Taking the common Lobster (fig. 136) 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. The so-called "head " is really composed of both the head, properly so called, and the thorax, which have coalesced so as to form 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, etc. The entire body, with the articulated appendages, is enclosed in a strong chitinous "shell," or exo-skeleton, and the cephalothorax is covered by a great cephalic shield or plate, which is termed the "carapace."
Fig. 135. - Theoretical figure illustrating the composition of the tegumentary skeleton of the Crustacea (after Milne-Edwards). D, Dorsal are: t t Tergal pieces; e e Epimeral pieces. V, Ventral arc : s s Sternal pieces; f f Episternal pieces; p p Insertion of the extremities.
Fig. 136. - The common Lobster (Homarus vulgaris), viewed from below. a The lesser antennae; a' The greater antennae; n The last pair of foot-jaws ; c The great claws, or first pair of legs; d e f g The last four pairs of walking legs; h i j k I m The six pairs of abdominal appendages, the last five being " swimmerets," and the last of all being greatly expanded ; t The last segment of the body, without appendages.
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. 137, 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 "epistema." 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, 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. 135).
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 " carapace " in the higher Crustacea is produced by an enormous development of the tergal pieces, or of the "epimera" or one or two of the cephalic segments : the tergal arcs of the remaining somites being overlapped by the carapace and remaining undeveloped.
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. 137, 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 modification - the 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. 136). The last thoracic somite - immediately in front of the abdomen - carries 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 "chelae." The next segment of the thorax carries a pair of chelate limbs, just like the preceding; and the next is furnished with appendages, which are essentially the same in structure, but are much larger, constituting the great claws. The next two segments of the thorax, and the segment in front of these (by some looked upon as belonging to the head, by others as referable to the thorax), bear each a pair of modified limbs, which are termed "maxillipedes," or "foot-jaws." These are simply limbs with the ordinary structure of protopodite, exopodite, endopodite, and epipodite, but modified to serve as instruments of mastication, the hindmost pair being less altered than the two anterior pairs (fig. 137, 3). The next two somites carry appendages, which are in the form of jaws, and are termed respectively the first and second pairs of " maxillae." Each consists of the parts aforementioned, but the epipodite of the first pair of maxillae is rudimentary, whilst that of the second pair is large, and is shaped like a spoon. It is termed the "scaphognathite," and its function is to cause a current of water to traverse the gill-chamber by constantly bailing water out of it. The next segment carries the biting jaws or "mandibles;" each of which consists of a large protopodite, and a small endopodite, which is termed the "palp," whilst the exopodite is undeveloped. The aperture of the mouth is situated between the bases of the mandibles, bounded behind by a forked process, called the "labium," or "metastoma," and in front by a single plate, called the "labrum" (upper lip). The next segment bears the long antennae, or feelers (fig. 136, a'), each consisting of a short protopodite, and a long, jointed, and segmented endopodite, with a very rudimentary exopodite. In front of the great antennae is the next pair of appendages, termed the "antennules," or smaller antennae (fig. 136, a), each composed of a protopodite, and a segmented endopodite and exopodite, which are nearly of equal size. Finally, attached to the first segment of the head are the eyes, each of which is borne upon an eye-stalk formed by the protopodite. The gill-chamber is formed by a great prolongation downwards of the pleurae of the thoracic segments, and the gills are attached to the bases of the legs.