112 ZOOLOGY. 



general characters of the order, and have seven tlioracic and five or six 

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

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

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

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

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

 walldno- and swimmino-. The male is much smaller than the female. 



Latreille divides the Isojmla into six families, corresponding to the 

 following names : 1, Bopyr'idiB ; 2, CymotlioidiQ ; 3, SjyhcBromidcB ; 4, 

 Idoteldce ; 5, Asellidce; 6, Oniscidce. 



Fam. 1. Bopyridce. Bopyrus crangorum is found affixed to the gills, and 

 beneath the shell of several large Crustacea, as Palcemon and Hippolyte. 

 Tlie 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 Qgg^ they are 

 much like those of Cyclops. 



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

 which they afiix 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 large and operculiform. The young leave the egg with only 

 six jiairs 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 Sei^oUnce., Clrolajiinm^ and Nerocilincie. 



Fam. 3. SpJuaromidcv. The genus Sphcero77ia {p>l. 78, Jig. 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 [ptl. 78, fig. 20) is allied to Sphseroma, but it is less 

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

 sub-families, Splicevomhioi and Ancininoi. 



Fam. 4. Idoteldce. In this family the four antennse 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 respii'atory 

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

 families, Arctui'inm and Idoteince. 



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

 antennae 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 fiuviatile. A species of Asellus is common in the fresh waters 

 of Europe, and A. communis., Say, Jour. Acad. Nat. Sci. i. 427, is 

 abundant in similar localities in the United States ; and as it diffei'S from 

 the European 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 a,Sa| a table, and ou^a the tail. The 

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

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

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