The members of this class are commonly known as Crabs, Lobsters, Shrimps, King-crabs, Barnacles, Acorn-shells, etc. They are nearly allied to the succeeding order of the Arachnida (Spiders and Scorpions); but may usually be distinguished by the possession of articulated appendages upon the abdominal segments, by the possession of two pairs of antennae, and by the presence of branchiae.
The body is composed of a number of definite rings or segments ("somites"), each of which may be provided with a pair of jointed appendages. With rare exceptions, some of the somites of the adult always carry appendages; and one or more pairs are almost invariably adapted for mastication. The nervous system of the embryo has the typical Annulose form of a chain of ventral ganglia, between the first two pairs of which the gullet passes. No water-vascular system is present; but there is generally a true blood-vascular system. The heart, when present, is placed on the opposite side of the alimentary canal to the ventral nerve-chain, and communicates by valvular apertures with a surrounding venous sinus - the so-called "pericardium." When differentiated breathing-organs are present, these are always in the form of branchiae or gills, adapted for respiring air dissolved in water.
In addition to these characters, the body in the Crustacea is always protected by a chitinous or sub-calcareous exoskele-ton, or "crust," and the number of pairs of articulated limbs is generally from five to seven. They all pass through a series of metamorphoses before attaining their adult condition, and every part that is found in an embryonic form, even though only temporarily developed, may be represented in a permanent condition in some member of a lower order.
As regards the classification of the Crustacea, the tabular view which follows embodies the arrangement which is most generally adopted, and the diagnostic characters of each order will be briefly given, a more detailed description being reserved for the more important divisions of the class. Before proceeding further, however, it will be as well to give a description of the morphology of a typical Crustacean, selecting the lobster as being as good an example as any.
The body of a typical Crustacean may be divided into three regions - a head, a thorax, and an abdomen, each of which is composed of a certain number of somites, though opinions differ both as to the number of segments in each region, and as to their number collectively. By the majority of authorities the body is looked upon as being typically composed of twenty-one segments, of which seven belong to the head, seven to the thorax, and seven to the abdomen. In many Crustacea, however, the segments of the head and thorax are welded together into a single mass, called the "cephalothorax;" in which case the body shows only two distinct divisions, of which the cephalothorax claims fourteen segments, whilst the remaining seven are allotted to the abdomen. By Professor Huxley, on the other hand, the terminal joint of the abdomen, termed the "telson." is regarded as an appendage, and not as a somite. Upon this view, the body of a typical Crustacean will consist of twenty segments only. Professor Huxley, further, differs from the above-mentioned view in the allotment of the somites, and he divides the body into six cephalic, eight thoracic, and six abdominal somites.* Fritz Muller and Claus deny that the eyes are limbs, or that there is an ocular segment. The telson, on the other hand, is regarded by the former as a true somite, chiefly because the intestine usually opens in this piece.
* In reality the five hindmost segments of the eight somites here allotted to the thorax, should alone be regarded as constituting the abdomen proper, - that is, the region corresponding to the "abdomen of insects and Arach-nida. The six somites allotted above to the abdomen belong to what is strictly called the "post-abdomen" of the Crustacea.
Whilst the normal number of segments in the body of any Crustacean may thus be regarded as being twenty-one, or twenty, there occur cases in which this number is exceeded, and others in which the number of somites is apparently less. In these latter cases, however, the apparent diminution in the number of segments is really due to some having been fused together, as is shown by the number of appendages, since each pair of appendages indicates a separate somite. In other cases, however, in which the number of somites is really less than the normal, this is due to an arrest of development. According to Milne-Edwards:
"In the embryo these segments are formed in succession from before backwards, so that, when their evolution is checked, the latter, rather than the earlier, rings are those which are wanting; and, in fact, it is generally easy to see in those specimens of full-grown Crustaceous animals, whose bodies present fewer than twenty-one segments, that the anomaly depends on the absence of a certain number of the most posterior rings of the body." According to Dana, however, the abortion of segments with their appendages almost always takes place at the posterior end of the cephalothorax.
In no single example can a general view be obtained of the different segments and their appendages in the Crustacea. "Indeed the only segment that may be said to be persistent is that which supports the mandibles, for the eyes may be wanting, and the antennae, though less liable to changes than the remaining appendages, are nevertheless subject to very extraordinary modifications, and have to perform functions equally various. Being essentially and typically organs of touch, hearing, and perhaps of smell, in the highest Decapods, they become converted into burrowing organs in the Scyllaridae, organs of prehension in the Merostomata, claspers for the male in the Cydopoidea, and organs of attachment in the Cirripedia. Not to multiply instances, we have presented to us in the Crustacea probably the best zoological illustration of a class, constructed on a common type, retaining its general characteristics, but capable of endless modification of its parts, so as to suit the extreme requirements of every separate species" (H. Woodward).