112 ZOOLOGY. 



general cluimcters of the order, and have seven thoracic and five or six 

 abdominal segments which are quite distinct, the females are misshapen, 

 with the segments of the body indistinct. Tiie thorax is narrow in the 

 male and wide in the female, and the eyes are present in the former and 

 absent in the latter. The antenna? are more or less rudimentary, and the 

 feet very short, snbmarginal, and formed for holding, but not adapted for 

 Walking and swimminor. The male is much smaller than the female. 



Latrcille divides the Isopoda into six ftimilies, corresponding to the 

 following names : 1, Bopyridfii / 2, Cymothoidce / 3, SjyJimromidm / 4, 

 IdoteidcB ; 5, Asellidce i 6, Oniscidce. 



Farn. 1. Bopyridiß. Bojyyrus crangorum is found affixed to the gills, and 

 beneath the shell of several large Crustacea, as Palcemoii and HlppolyU. 

 The male is only one fifth or one sixth the size of the female, and is found 

 under the abdomen of the latter. When the young leave the egg, they are 

 much like those of Cyclops. 



Fam. 2. Cymothoidre. Most of these are parasitic upon marine fish, to 

 which they affix themselves with the aid of their strongly hooked feet. The 

 body is lengthened oval, narrowing towards each end, the head is small, and 

 the feet are lai-ge and operculiform. The young leave the egg with only 

 six pairs of feet, and at this period the abdomen is adapted for swimming. 

 Some are sedentary, and others possess the power of walking. The sub- 

 families are the ScroUna^ Cirolaninm^ and NerociUncp. 



Fam. 3. SpTiOiromida^. The genus Sphmroma {pi. 78, fig. 21) has the 

 thoracic segments nearly alike in form and size, the feet slender, and- the 

 false feet (except the last pair) received into a cavity beneath the abdomen. 

 The species live among submarine plants, and they can roll themselves into 

 a ball. Cymadocea {pi. 78, fig. 20) is allied to Sphteroma, but it is less 

 flexible, and therefore not able to roll itself into a ball. It contains the two 

 sub-families, Sphmrominm and Ancinhuv. 



Fam. 4. Idoteidcß. In this family the four antennae are placed in the 

 same line, and the first pair are very small. The body is slender, not much 

 thicker in the middle, and truncated or concave posteriorly. The respiratory 

 false feet are concealed in an opercular cavity beneath the abdomen. Sub- 

 families, Arctainnm and IdotehKc. 



Fam. 5. Asellklce. Body elongated and flattened above ; the two pairs of 

 antennse are setaceous, and arranged in two lines, the first pair small. The 

 abdomen is composed of one large scutiform segment without lateral 

 swimmerets, and with two terminal stylets. Some of the species are marine, 

 and others fluviatile. A species of Asellns is common in the fresh waters 

 of Europe, and A. cormmmis., Say, Jour. Acad. Nat. Sei. i. 427, is 

 abundant in similar localities in the united States ; and as it differs from 

 theEuropean representative of the genus in having the sides rectilinear, entire, 

 and gradually diverging posteriorly, and the posterior segment being the 

 widest as well as the largest, and transversely quadrate ; we propose to give 

 it the generic name of Abacura., from aßal a tal)le, and ou^a the tail. The 

 antennae and caudal appendages are as in Asellns. It is half an inch or 

 less in length, and may be found walking upon the bottom of springs. 

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