CRUSTACEA 



93 



segments are sometimes so fused as to puzzle one to distinguish 

 twenty segments, as in the crayfish, but by regarding one pair 

 of appendages to each segment one is able to count the number 

 of segments present in the specimen. There is a number of 

 orders under this sub-class, but only a few can be mentioned. 



Order I. Phyllocar'dia is marine. The genus Nebalia, with 

 its bivalve carapace, its leaf-like thoracic feet, and biramous 



Fig. 67. — Three adult crustaceans and their larvae, a, Prawn {Pe?ieus), 

 active and free living; b, larva of prawn; c, Sacculina, parasite; d, larva of 

 SarcHlina; e, barnacle {Lepas), with fixed quiescent life; /, larva of barnacle. 

 (After Hackel.) (From Jordan and Kellogg, " Animal Life," D. Appleton 

 ami Co., Publishers.) 



abdominal appendages, may be taken as an example of this 

 order. 



Order II. Decap'oda. — This order consists of both marine 

 and fresh-water crustaceans. It contains the best-known forms 

 as well as the most useful ones to man, as the crayfish, lobster, 

 shrimp, prawn (Fig. 67), and crab. As the ordinal name sug- 



