ON THE ANATOMY OF HISTRIOF.DELLA HOMAEI. 847 



of development, renders its relationship hard to determine, 

 and hides the primitive characteristics of its organisation. 

 That the Rotifers themselves are likewise a highly specialised 

 class of somewhat uncertain affinities is an opinion that is 

 gaining ground, since so much doubt has been thrown on their 

 supposed relation to the Annelid trochophore. The work of 

 Wesenberg-Lnnd (20) has shown that the most simple and 

 trochophore-like of the Rotifers are probably the most highly 

 specialised and the farthest removed from the Annelids. Yet 

 the clearly segmented plan of both Dinophilus and His- 

 triobdella, it must be admitted, is essentially similar to that 

 of a Chsetopod. This, combined with the clearly Polychajt 

 nature of egg-segmentation in Dinophilus, is sufficient to 

 place these forms in direct connection with the Annelids, 

 quite apart from either Protodrilus or Polygordius. 



Under the heading of the various organs I have already 

 gone into a more or less detailed comparison of Histrio- 

 bdella with Dinophilus, so that it is only necessary to 

 review the subject here from a more general standpoint. 

 In both forms the animal consists of a distinct head and 

 trunk, the latter composed of relatively few segments. In 

 both the nervous system consists of a well-defined brain or 

 peuropile, and a double ventral nerve-cord, with metameri- 

 cally arranged ganglia. In Dinophilus these are formed 

 by transverse commissures, while in Histriobdella the two 

 parts of the cord unite directly to form the ganglia. The 

 external segmentation corresponds with that of the nervous 

 system. Dinophilus does not possess the feet, cirri, or 

 tentacles that so clearly mark segmentation in Histrio- 

 bdella. But the metamerism is less definitely shown by the 

 ciliated bands, mucus glands, and the ring-like constriction 

 of the body into a series of segments. On the other hand the 

 nephridia show a more matamerically placed arrangement than 

 they do in Histriobdella. In both (with the exception of 

 Stratiodrilus) the nephridia open to the exterior in the seg- 

 ment following that in which they arise, as in Annelids. In 

 Histriobdella the muscular system shows a very high 



