IQ CRUSTACEA. 



fication ; but in fact it is confined to the lower grades, and it indicates 

 only subordinate divisions of the inferior group. "We deem it of so 

 little taxonomic importance, that we do not assume it as a basis of a 

 grand distinction among the Edriophthalmia ; for it appears evident 

 that the characters of the Amphipoda and Isopoda separate them 

 widely from the rest of the species. Indeed, the sucking Caligi are 

 so like the Sapphirinae among the Cyclopacea in every point of struc- 

 ture, except the mouth, and so close even in this organ, that they seem 

 to be only related groups of the same subdivision that of the Ento- 

 mostraca. 



Besides the species alluded to, there are also in this sub-class the 

 Trilobita and Rotifera. The latter evidently have the lowest place. 

 The former have been arranged both with the Entomostraca and Iso- 

 poda; but the opinion of most authors places them at present in 

 an intermediate group. A few brief considerations on this point are 

 offered on a following page, after our observations on the Entomo- 

 straca. The subdivisions of the sub-class Edriophthalmia are, therefore, 

 as follows : 



Order I. CHOEISTOPODA (or Tetradecapoda), including the Amphi- 

 poda and Isopoda. 



Order II. TRILOBITA. 



Order III. ENTOMOSTRACA. 



Order IV. ROTIFERA. 



I. CHORISTOPODA. The Choristopoda are so called from x*p rTi >i sepa- 

 rate, and , foot, alluding to the most striking peculiarity of these 

 species, separating them both from the Macroura and the Entomo- 

 straca, viz. : that the thorax consists of a series of segments exposed 

 to view and corresponding each to a pair of thoracic feet, which feet 

 are ambulatory or prehensile. This division of the body into distinct 

 segments from the head to the abdomen has scarcely an exception. 

 Yet sometimes one or two pairs of the feet are rudimentary or wanting, 

 and one or two of the anterior segments of the thorax, adjoining the 

 head, are obsolescent or concealed; and in a very rare case two seg- 

 ments are coalescent. 



Among the Choristopoda, there are two prominent groups, the Am- 

 phipoda and Isopoda. 



In one group, the Amphipoda, the abdomen is elongated, with flexi- 

 ble articulations ; the three anterior pairs of appendages are natatory ; 

 the three posterior pairs styliform; the branchial vesicles are attached 

 to the thoracic legs at base. 



