PHYLUM ARTHROPODA 



295 



.....m.iit-.Ki ■,■,.. mm. 



First anlrenna- 



Exo. 



Second- raayilliped- 



First mcocilla. 



First ma.x'illipcd. 



Fourth pgreiopod 



End- 

 Third S^A7immgret 



Ezxro. ^Prot 



First sv\7immer(z.t: 

 in Taa.le 



Uropod 



Figure 16.5. Appendages of the crayfish. Prot. = protopodite. end. = endopodite, 

 exo. = exopodite. Those on the left are drawn to a larger scale than those on the right. 

 (After Howes.) 



also used as plows for digging. The other chelate legs are used for 

 picking up bits of food ancl handing them to the mouth parts. The last 

 four pairs of pereiopods are the primary walking legs. Crayfishes cannot 

 run, but they use the tail fan for swift escape. 



The anterior three pairs of thoracic appendages (segments 7 to 9) 

 and the posterior three pairs of cephalic appendages (4 to 6) form the 

 mouth parts. These overlap each other so that the most posterior pair 

 covers those in front. The thoracic legs are three pairs of maxillipeds, 

 with endopodites modified as small arms to hold, manipulate and tear 

 the food, and exopodites modified as tactile palps. The two pairs of 



Hinge 



jcccnsor 



~Flz.Dcors 



Figure 16.6. Dissected pereiopod of Astacus showing muscle arrangement. The 

 terminal joint moves up and down, the next joint fore and aft, and the third joint up and 

 down. (.After Parker and Haswell.) 



