302 ZOOLOGY. 



(abdomen). It cannot, however, swim like a fish, the body 

 being flexible only in the vertical and not in the horizontal 

 plane. 



2. Exoskeleton. This limited flexibility is due to the 

 high development of the exoskeleton. As in the earth- 

 worm and mussel this is a cuticle secreted by the epidermis, 

 but instead of being equally thin throughout as in the 

 former, or merely thickened and calcified in two valves 

 which cover in the whole animal as in the latter, we have 

 an alternation of thickened and calcified regions (sclerites) 

 with thin uncalcified flexible areas. In this way the exo- 

 skeleton combines rigidity with movableness. 



Hair-like setce are abundant on the cuticle of various 

 parts: unlike those of the earthworm, these are not de- 

 veloped in sacs of the epidermis, but the epidermis extends 

 up for some way into them. 



The front half of the body is encased in a single unjointed 

 sclerite called the carapace. The region covered by this is 

 called the cephalo-thorax, and vaguely divided into " head " 

 and " thorax," the remainder of the body being called 

 "abdomen" (fig. 148). The head so far resembles the 

 Vertebrate head that it bears the chief sense-organs, but 

 otherwise no comparison can be made between these three 

 divisions of the body and the similarly named divisions of 

 a rabbit : the organs they respectively contain differ 

 altogether. The carapace runs out in front into a pointed 

 median rostrum projecting beyond the real front end of the 

 body. An oblique groove in the carapace the cervical 

 groove marks off the " head " from the " thorax." In front 

 of this groove the carapace forms a simple circle round the 

 body ; but behind the groove it is extended down on either 

 side as a gill-cover or branchiostegite, and ends apparently 

 in a free edge. Really the branchiostegite is double there 

 being an inner thin uncalcified layer on its inner face (see 

 fig. 157). The line of origin of the branchiostegites is in- 

 dicated on the dorsal surface by the pair of branchio- cardiac 

 grooves. 



f 3. Structure of Abdominal Region. The abc|omen is 



