326 



THE CRAYFISH AND THE ARTHROPODA 



material among metameric animals for illustrating this great 

 principle of organization. 



The simplest appendages of the crayfish's body are the swim- 

 merets on the abdomen. A swimmeret (Fig. 157 A) consists of a 

 proximal segment, the protopodite, which bears two distal seg- 



FiG. 157. — Appendages of American lobster, Homarus americanus. 



A, second swimmeret. B, mandible. C, first maxilliped. D, third maxilliped. E, 

 first pereiopod. 



en, endopodite; e-p, epipodite; ex, exopodite; y, gill; pr, protopodite. 



ments, the endopodite toward the median plane and the exopodite 

 toward the outside, as the names imply. The endopodites of 

 the swimmerets are further divided into proximal and distal seg- 

 ments. The uropods, which are borne on the most posterior 

 abdominal segment, are much larger, yet they exhibit this same 

 type of structure save that the exopodite, instead of the endopodite, 



