ON A NEW HISTRIOBDELLID. 331 



tions of a pair of nephridia in the genital segment to the 

 reproductive apparatus is a special development following 

 upon the establishment of metamerism, such as we could not 

 expect to see foreshadowed among the Rotifera. The secon- 

 dary male character of the presence of the claspers seems to 

 be paralleled in some species of As planchna, in which there 

 is present only in the males a pair of lateral processes which 

 seem to have glands at their bases. 



If, as the evidence adduced above seems to indicate, the 

 Histriobdellidse are primitive Annulates which retain certain 

 Rotiferan features that have become lost or disguised among 

 higher forms, it becomes necessary to account for Dinophilus 

 in a similar manner, unless Ave are to suppose that the annu- 

 late features of the latter have had an independent origin. 

 Now Dinophilus, while in most respects distinctly less 

 advanced than the Histriobdellidas towards the normal 

 annulate type, has at the same time fewer Rotiferan features, 

 and is looked upon by most of the zoologists who have paid 

 special attention to it as directly related to the Turbellaria. 

 If, however, the evidence in favour of the derivation of the 

 Histriobdellid^ from primitive Rotifers is sufficiently strong, 

 we must look on Dinophilus as having diverged from the 

 direct line of descent between the two groups, and as having 

 lost some of the Rotiferan features that in the Histriobdel- 

 lidee have undergone a further evolution. 



The phase of metamerism which the Histriobdellida3 

 exhibit is of great interest. The metamerism of Stratio- 

 drilus and that of a Chaetopod must be acknowledged to be 

 of the same nature, though the former is less complete in 

 certain respects. In an animal which, Avhatever may be the 

 nature of its food, is of extremely alert and active habits, 

 great degeneration does not seem to me probable; and yet, 

 unless we are to look upon the Histriobdellidse as degenerate, 

 it is impossible to avoid the conclusion that in them we have 

 a metamerism tending in the direction of that of the Annu- 

 lata, but in an incipient condition . If this be granted, it 

 carries with it the conclusion that the metamerism of the 



