234 ANIMAL BIOLOGY. [Part II. 



those of the first and third ; the third overlaps the fourth ; the 

 fourth, the fifth ; and the fifth, the sixth. Thus lateral motion 

 of the abdomen is prevented. 



Fig. 73, D., shows a transverse section of the cephalo-thorax. 

 The pleura are seen to be enormously enlarged, and are here 

 called the branchiostegites. The space between the point of 

 articulation of the limb and the pleuron is also much enlarged. 

 Between this (epimeron) and the branchiostegite is enclosed the 

 branchial chamber, in which lie the gills. In a dorsal aspect 

 (Fig. 73, A.) the terga are seen to have coalesced into a con- 

 tinuous carapace. 



Viewed from the ventral aspect, however, the sterna, though 

 they tend to run together in the mid-line, give evidence that 

 the thoracic region is compounded of coalesced somites homo- 

 logous with those in the abdomen. Six sterna may be readily 

 made out, of which the most posterior is a transverse plate, the 

 next two triangular, and the other three longitudinally elongated. 

 External to these sterna are the articular spaces for the four 

 legs, the pincers, and, anterior to the pincers, the posterior 

 maxillipeds. Between these articular spaces are transverse 

 calcified inter-articular bars, or arthrophragms. The sterna in 

 front of these six (i.e. between them and the mouth), are very 

 delicate. In front of the mouth the sternal region forms a 

 broad plate, with anterior spear-shaped process, the epistoma. 



Projecting inwards from the ventral aspect of the thoracic 

 region is a complicated system of "pillars and bulkheads," 

 constituting the endophragmal system, which (1) gives attachment 

 to certain muscles ; (2) serves to protect the nerve-chain ; and 

 (3) assists in consolidating the cephalo-thorax. Fig. 73, E., shows 

 diagrammatically the infoldings of the cuticle, which are seen 

 in a transverse section of the region of the first pair of legs. 

 Where the arthrophragm (arth.) joins the sternum (st.) there 

 rises a little pillar, the endosternite (en. st.), with an expanded 

 capital. From the outer end of the arthrophragm and the 

 adjoining epimeron (ep.) 9 there projects inward a plate, the 

 endopleurite (en. pi.). Viewed from above the endopleurite is 

 seen to split into two horizontal processes, as shown diagram 



