appendages upon the abdominal segments, by the possession of two pairs of antennæ, and by the presence of branchiæ.

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 branchiæ 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 exoskeleton, 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 twentyone 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 Müller 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.

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 antennæ, 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

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 Arachnida. The six somites allotted above to the abdomen belong to what is strictly called the "post-abdomen” of the Crustacea.

touch, hearing, and perhaps of smell, in the highest Decapods, they become converted into burrowing organs in the Scyllarida, organs of prehension in the Merostomata, claspers for the male in the Cyclopoidea, 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).

Taking the common Lobster (fig. 122) as a good and readily obtainable type of the Crustacea, the body is at once seen to be composed of two parts, familiarly called the "head" and the "tail," the latter being jointed and flexible. called "head" is really composed of both the head, properly so called, and the thorax, which have coalesced so as to form

D

V

The so

a single mass, technically called the "cephalothorax." The so-called "tail," on the other hand, is truly the "abdomen." The various appendages of the animal are arranged along the lower surface of the body, and consist of the feelers, jaws, claws, legs, &c. The entire body, the tegumentary skeleton of with the articulated appendages, is wards). D, Dorsal arc: tt enclosed in a strong chitinous shell," or exoskeleton, and the cephalothorax is covered by a great cephalic shield or plate, which is termed the "carapace."

Fig. 121.-Theoretical figure il

lustrating the composition of

the Crustacea (after Milne-Ed

Tergal pieces; ee Epimeral pieces. V, Ventral arc s s Sternal pieces; ff Episternal pieces; Insertion of the extremities.

Each segment of the body may be regarded as essentially composed of a convex upper plate, termed the "tergum," which is closed below by a flatter plate called the "sternum," the line where the two unite being produced downwards and outwards, into a plate, which is called the "pleuron," or pleura" (fig. 123, 2).

Strictly speaking, the composition of the typical somite is considerably more complex, each of the primary arcs of the somite being really composed of four pieces. The tergal arc is composed of two central pieces, one on each side of the middle line of the body, united together, and constituting the "tergum" proper. The superior arc is completed by two lateral pieces, one on each side of the tergum, which are termed the "epimera." In like manner the ventral or sternal arc is composed of a central plate, composed of two pieces united together in the middle line, and constituting the "sternum proper; the arc being completed by two lateral pieces, termed the "episterna." These plates are usually more or

less completely anchylosed together, and the true structure of the somite in these cases is often shown by what are called "apodemata." These are septa which proceed inwards from the internal surface of the somite,

Fig. 122.-The common Lobster (Homarus vulgaris), viewed from below. a The lesser antennæ; a' The greater antennæ; The last pair of foot-jaws; c The great claws, or first pair of legs; defg The last four pairs of walking legs; h ijklm The six pairs of abdominal appendages, the last five being merets," and the last of all being greatly expanded; t The last segment of the body, without appendages.

swim

penetrating more or less deeply between the various organs enclosed by the ring, and always proceeding from the line of junction of the different pieces of the segment (fig. 121).

66

It must be borne in mind that though the so-called "head"—that is to say, the " cephalothorax "-of the Lobster is produced by an amalgamation of the various somites of the head and thorax, this is not the case with the great shield which covers this portion of the body. This shield-the so-called "cephalic buckler," or carapace "-is not produced by the union of the tergal arcs of the various cephalic and thoracic segments, as would at first sight appear to be the case. On the contrary, the pace" in the higher Crustacea is produced by an enormous development of the tergal pieces, or of the "epimera" of one or two of the cephalic segments: the tergal arcs of the remaining somites being overlapped by the carapace and remaining undeveloped.

cara

Examining the somites from behind forwards (for simplicity's sake), the last segment comes to be first described. This is the so-called "telson," which forms the last articulation of the abdomen, and never bears any appendages. For this reason, many authorities do not regard it as a somite, properly speaking, but simply as an azygous appendage-that is to say, as an appendage without a fellow. In the next segment (the last but one, or the last, of the abdomen, according to the view which is taken of the "telson"), there is a pair of natatory appendages, called "swimmerets." Each swimmeret (fig. 123, 2) consists of a basal joint, which articulates with the sternum, and is called the "protopodite" or propodite, and of two diverging joints, which are attached to the former; the outer of these being called the "exopodite," and the inner the "endopodite." In this particular segment, the exopodite and the endopodite are greatly expanded, so as to form powerful paddles, and the exopodite is divided into two by a transverse joint. In the succeeding somites of the abdomen-with the exception of the first, in which there is some modificationthe appendages are in the form of swimmerets, essentially the same as those attached to the penultimate segment, and differing only in the fact that the exopodite and endopodite are much narrower, and the former is undivided (fig. 122). The last thoracic somite-immediately in front of the abdomencarries a pair of the walking or ambulatory legs, each consisting of a short basal piece, or "protopodite," and of a long jointed "endopodite," the "exopodite" not being developed. The next thoracic segment carries another pair of ambulatory limbs. quite similar to the last, except for the fact that the protopodite bears a process which serves to keep the gills apart, and is termed the "epipodite." The succeeding segment supports a pair of limbs similar to the last in all respects, except that its extremities, instead of being simply pointed, are converted into nipping claws or "chelæ." The next segment of the thorax carries a pair of chelate limbs, just like the preceding;