296 THE INVERTEBRATA 



with the presegmental region so as to form a head, and oftenthere is also 

 fusion of them elsewhere. 



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

 ferentiated 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 

 antennae, 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, presently to be mentioned, 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. 256, and Mysis, Fig. 253) 

 by a groove which crosses the cheek immediately behind the man- 

 dible. This mandibular groove is distinct from the true cervical groove 

 which often (as in Astacus^ Fig. 269) marks the boundary between 

 head and thorax: the two grooves may co-exist, as in Apus and in 

 Nephrops. The Crustacea, indeed, admirably 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 shifting 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 com- 

 missure which unites the ganglia of the antennae still passes behind 

 the mouth, and may usually be seen, as in Astacus (Fig. 214), crossing 

 from one of the circumoesophageal commissures to the other. 



The head, though it varies in extent, is of the same nature through- 

 out the group, being, like the heads of other animals, the seat of the 

 principal organs of special sense and of manducation. On the other 

 hand, the two tagmata known as the thorax and abdomen, which 

 usually can be recognized in, and together compose, the postcephalic 

 part of the body or trunk, vary much more in extent, and each of 

 them has in the several groups no constant feature save its position 

 relative to the other. The precise boundary between thorax and 

 abdomen is sometimes difficult to fix. The names, as they are com- 

 monly used, are in this respect inconsistently applied, denoting in 

 some groups limb-bearing and limbless regions, in others the sections 

 of the trunk which lie before and behind the genital openings. For 

 the sake of consistency we shall adopt the convention that the somite 



