ON THE ANATOMY OF HISTKIOBDELLA HOiMAKI. 351 



noteworthy that Paraseison, like Histriobdella, is para- 

 sitic, being found on the gills of the Crustacean Nebalia. 

 It is undoubtedly with such forms as Paraseison among the 

 Rotifers that Histriobdella must be compared. The 

 greatest objection to the comparison of Histriobdella with 

 the Rotifer is encountered in regard to the nervous system. 

 This in Histriobdella is already so elaborated, and of that 

 type found among the higher Annelids, as to be hardly com- 

 parable to the diffuse, and less differentiated, and centralised 

 system of Rotifers. 



I cannot agree with Haswell that Zelinka's (36) discovery 

 of a sub-oesophageal ganglion in Callidina and Discopus 

 renders this comparison more easy. A further difficulty is 

 found in the absence of any true metamerism in the Rotifers. 

 This difficulty is possibly not so great when we consider the 

 arrangement of the transverse muscle-cells in such a rotifer 

 as Discopus synaptas. Leaving aside any comparison, 

 therefore, of the nervous system, it nevertheless remains a 

 fact that Histriobdella undoubtedly resembles the Rotifers 

 more closely than any other group of animals. 



If Histriobdella is related to the Rotifers it becomes 

 necessary to determine the relationship of Dinophilus to 

 the same class. Schimkevvitsch (28) was the first to point out 

 the similarity of the caudal appendage in Dinophilus to 

 the foot of the Rotifer. In Dinophilus, as in the Rotifer, 

 this is used in attaching the animal. In both forms there is 

 a marked sexual dimorphism. But as Nelson (25) has pointed 

 out, the caudal appendage in Dinophilus i-esembles more 

 that of some of the polytrochal annelid larv« than the foot of 

 the Rotatoria, and the sexual dimorphism can have arisen 

 within the genus, as it is found in other groups of the 

 Annelida besides the Rotifers. One striking difference 

 between the Rotifers and Dinophilus is the apparent total 

 absence of a definite mesoblast in the Rotifers, while it is 

 clearly present in Dinophilus, where it has the same cell- 

 oi'igin as in Polychasts. In Rotifers the mesoblast would 

 seem to be represented by the germ-cells alone, and it is 



