266 



SECTION D. ENTOMOSTRACA. 



ORDER VII. PHYLLOPODA. 



876. We are now arrived at the fourth Section of the class, 

 that of ENTOMOSTRACA j in which the union between the legs 

 and respiratory organs, which we have seen to be gradually 

 taking place in the preceding Orders, becomes more complete, 

 there being now no distinct gills, and the anterior legs being 

 converted into respiratory organs, by an expansion of their sur- 

 face. In the greater part of this and the succeeding groups, 

 the body is inclosed in a sort of horny shell ; which sometimes 

 appears composed of but a single piece, and in other instances 

 is formed like the shell of a bivalve Mollusk : hence they are 

 commonly known as Entomostracous Crustacea (this term 

 meaning, inclosed in a shell). The eyes are generally placed near 

 each other ; and are sometimes so close, as to run together (so to 

 speak) into one mass. The foot-jaws are all converted into 

 true legs, being, like them, adapted for swimming ; and even 

 the antennae sometimes become locomotive organs. The number 

 of legs varies greatly ; reaching in some 

 species to above a hundred ; but being 

 commonly much less. All these animals 

 are aquatic, and most of ihem inhabit fresh 

 water. They are for the most part extremely 

 active in their habits ; and are all of small size, 

 some of them being even of microscopic minute- 

 FIQ 599. CYCLOPS ness< This active locomotion is doubtless in 

 VULGARIS, magnified, part connected with the act of respiration, and 

 serves to produce a constant interchange in the 

 water in contact with the surface of the body, and especially 



