[Illustration: FIG. 4.--Diagram of an Amphipod. (After Spence Bate and Westwood.)
C, cephalon. Th, thorax. (Only seven of the eight thoracic somites are visible, the first being fused with the cephalon.) Ab, abdomen.
The numbers appended to the somites do not correspond to the enumeration adopted in the text. 21 is the telson.]
A remarkable feature found only in the Stomatopoda is the reappearance of segmentation in the anterior part of the cephalic region. Whether the movably articulated segments which bear the eye-stalks and the antennules in this aberrant group correspond to the primitive head somites or not, their distinctness is certainly a secondarily acquired character, for it is not found in the larvae, nor in any of the more primitive groups of Malacostraca.
The body proper is usually divisible into two regions to which the names _thorax_ and _abdomen_ are applied. Throughout the whole of the Malacostraca the thorax consists of eight and the abdomen of six somites (fig. 4), and the two regions are sharply distinguished by the character of their appendages. In the various groups of the Entomostraca, on the other hand, the terms thorax and abdomen, though conveniently employed for purposes of systematic description, do not imply any homology with the regions so named in the Malacostraca. Sometimes they are applied, as in the Copepoda, to the limb-bearing and limbless regions of the trunk, while in other cases, as in the Phyllopoda, they denote, respectively, the regions in front of and behind the genital apertures.
[Illustration: FIG. 5.--Phyllopoda and Phyllocarida.
1, _Ceratiocaris papilio_, U. Silurian, Lanark. 2, _Nebalia bipes_(one side of carapace removed). 3, _Lepidurus Angassi_: a, dorsal aspect; b, ventral aspect of head showing the labrum and mouth-parts. 4, larva of _Apus cancriformis_. 5, _Branchipus stagnalis_: a, adult female; b, first larval stage (Nauplius); c, second larval stage. 6, Nauplius of _Artemia salina_.]
A character which recurs in the most diverse groups of the Crustacea, and which is probably to be regarded as a primitive attribute of the class, is the possession of a carapace or shell, arising as a dorsal fold of the integument from the posterior margin of the head-region. In its most primitive form, as seen in the _Apodidae_ (fig. 5, 3) and in _Nebalia_ (fig. 5, 2), this shell-fold remains free from the trunk, which it envelops more or less completely. It may assume the form of a bivalve shell entirely enclosing the body and limbs, as in many Phyllopoda (fig. 6) and in the Ostracoda. In the Cirripedia it forms a fleshy "mantle" strengthened by shelly plates or valves which may assume a very complex structure. In many cases, however, the shell-fold coalesces with some of the succeeding somites. In the Decapoda (fig. 3), this coalescence affects only the dorsal region of the thoracic somites, and the lateral portions of the carapace overhang on each side, enclosing a pair of chambers within which lie the gills. The arrangement is similar in Schizopoda and Stomatopoda (fig. 7), except that the coalescence does not usually involve the posterior thoracic somites, several of which remain free, though they may be overlapped by the carapace.