350 CRRSSWKLL SHEARER. 



Apart from any degeneration I agree with Haswell (13) 

 that the relationship of Histriobdella with Polygordius 

 *'is extremely remote, and not such as to justify their inclu- 

 sion in the same class." The absence in Histriobdella of 

 a blood-vascular system, a distinct prostomium and peri- 

 stomium, the presence of mouth opening well forward in the 

 head, chitiuous jaws, and complicated generative apparatus 

 in the male, paired limbs, and mucous glands, clearly 

 separate it from Polygordius and Protodrilus, placing 

 it quite apart from these forms. With the Rotifers, on the 

 other hand the relationship is undoubtedly more pronounced. 

 Haswell has pointed out that all the main features of His- 

 triobdella can be traced to this class, although in general 

 features the resemblance is greater perhaps with the 

 Gastrorichia than with the Rotifers proper. The 

 chitinous jaws of Histriobdella can be readily homo- 

 logised with the mastix of Rotifers. In the absence of 

 solenocytes and the general similarity of the nephridia of 

 Histriobdella to the flame-cell type nephridia of Rotifers, 

 we have a further resemblance. In both the cuticle is firm 

 and shows a tendency to contract into ring-like folds. In 

 both, also, the generative organs, especially in the male, can 

 be reduced to the same plan. 



In Paraseison we have a Rotifer not unlike Histri- 

 obdella in many of its features. The body is elongated and 

 worm-like, with a distinct head bearing the mouth at its 

 anterior extremity. In the middle of a very rudimentary 

 coronal disc which bears no ciliated apparatus ai-e four small 

 bundles of hairs, placed in two pairs. Behind the mouth are 

 found the orifices of two glands, similar to those found on the 

 anterior feet of Histriobdella. On the top of the head is 

 a small tubercle representing the dorsal median tentacle of 

 Histriobdella. There is a narrow oesophagus, which leads 

 into a large cylindrical stomach. There is no gut, and the 

 stomach, which is not ciliated, is definitely closed. But this 

 condition has plainly been evolved within the limits of the 

 ffenus, as it is not characteristic of other Rotifers. It is 



