12 CRUSTACEA. 



racic feet eight in number, and used like arms, the six posterior am- 

 bulatory or affixing. 



3. Isopoda. Branchial vesicles abdominal; forward series of thoracic 

 feet six in number, and all ambulatory (except that one or two ante- 

 rior pairs are sometimes prehensile). 



The name Anisopoda, from the Greek */, unequal, and f,foot, 

 alludes to the unlike functions and size of the anterior and posterior 

 feet; the six posterior feet serve as feet for affixing themselves and 

 standing; while, the eight anterior are used like arms, and are 

 stretched out in search of food. Unlike most Amphipods and Isopods, 

 therefore, the two sets of thoracic feet are strikingly different in func- 

 tion ; and from the latter, they differ in that only the six posterior feet 

 are ambulatory, and these have the additional function of enabling 

 the animal to hold on to objects with an erect body, while the anterior 

 members are free to move in every direction. 



II. ENTOMOSTRACA. The Entomostraca agree in a general degrada- 

 tion of character (by which they differ from the Choristopods) , rather 

 than in any similarity of form ; yet, there are strong points which 

 unite them. Unlike the Choristopods, the thorax does not consist of 

 a series of seven segments following the head, with as many pairs of 

 ambulatory, or ambulatory and prehensile feet. The abdomen, more- 

 over, is without a regular series of appendages, either natatory or 

 branchial, a caudal pair being usually the only one present, though 

 sometimes, one or two preceding pairs, of peculiar structure, exist 

 connected with the egg-system. Of the thoracic members, the pos- 

 terior two pairs are, with few exceptions, obsolete, and in these excep- 

 tions they are natatory ; and the three to five pairs preceding, when 

 present, are natatory, excepting when one of them is genital in its 

 use. These natatory feet are well seen in the groups, Cyclops, Sap- 

 phirina, Caligus, Daphnia, and others ; and they are not found in the 

 Cypris group, because three of these pairs of legs, elsewhere natatory, 

 are here obsolete. These are striking peculiarities, removing the 

 species far from the Choristopods; and they as closely bind the 

 species together into a common family. Other points of resemblance 

 are as follows : 1, the absence in general of arterial vessels ; 2, the 

 frequent diversion of the posterior antennas to a natatory or prehensile 

 purpose ; 3, the diverse forms often presented by the anterior thoracic 

 members ; 4, the reduction of the nervous system, in most cases to a 

 single ganglion, encircling the oesophagus, which gives out all the 



