No. 26.] ARTHROSTRACA OF CONNECTICUT, 185 



PART II. ISOPODA. 



The Isopoda are divided into six suborders by Sars, whose 

 classification of the group is usually followed and upon whose 

 key the following one is based. 



KEY TO THE SUBORDERS OF ISOPODA. 



A. First gnathopods chelate; uropods terminal; pleopods when dis- 

 tinctly developed, exclusively natatory TANAIOIDEA p. 193 



AA. First gnathopods not chelate. 

 B. Uropods lateral. 



C. Uropods forming together with the terminal segment of the 

 abdomen a caudal fan; pleopods 'for the most part natatory 



CYMOTHOIDEA p. 197 

 CC. Uropods valve-like, inflexed, arching over the pleopods 



which to a great extent are branchial IDOTHEOIDEA p. 214 



BB. Uropods terminal. 

 C. Pleopods exclusively branchial, generally covered by a thin 



opercular plate (the modified first pair) ASELLOIDEA p. 230 



CC. Pleopods fitted for air-breathing ONISCOIDEA p. 237 



CCC. Pleopods, when present, exclusively branchial in the adult 



and not covered by an operculum BOPYROIDEA p. 235 



EXTERNAL ANATOMY. 



The Isopoda differ most strikingly from the Amphipoda in 

 the general form of the body which is always depressed or, at 

 least, not compressed laterally. The Anthuridse are very slender 

 and elongate and represent an extreme condition which is com- 

 parable with that of the Caprellidea. With the exception of the 

 Bopyridas, which are parasitic, the Isopoda are all bilaterally 

 symmetrical. In this one aberrant family the males and females 

 are exceedingly dift'erent in form, the females being four or five 

 tinies larger than the males and exhibiting marked degenerative 

 changes while the males are minute and remain permanently 

 attached to the body of the female in the region of the latter's 

 genital openings. 



The segmentation of the body is less regular than among the 

 Amphipoda ; the head is fused with the first thoracic segment in 

 the Tanaioidea and the segments of the abdomen suffer various 



