DISTINCTIVE CHAEACTERS OF THE CRAYFISH. 241 



longest : and when the latter are spread out at right 

 angles to the body, the distance from tip to tip of the dac- 

 tylopodites is equal to, or rather greater than, the extreme 

 length of the body from the apex of the rostrum to the 

 posterior edge of the telson, in both sexes. In both sexes, 

 the length of the swimmerets hardly exceeds half the trans- 

 verse diameter of the somites to which they are attached. 



The exopodites of the appendages of the sixth abdo- 

 minal somite (the extreme length of which is rather 

 greater than that of the telson) are divided into a larger 

 proximal, and a smaller distal portion (fig. 37, F, p. 144). 

 The latter is about half as long as the former, and has a 

 rounded free edge, setose like that of the telson. There 

 is a complete flexible hinge between the two portions, 

 and the overlapping free edge of the proximal portion, 

 which is slightly concave, is beset with conical spines, 

 the outermost of which are the longest. The endopodite 

 has a spine at the junction of its outer straight edge 

 with the terminal setose convex edge. A faintly marked 

 longitudinal median ridge, or keel, ends close to the 

 margin in a minute spine. The tergal distal edge of 

 the protopodite is deeply bilobed, and the inner lobe 

 ends in two spines, while the outer, shorter and broader 

 lobe, is minutely serrated. 



In addition to the characters distinctive of sex, which 



have already been fully detailed (pp. 7, 20, and 145), there 



is a marked difference in the form of the sterna of the three 



posterior thoracic somites between the males and females. 



17 



