BIVALVE CRUSTACEANS AND BARNACLES. 



147 



crustaceans, one of which is figured on page 145. The ceph- 

 alo-thorax is composed of fourteen segments, seven of these 

 belonging to the head, judging from the number of append- 

 ages which arise from that part. The abdomen is supposed 

 to possess seven segments, though the last one is so rudimen- 

 tary that its existence as a true segment has been denied by 

 some. 



The deep line running across the back of the carapace, in 

 the crawfish and lobster, is called the cervical suture. 



The following outline represents a sowbug, with the 

 regions of the body marked. Compare this with Fig. 125. 



head, 

 tliorax- 



abdomen. 

 FIG. 137. SOWBUG, WITH REGIONS OF BODY MASKED. 



CHAPTER XIX. 



BIVALVE CRUSTACEANS AND BARNACLES. 



134. THE crustaceans during their growth shed their hard 

 outer covering at intervals, and, in this as in many other re- 

 spects, resemble the spiders. In the possession of gills, in- 



