EXTERNAL STRUCTURES OF THE LOBSTER 167 



or gill protectors. On the under side of the body, especially in 

 the abdomen, the chitin is thin and transparent, with heavier 

 ribs of chitin for muscular support, while, in the region of the 

 cephalothorax, these heavier bars are united to form an internal 

 skeleton-like structure, termed the endophragmal skeleton. 

 This forms the floor of the body cavity, and protects the ven- 

 tral chain of ganglia (Fig. 69, p. 171). 



APPENDAGES AND SERIAL HOMOLOGY. The metameric struc- 

 ture of the body is well indicated by the appendages, of which 

 there is one pair to each somite. The relation of the appendages 

 to the somite is clearly shown in the abdominal region where the 



FIG. 67. The American lobster, Homarus americanus, showing regions of the 



body. 



somites are free, but in the cephalothorax where the somites are 

 fused, their external signs are given only by the several pairs of 

 appendages lying closely packed over one another. Some of 

 these belong to the head, and some to the thorax. The number 

 of somites in lobsters and in all of the higher types of Crustacea 

 is limited to 20, of which 5 form the head, 8 the thorax, and 6 the 

 abdomen (some zoologists allow 6 for the head) . Correspond- 

 ing to these somites, there are 19 (or 20) pairs of appendages 

 which are named according to their functions, and are distrib- 

 uted as follows: 



