COMMON CRAYFISH. 309 



w 6. Appendage of sixth abdominal somite, forming the right lateral 

 element of the caudal fin. The telson is interposed mesially between 

 the two appendages of the sixth abdominal somite. 



x. Flexor muscles acting on the swimmeret and abdominal somites 

 in the animal's rapid movements ; its slower movements being de- 

 pendent upon the ambulatory feet. 



y. Extensor muscles, in two layers like the flexors, but of much smaller 

 size. 



