580 ZOOLOGY 



head has the same composition as in the Entomostraca, but the 

 thorax is invariably formed of eight segments, and, except in 

 the Phyllocarida, the abdomen of six segments and a telson. 



Fig. 450. -Sacculina carcini, on abdomen of crab. br. branchial region of crab ; I, hepatic 

 region ; d, intestinal region ; ks, body of parasite ; p. peduncle ; mb, basilar membrane, 

 giving off root-like processes which are seen extending through the body of the host. (From 

 Lang's Comparative Anatomy, after Delage.) 



The limbs are strikingly modified for the performance of various 

 functions. 



The Phyllocarida are interesting from the fact that they are 

 annectent or linking forms between the Branchiopoda and the 

 Copepoda on the one hand, and the higher Crustacea, particularly 

 the Schizopoda and Decapoda, on the other. The order contains 

 only three genera, the commonest of which, Nebalia (Fig. 457), 

 is a little shrimp-like marine Crustacean about 6-8 mm. in 

 length. The body is divisible into head, thorax, and abdomen, 

 all having the normal malacostracan number of segments ex- 

 cept the abdomen, which is formed of eight segments, the last 

 bearing caudal styles — structures not found elsewhere in the 

 sub-class. There is a bivalved cephalic carapace (s), closed by 

 an adductor muscle (sm) and extending backwards to the fourth 

 abdominal segment : it is terminated in front by a movable 

 rostrum (r). 





