586 FRESH-WATER BIOLOGY 



tunity lies in the study of the various problems connected with 

 reproduction and the diversity of the sexes. Few groups of organ- 

 isms present conditions so favorable for the study of these funda- 

 mental matters. 



The relationships of the Rotifera to other groups, and their 

 interrelationships among themselves are subjects which have been 

 much discussed and on which there is wide divergence of opinion. 

 As a result, the classification of the group differs greatly with 

 different authors. The classification perhaps most commonly 

 employed is that given in Hudson and Gosse's Monograph of the 

 Rotifera. Wesenberg-Lund's classification, based on that view of 

 the interrelationships of the Rotifera set forth in the foregoing 

 paper, has been little used; yet it appears to be that to which a 

 careful and unprejudiced study of the members of the group leads. 

 Most earlier classifications have found their guiding principles in 

 matters quite extraneous to the Rotifera as such. Led by theo- 

 retical considerations, the primitive rotifers have been looked for 

 among highly specialized species. Huxley compared the two ciliary 

 wreaths of Lacinularia to the two wreaths of certain larvae of other 

 groups, of echinoderms, annelids, and the like, thus indicating 

 a possible close relationship between them. This suggestion was 

 eagerly followed up, and the primitive organization of the Rotifera 

 has been sought in such highly differentiated, untypical forms as 

 the Melicertidae, the Philodinidae, and the like. Even that bizarre 

 side-shoot of one of the most highly specialized families, Trocho- 

 sphaera (Fig. 947), has been considered a primitive rotifer of special 

 significance, from its superficial resemblance to the trochophore 

 larvae of annelids, etc. Less popular, but still enjoying consider- 

 able repute, have been the theories which held that such forms as 

 Pedalion (Fig. 946) show a close relationship of the Rotifera with the 

 larvae of Crustacea. Careful comparative study of the Rotifera 

 themselves seems to show clearly that Lacinularia and the Meli- 

 certidae, Trochosphaera and Pedalion are alike terminal twigs of 

 the rotatorian tree highly specialized forms, whose origin is to 

 be sought in such rotifers as the primitive Notommatidae. 



Note. For recent changes in the names of many rotifers, in accordance with the 

 strict rules of priority, the paper of Harring ('13) should be consulted. 



