, and to the abdominal appendages, it might be very difficult to say whether a given crustacean belonged to the Astacine, or to the closely allied Homarine group. ”

“ Our present purpose, however, is not to compile a faunistic catalogue, but merely to give some idea of the endless diversity of form, and to note a few of the "shifts for a living"—of the ways in which structure and habit are adapted to the conditions of life in the Crustacea of the shore and of shallow water. ”

“ The choice was necessarily limited to those the chief forms of which were easily to be obtained alive in some abundance. The Crabs and Macrurous Crustacea, the Stomapoda, the Diastylidæ, the Amphipoda and Isopoda, the Ostracoda and Daphnidæ, the Copepoda and Parasita, the Cirripedes and Rhizocephala of our coast, representing the class of Crustacea with the deficiency only of the Phyllopoda and Xiphosura, furnished a long and varied, and at the same time intimately connected series, such as was at my command in no other class. ”

“ Onychophora, including the curious worm-like Peripatus; (v.) Arachnida, or Scorpions, Spiders, Mites, and their allies; and (vi.) Crustacea.
It is not easy to summarize in a few words the characters common to all Crustacea, and distinguishing them from the other groups of Arthropoda. As [3] a rough guide to classification, it is useful to remember that an Insect can generally be recognized by having three pairs of walking legs, an Arachnid by having four pairs, and a Centipede or Millipede by having a great many pairs, all nearly alike. ”

“ Formalin is very cheap and readily obtained, but it is much less suitable than spirit for most Crustacea, as it tends to make them stiff and fragile, and small forms containing much lime, such as Cumacea, may become decalcified. ”

“ In nearly all Crustacea the antennules and often also the antennae bear groups of hair-like filaments in which the chitinous cuticle is extremely delicate and which do not taper to a point but end bluntly. These are known as olfactory filaments or aesthetascs. They are very often more strongly developed in the male sex, and are supposed to guide the males in pursuit of the females.
Glands.—In addition to the digestive and excretory glands already mentioned, various glandular structures occur in the different groups of Crustacea. ”

“ The Crustacea form the third class of the Articulata, they comprise animals covered with a hard shell or carapace, like a case made up of rings, with joints allowing the pieces to move upon each other, except where several of them are consolidated to form the principal covering. These rings generally amount to twenty-one in number, some composing the claws, others the legs, &c. The shell is chiefly made [Pg 230] of carbonate of lime (chalk) held together by animal matter. The greater part of the Crustacea inhabit the waters, and they cast their shells from time to time as they grow ”

“ While the Copepoda seem to have been derived directly from the Hypoparia, the remainder of the Crustacea apparently branched off after the compound eyes became fully developed, unless, as seems entirely possible, compound eyes have been developed independently in various groups. Most Crustacea were derived from crawling trilobites (Lower Cambrian or pre-Cambrian Opisthoparia) , for they lost the large pygidium, and also the major part of the pleural lobes. In all Crustacea, too, other than the Copepoda and Ostracoda, there is a tendency to lose the exopodites of the antennæ. ”

“ One by one the species found near the shore become rare and disappear, and their places are taken by others characteristic of the intermediate depths. These in their turn give way to others, till in the abysses of the great oceans we find an assemblage of strange animals adapted to the conditions of life in the great depths, and differing widely in many respects from the more familiar inhabitants of the coastal waters. In this "fauna of the deep sea," which extends to the greatest depths reached by the dredge or trawl, the Crustacea occupy a prominent place. ”

“ 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. ”

“ This uniformity is well illustrated by many of the smaller Crustacea. In a gathering of Cladocera, Copepoda, and Ostracoda, from Central Africa or from Australia, we find that most of the genera, and even some of the species, are identical with those found in similar situations in this country. It is by no means the case that all the species and genera are thus universally distributed, for there are many, especially among the larger forms, which have a very restricted range; but this does not render less striking the general uniformity of the fauna over very wide areas. ”

“ A good deal of reconnoitring is done before they make up their minds to come close, and then the smallest specimens always come first. Altogether this species forms one of the most interesting and amusing of the Crustacea. Port Jackson; New Zealand ”