MALACOSTEACA 363 



mentai-y males: 2 families with about 6 genera and 35 species. The best- 

 known genera are Sacculina Thompson, with about 6 species, which live 

 on decapod crabs, forming a thick sac between the abdomen and thorax, 

 and Peltogaster Rathke, w4th about 7 species, which live on hermit crabs. 

 They occur in the North Atlantic off the European coast. 



Subclass 2. MALACOSTRACA.* 



Crayfish, lobsters, and crabs, and the other higher and larger crusta- 

 ceans. In all, excepting the Phyllocarida, the body is made up of 20 

 somites, of which 5 form the head, 8 the thorax, and 7 the abdomen. The 

 head is invariably fused with one or more thoracic somites, and together 

 with these, in the Phyllocarida and Thoracostraca, is covered by a shield- 

 like carapace (Fig. 625,5). The number of pairs of appendages is 

 typically 19, of which 5 are cephalic, 8 are thoracic, and 6 are abdominal : 

 the last abdominal somite, which is called the telson, bears no appendages. 

 The cephalic appendages are the first and second pairs of antennae, one 

 pair of mandibles, and two pairs of maxillae. The terminal portion of 

 the antenna is called the flagellum (Fig. 576,1): in many cases more 

 than one may be present, one of which may be a smaller or so-called 

 secondary flagellum. The basal portion of the antenna is called the 

 peduncle (Fig. 576,2). The exopodite of the second antenna is broad 

 and flat in many forms and is called the antennal scale (Fig. 615, 3). 



From one to five of the anterior pairs of 

 thoracic appendages are specially modified to assist 

 in eating and are called maxillipeds, while the 

 posterior pairs are called periopods and are used 

 principally for locomotion (Fig. 576,5). The 

 thoracic appendages are often prehensile and used 

 for gTasping, in which case the distal segments may 

 be modified in one of three ways : (1) the terminal 

 segment may form with the next one a forceps-like 



pinching claw or chela as in the crayfish or crab, pjg. 573 Diagrams of 



when the appendage is spoken of as chelate (Fig. rsubcheiato°ciaw^(Smaii- 

 573, B); (2) the terminal segment may simply 7g°e?rick')'.^ '^'^^*' '^*'' 

 bend back on the next one as in the Amphipoda, 



when the appnendage is spoken of as subchelate (Fig. 573, A); (3) the 

 terminal segment may be sharp and spine-like, as in the parasitic isopods. 

 The abdominal appendages are called pleopods or swimmerets (Fig. 



* See "Natural History of Economic Crustaceans," etc., by R. Rathbun, Bull, of 

 U. S. Fish Com. for 18S9, p. 763. "Higher Crustacea of New York City," by F P. 

 Paulmeier, Bull. 91 of N. Y. St. Mus., 1905. "Die Siisswasserfauna Deutschlands." 

 Heft 11, 1910. 



