The body of this Crustacean, like that of all Podophthalmata, consists of two great divisions, an anterior, the cephalothorax, covered dorsally and at the sides by a large continuous shield, the carapace, and a posterior, the abdomen, consisting of six separate metameres or somites, and of an azygos terminal flap, the telson, the last somite of the body, upon which the anus is situated ventrally. The cephalothoracic carapace is divisible into two regions by a well-marked curved line, with its concavity looking forwards, which is known as the cervical groove. The part anterior to this line corresponds to the head, the part posterior to it to the thorax, and they are known respectively as cephalo- and omo-stegite. The omo-stegite is marked dorsally by two longitudinal and short branchio-cardiac grooves connected anteriorly by a curved transverse groove. Within the area inclosed by these three grooves lies the heart. The lateral areae of the omostegite, known as the branchiostegites, roof in the branchial chamber.
They are formed by two flaps, right and left, which are homologous with the pleura of the abdominal somites, and like those pleura have an outer and inner lamella and a free ventral edge. The cephalostegite bears a median anterior projection, the rostrum, and to either side of the base of this rostrum an eye is visible, pedunculate as in all Podophthalmata.
The two first antennae (antennules), each with an outer and inner division, the exopodite and endopodite, project forwards in front of the rostrum, and have to either side the long annulated second antenna (antenna) with a pointed scale or squame, the exopodite, at its base. The appendages in relation with the mouth, the two mandibles, the two pairs of maxillae, and three anterior pairs of thoracic limbs or maxillipeds, can only be clearly identified by dissection. The third pair of maxillipeds, however, is conspicuous and lies between the first and largest pair of ambulatory thoracic limbs. Of these ambulatory thoracic limbs there are five pairs. Unlike the three pairs of maxillipeds they consist of a single stem, the endopodite, the exopodite being lost in all Decapoda. Each limb of the first pair is very large and is often spoken of as 'Chela' but is perhaps better termed, with Professor Huxley, 'Forceps.' It is chiefly used in prehension not in crawling, the function of the remaining limbs.
These limbs are slender and consist each of seven joints, the typical number in the higher Crustacea, and known, counting from base to tip, as coxopodite, basipodite or basis, ischiopodite, meropodite, carpopodite, propodite, and dactylopodite.
One of the coxopodites bears the genital aperture in both sexes in all Macrura, the group to which Astacus and the Lobster belong, in the Hermit Crabs and the Arthrostraca or Sessile-eyed Crustacea. The oviducal aperture in the Macrura is on the third coxopodite, and in this specimen a black bristle has been inserted into it. In the male, the aperture of the vas deferens is on the fifth coxopodite. The membrane connecting the coxopodite and basipodite is the spot at which the limb separates when the animal throws it off in consequence of either fright or injury. The basi- and ischio-podite are united in the forceps, so that the joints of this pair of limbs are reduced to six. The three first pairs of ambulatory limbs are chelate. The chela or claw is produced by the enlargement of the outer angle of the propodite into a process equal in length to the dactylopodite. Foreign objects are seized between this process and the moveable terminal joint. The chela of the Scorpion is fashioned on the same plan, but the produced angle of the propodite is in this animal the internal angle. The claw in the Squillidae is of a different type. The two terminal joints of the limb are elongate, the propodite is more or less grooved, and the dactylopodite bends backwards and fits into the groove.
Note that the space between the bases of the thoracic limbs becomes wider and wider posteriorly, and that the sternum of the last pair is separate and moveable; whereas the anterior sterna are calcified as a continuous whole.
The six abdominal somites are all free and connected one to another by soft intersegmental membranes. Each somite bears attached a single pair of appendages, swimmerets or pleopoda. The somite forms an unbroken ring. Its ventral region between the attachments of the limbs is the sternum, while the opposite dorsal area is the tergum. The flap projecting ventrally and laterally is the pleuron, and a small space between the socket for the limbs and the base of the pleuron is known as epimeron. A typical swimmeret, e. g. that of the fourth somite, consists of a basal protopodite bearing two processes, an inner, the endopodite, and an outer, the exopodite, both fringed with setae. There are two joints in the protopodite, a small basal coxopodite and a larger basipodite. The endopodite consists of a simple unjointed basal and a terminal jointed or annulated portion. The exopodite is similar but its parts are smaller. The last pair, often termed par excellence the swimmerets, are somewhat modified. The protopodite consists of a single joint. The endopodite and exopodite are expanded into broad thin plates, and the latter is divided by a transverse joint.
This pair of limbs together with the telson make up the caudal fin, by means of which the animal deals a powerful stroke upon the water, and darts backwards whenever the tail is suddenly flexed. The first pair of abdominal limbs is either wanting altogether in the female, or one limb is present without the other. It is rare for both to be present. When present they are reduced to slender filaments with a minute basal joint or protopodite, and a jointed terminal portion which perhaps represents an endopodite. The two first pairs of these appendages are modified in the male for sexual purposes, and are to be seen in the two following preparations.