EXTERNAL STRUCTURES OF THE LOBSTER 163 



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 ventral 

 chain of ganglia (Fig. 69, p. 167). 



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 



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



body. 



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

 are fused some of the appendages are closely packed together 

 and the external signs of the somites are given only by the 

 several pairs of appendages lying closely packed over one an- 

 other. The number of somites in lobster and in all of the higher 

 types of Crustacea is limited to 20 of which 5 form the head, 

 8 the thorax, and 7 the abdomen (some zoologists allow 6 for the 

 head). Corresponding to these somites there are 20 (or 21) 

 pairs of appendages which are named according to their func- 

 tions and are distributed as follows: 



