198 ELEMENTARY BIOLOGY. [CHAP. 



All are, like d, destitute of exopodite. All are 

 elongated, the proportionate lengths of the several 

 segments being identical with those of d. The first 

 and second pairs are chelate, and the first three 

 bear both a gill and an epipodite. 

 The fourth; destitute of both gill and epipodite: 

 when at rest it is backwardly directed. 



In the Lobster, the gill borne by each of the above 

 appendages is, like that of b, distinct from the epi- 

 podite. 



The genital orifices are borne upon these ap- 

 pendages. They have been described at A. c. y. 



/. The abdominal appendages, other than the third pair 

 (described at a). 



a. The fourth and fifth pairs : closely resembling 

 the third, functional as swimmerets. 



(3. The sixth pair, modified to form, together with 

 the telson, a tail-fin. The protopodite : repre- 

 sented by a single short strong segment. (In 

 the lobster there is a second incomplete basal 

 segment.) The exopodite and endopodite: 

 wide plates fringed with setae. The exopodite ; 

 divided into two portions by a transverse joint : 

 the free edges of its proximal portion are markedly 

 serrated. The endopodite ; continuously calci- 

 fied; its proximal internal surface is beset, in 

 the Crayfish, by a patch of short setae, which 

 play, during life, upon a corresponding hirsute 

 area of the under surface of the telson. 



y. The second pair. Cf. Sect A. c. y. 



Closely resembling the third in the female. In 

 the male the endopodite is much modified ; its 



