332 THE INVERTEBRATA 



relative to the size of the animal, but is thinner and flexible in many 

 parasitic genera. It is often strengthened by calcification, and in 

 certain ostracods, barnacles, and crabs this gives it a stony hardness. 

 In each somite there mayor may not be distinguishable the dorsal plate 

 or tergite (tergum) and ventral sternite {sternum) usual in arthropods. 

 The tergite may project at each side as a pleuron. 



There are embryological indications that the body should be re- 

 garded as containing, besides the somites, an anterior presegmental 

 region, to which the eyes belong, corresponding to the prostomium of 

 a worm, and a. postsegmental region or telson, on which the anus opens. 

 Each somite, except the first, which is purely embryonic, may bear a 

 pair of appendages, though it is rarely that the appendages of all the 

 somites are present at the same time. The somites never all remain 

 distinct in the adult. Always some of them are fused together and 

 with the presegmental region so as to form a head, and often there is 

 also fusion of them elsewhere. 



Nearly always the somites are grouped into three tagmata, differ- 

 entiated by peculiarities of their shape or appendages, and known as 

 the head, thorax, and abdomen. These, however, are not morpho- 

 logically equivalent in different groups. The head always contains, 

 besides the region of the eyes and the embryonic first somite, the 

 somites of five pairs of appendages — two, the antennules and an- 

 tennae, preoral; and three, the mandibles, maxillules, and maxillae, 

 postoral. More somites are often included in the actual head, but as 

 the additional appendages (maxillipeds) then usually show features 

 of transition to those behind them, and as the fold of skin which forms 

 the carapace first arises from the maxillary somite, the true head is 

 held to consist only of the anterior portion of the body as far as that 

 somite inclusive. There is evidence of an earlier head, carrying only 

 the first three pairs of limbs which alone exist in the Nauplius larva, 

 and still indicated in some cases (as in Chirocephalus , Anaspides, 

 Fig. 269, and My sis, Fig. 265) by a groove which crosses the cheek 

 immediately behind the mandible. This mandibular groove is distinct 

 from the true cervical groove which often (as in Astacus, Fig. 283) 

 marks the boundary between head and thorax : the two grooves may 

 co-exist, as in Apus and in Nephrops. The Crustacea, indeed, ad- 

 mirably illustrate the way in which the process of *'cephalization" 

 tends, in arthropods as in vertebrates, to extend backwards and to 

 involve more and more segments. With it has gone a backward shift- 

 ing of the mouth, which in the Crustacea now stands behind the third 

 somite, with two pairs of appendages (antennules and antennae) in 

 front of it. The commissure which unites the ganglia of the antennae 

 still passes behind the mouth, and may usually be seen, as in Astacus 

 (Fig. 225), crossing from one of the circumoesophageal commissures 



