Phylum Arthropoda 495 



the thoracic region by the dorsal cervical groove. Laterally the thoracic 

 portion of the carapace, the branchiostegite, covers the gills. This is 

 separated from a narrow central strip, the areola, by the branchiocardiac 

 groove. On either side of the rostrum is the stalked eye. 



The abdomen is composed of six segments and a modified posterior 

 extension, the telson. This latter may or may not be considered a true 

 segment. 



Ventrally several openings may be seen. The mouth is in the head 

 region and is surrounded by the jazvs and other mouth parts. The anus 

 is in the telson while the openings of the ducts of the reproductive 

 system are at the posterior portion of the thorax. The openings of 

 the oviducts are at the base of the third pair of walking legs, while those 

 of the male are at the base of the fifth pair. 



Primitively each segment of the arthropods has a pair of appendages ; 

 this situation is very nearly approached by the crayfish in which every 

 segment but the fourteenth of the female has a pair. The basic structure 

 of these appendages is very clearly demonstrated by those of the ab- 

 dominal region. Each of these swimmcrets, as they are termed, is 

 formed of a basal portion the protopodite and two branches, an inner 

 cndopodite, and an outer cxopodite. The protopodite usually has two 

 joints, a coxopodite and a basipodite. All of the appendages have been 

 modified from this basic type. The appendages of the head region are 

 the most highly modified. Of the five pairs of head appendages, the 

 first are the antenniiles and the second, the antennae, both of which are 

 sensory in function. The third, fourth, and fifth pairs of appendages 

 are associated with the mouth and are important for handling food. 

 These are the mandibles and the two maxillae. The thorax has eight 

 pairs of appendages. The first three, the maxillipeds, are concerned 

 with food handling while the remaining five are the large conspicuous 

 walking legs. The first of the walking legs are modified into heavy 

 pincers, the chelipeds, which mainly grasp prey. The last two maxilli- 

 peds and the first four walking legs bear gills attached to their coxop- 

 odites. In the male, the first two swimmerets are modified for sperm 

 transfer and the first is lacking in the female. In both, the remaining 

 swimmerets are similar. The nineteenth segment lacks swimmerets but 

 has the broad flat uropods which are useful for swimming. 



Even though most of the appendages of the crayfish are highly modi- 

 fied from the basic plan illustrated by the swimmerets, the parts are 

 recognizable as having the same fundamental structure and it can be said 

 that they are homologous. Inasmuch as the basic plan is repeated with 



