SUBORDER II. 



CORMOSTOMATA. 



There are two widely different types of structure among the sucto- 

 rial Crustacea. 



In one type, characterizing by far the larger part of the species, 

 the general arrangement of the organs is identical with that in the 

 Cyclopoidea, the only essential variation from that group being found 

 in the mouth. There are species like the subcylindrical Cyclops, 

 and others depressed, like the Sapphirinse. The succession of parts, 

 the natatory thoracic legs both as to form and number, and the parti- 

 cular structure of the abdomen, are represented exactly among the 

 species ; and a change in the mouth to the ordinary sessile form 

 would bring the species strictly within the limits of the Cyclops sec- 

 tion of Lophyropods. Only the lower divisions, in which the members 

 are partly obsolete, would stand apart; and thus separated would still 

 form a closely related group. 



The second type— exemplified in Nymphon and Pycnogonum,-- 

 presents an Arachnoid form, and most nearly approaches in general 

 outline the genus Cyamus among the Isopods. The abdomen is 

 obsolete; the body short and annulate throughout without a carapax; 

 the legs long and spreading, spider-like, and the only appendages to 



the thorax. 



The species of the first type constitute the section Pcecilopoda, 



those of the second, the section Arachnopoda. 



The most important and fundamental point of distinction between 

 the Pcecilopoda and the preceding group, next to the trunk form ot 

 the mouth, consists in the fact that throughout the characteristic 

 species of Poecilopods, the second maxilla (or the maxillipeds) ot tne 



