ON A NEW HISTRIOBDELLTD. 829 



It will be observed that in order to counect Dinophilus 

 genetically with the Rotifera we have not only to assume the 

 development of metamerism, but also the loss of the mastax 

 and the appearance of the external ciliation. 



Let us now consider what assumptions must be made if we 

 are to regard the Histriobdellidfe as direct descendants of 

 the Rotifera. The evolution of an incomplete metamerism 

 must of course be assumed. But there are indications of 

 such a tendency not only externally, but in both the muscular 

 and nervous systems of the Rotifera. 



In the general shape the Histriobdellidte more nearly 

 resemble the Grastrotricha than the Rotifera proper — in 

 the narrow body, in the presence of a distinct head region 

 having the mouth at its anterior end, and in the foot being 

 represented by a pair of processes each with its pedal gland, 

 with the anus situated nearly between them on the dorsal 

 side. But in Paraseison (23) we have a true Rotifer in 

 which the trochal disc is not developed, and in which there 

 is a definitely separated head region, containing the brain 

 and the mastax, and having the mouth at its anterior end. 



The tail region of the Histriobdellidaa corresponds to the 

 tail of the Rotifer ; the posterior legs of the former to the 

 ''toes" of the latter. The tail of the Rotifer is always 

 entirely post-anal; but, as sliown by Tessin and by Zelinka 

 (27), its interior is filled at an early stage with endoderm 

 cells, from which circumstance the latter author comes to the 

 conclusion that the anus originally opened at the end of the 

 tail. In the Histriobdellid^e the terminal position of the 

 anus was retained. 



The resemblance between the glands in the bases of the 

 posterior legs of the Histriobdellidae and the foot glands of 

 the Rotifers will be obvious. In some cases the latter would 

 appear to consist of homogeneous masses of protoplasm, not 

 divided into cells, with scattered nuclei (Plate, 24) ; but in 

 others (Callidina), as in Stratiodrilus, each is a group of 

 distinct, apparently dust bearing, cells (Zelinka, 27). 



In Paraseison, as already noted, there is a head region 



