DAPHNIOIDEA. |263 



except in the group Bosminidae, in which they are multiarticulate 5 

 often they are quite obsolete. The inferior antennae, on the contrary, 

 are prominent organs, ending in two or three few-jointed branches. 

 The foliaceous legs are partly branchial in character, and have in 

 most genera a small appendage, especially adapted for this function. 



There is usually a pair of minute appendages, or, at least, a pair of 

 setae, near the base of the abdomen, having a dorsal position. 



Although approaching the Cyproidea in general habit, the presence 

 of the pairs of subnatatories serves to distinguish them, the corre- 

 sponding organ in the Cyproidea being obsolete. This character is of 

 more importance than the existence of a separate head in the Daphni- 

 oidea, although the latter is the more obvious character. 



We do not undertake to draw out an account of the structure of the 

 animals in this tribe, as our own observations have been comparatively 

 few ; and these will be given with the descriptions of the species .* 



The known species of Daphnioidea belong to four families, distin- 

 guished by the number of pairs of legs and the antenna. These are 

 the Penilidae, with six pairs of legs and obsolescent anterior antennae ; 

 the Daphnidae, with five pairs of legs and obsolescent anterior an- 

 tennae; the Bosminidae, with five pairs of legs and multiarticulate 

 anterior antennae ; the Polyphemim, with four pairs of legs. 



These groups appear to be based on important characteristics. 

 The presence, as in the Bosminidce, of multiarticulate superior an- 

 tennae, in a tribe which through nearly all its species has these an- 

 tennae obsolescent, is a characteristic of considerable importance. 



The Polyphemidce constitute a trenchant group, remote from the 

 other Daphnioidea. They carry in front a large head, full of eyes ; 

 the legs are subterete, rather than foliaceous, and are destitute of the 

 branchial appendage, and moreover they are not wholly covered by 

 the shell ; the body behind inclines downward, and leaves a very large 

 cavity for the young or ova. 



The Daphnidae and Penilidae are more nearly related, as they are 

 mostly similar in the characters of the legs, and in most points of 

 structure. Yet they differ in the number of legs, and these legs are 

 rather narrower in the Penilidae than in the Daphnidae. The poste- 



* A general review of the subject, with many original observations, is contained in 

 Baird's British Entoniostraca. The extended aud thorough investigations of E. 

 Schodler, on the Acanthocercus rigidus (Archiv f. Nat., 1846, 301-374 pp.), have thrown 

 great light upon the structure of this species, as well as the Daphnioidea generally. 





