PHYLUM ARTHROPODA 



535 



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 ordinary segments and a telson. 



FIG. 421. Sacculini carcini, 011 abdomen of crab. 1>,\ branchial region of crab ; I, hepatic- 

 region ; (/, intestinal region ; /-., body of Peltogaster ; ;). peduncle ; mlt, basilar membrane, 

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

 Lang's Comparativi Anatomy, after Delage.) 



The limbs arc strikingly modified for the performance of various 

 functions. 



The Phyllocarida are interesting from the fact that they are 

 annectent or linking forms between the Phyllopoda 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. 422), 

 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). 



