274 THE ANATOMY OF INVERTEBRATED ANIMALS. 



with the axis of the body, a similar line drawn through the 

 sterna of the somites, in front of the mouth, ascends as it 

 passes through the antennary, antennulary, and ophthalmic 

 sterna, and thus takes a position at right angles to the former 

 line (Fig. 72, JB). The sterna of the somites, in front of the 

 mouth, are, therefore, bent up so as to look forward instead 

 of downward ; and it is of essential importance to bear in 

 mind this cephalic flexure, in considering the structure of the 

 head in these and other Arthropoda. 



Just as the lateral regions of the abdominal somites are 

 produced into the pleura, so are the lateral regions of the 

 cephalo-thorax similarly prolonged. Thus the membranous 

 lateral walls of the posterior cephalo-thoracic somite are re- 

 flected superiorly, and bent down again to the level of the 

 bases of the legs, where they become continuous with a calci- 

 fied layer corresponding with the tergal half of the pleura, 

 and forming the posterior part of the carapace. In like man- 

 ner, the more or less calcified epimera of all the other somites 

 are reflected superiorly into a membrane which passes down- 

 ward, and the free lower edge of which is continuous with 

 the edges of the carapace. The carapace, therefore, corre- 

 sponds in position with the terga and tergal halves of the 

 pleura of all the somites which are thus reflected into it, and 

 these somites include all, without exception, from the last 

 thoracic to the ophthalmic. Posteriorly, the edges of the 

 carapace are a little prolonged beyond the last thoracic somite, 

 and take the form of a fold, with an under layer distinct from 

 the upper. Anteriorly, in the middle line, the carapace is 

 prolonged in a similar manner, but to a much greater extent; 

 it thus gives rise to the long rostrum, which overhangs the 

 sterna of the ophthalmic and antennulary somites. At the 

 sides of the antennulary and antennary somites the rostral pro-- 

 longation of the carapace is the direct continuation outward of 

 the epimera of those somites, and there is nothing to be com- 

 pared to an apodeme ; but the sternum of the ophthalmic so- 

 mite, after giving off the lamella which forms the inferomedian 

 region of the rostrum, is prolonged on each side of the middle 

 line backward and outward into a free, expanded, thin, cal- 

 cified process, which applies itself against the carapace by its 

 upper surface, and by its under surface gives attachment to 

 the anterior gastric muscles. Corresponding processes are 

 developed from the carapace itself, in some Podoplithalmia 

 (e. g., Galathea, Carcinus), for the attachment of the poste- 

 rior gastric muscles. From the last thoracic to the maxilli- 



