112 ZOOLOGY. 



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

 ahdoiriinal 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 tlie hitter. The antennic are more or less rudimentary, and the 

 feet very short, suljmarginal, and formed for liolding, but not adapted for 

 walking and swimming. The male is mucli smaller than the female. 



Lati-eille divides tlie Impoda into six families, corresponding to the 

 foHowing names: 1, liopijndm ; 2, Cymothoidte ; 3, Spli(XiTomid(B ; 4, 

 Idoteidm; 5, AselUdoe ; G, Oniscidm. 



Fain. 1. Bo])ijndm. Bopyrun crangorum' is found affixed to the gills, and 

 beneath the siiell of several large Crustacea, as Pal(jemon and Ilipjjolijte. 

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

 undei- the abdomen of the latter. When the young leave the agg., they are 

 mucli like those of Cyclops. 



Fam. 2. C'lpnotJimdai. Most of these are parasitic ui)on 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 large and operculiibrm. Tlie young leave tlie egg with only 

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

 Some are sedentary, and others possess the j)ower of walking. The sub- 

 families are the Serolinm^ ClrolanincB, and Nerocillnm. 



Fain. 3. ISphairomidai. The genus Sph.aiToma {pjl. 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 abdptnen. 

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

 a ball. Ci/madocea {pi. 78, fig. 20) is allied to Sphieroma, but it is less 

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

 sub-i'amilies, Sphm'omhim and Ancininm. 



Fain. 4. Idote'ulai. In this family the four antennce 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, ArcturhuB and Idoteinm. 



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

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

 abdomen is composed of one large scutiform segment without lateral 

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

 and others fluviatile. A species of A'Sellus 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 difters from 

 theEuro])can re])iv,seiitative of the genus in having the sides rectilinear, entire, 

 and gradually divei-ging 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 Ahacura.., from «,/?«? a table, and ou^a the tail. The 

 anteima^. and caudal appendages ai-c 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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