PHYLUM ARTHROPODA 



201 



' Protopodite 



Swimmeret - 



Exopodite 



Tergum 



Pleuron 



^ Endopodite 



Figure 113. Diagram of a cross section of the third abdominal segment of the crayfish. 



ally by a cuticular shield, the carapace. An 

 indentation, termed the cervical groove, 

 runs across the middorsal region of the cara- 

 pace and obliquely forward on either side. 

 The anterior pointed extension of the cara- 

 pace is known as the rostrum. Beneath this 

 on either side is an eye at the end of a mov- 

 able stalk. The mouth is situated on the ven- 

 tral surface near the posterior end of the 

 head region. It is partly obscured by the 

 neighboring appendages. The carapace of 

 the thorax is separated into three parts by 

 branchiocardiac grooves: a median dorsal 

 longitudinal strip, the areola, and two large 

 convex flaps, one on either side, the bran- 

 chiostegites, which protect the gills beneath 

 them. 



Abdomen 



In the abdomen there are 6 segments 

 and a terminal body extension, the telson, 

 bearing on its ventral surface the longitud- 

 inal anal opening. Whether or not the tcl- 



of the annelids and is a region, in a phylogenetic 

 sense, that has not come under the influence of 

 metamerism; therefore the first pair of serially me- 

 tameric appendages of the crayfish is the antennae. 

 The antennules are actually prostomial sense organs, 

 like the eyes, and hence not homologous with the 

 other true appendages. It will be noted that the 

 numbers used in the discussion of the appendages 

 are 1 less in value than those of texts which list 19 

 pairs of homologous appendages. 



son is a true segment is still in dispute; we 

 shall adopt the view that it is not. The first 

 abdominal segment (13) is smaller than the 

 others and lacks the pleura. Segments 14-18 

 are sheathed as described above. 



Appendages 



Every segment of the body bears a pair 

 of jointed appendages. These are all varia- 

 tions of a common type consisting of a 

 basal region, the protopodite, which bears 

 2 branches, an inner endopodite, and an 

 outer exopodite. Beginning at the anterior 

 end, the appendages are arranged as follows 

 (Fig. 115). In front of the mouth are the 

 antennae; the mouth possesses a pair of 

 mandibles, behind which are the first and 

 second maxillae; the thoracic region bears 

 the first, second, and third maxillipeds, the 

 chelipeds (pincers), and 4 other pairs of 

 walking legs; beneath the abdomen are 5 

 pairs of swimmerets, some of which are 

 much modified. The sixth abdominal seg- 

 ment bears greatly flattened appendages 

 termed uropods. Tlie accompanying table 

 (pp. 206) gives brief descriptions of the dif- 

 ferent appendages, and shows the modifica- 

 tions concerned with differences in function. 

 The functions of some of the appendages 

 are still in doubt. 



Three kinds of appendages can be dis- 

 tinguished in the adult crayfish: (1) foliace- 



