STRUCT[JI{E OF THE NEPHRIDIA OF DINOPHILUS. 521 



Another point of resemblance between these groups is that 

 in the majority of marine and fresh-Avater Turbellaria deve- 

 lopment is simple and direct, as in Dinophilus and His- 

 triobdella. While it is true other members of the Archian- 

 nelida, as Protodrilus, Polygordius, and Saccocirrus, 

 may possess larval stages, these forms are more nearly 

 related to the Polycheetas than to either Dinophilus^ or 

 Histriobdella. Saccocirrus is undoubtedly a Poly- 

 chgetj and can hardly be considered at all an Archiannelid, 

 as Goodrich (5) has shown. Protodrilus is also evidently 

 closely related to the same class, foi* young larvae I obtained 

 at Naples in 1903 plainly showed the presence of a well- 

 marked ciliated ring, and in external appearance bear a 

 certain resemblance to young Nereis larvae.^ The larval 

 form of Polygordius is of course well known. Moreover, 

 Haswell (9), in a paper on Histriobdella, has advanced 

 strong reasons for including Dinophilus and Histriob- 

 della in one class separate from that of Polygordius and 

 Protodrilus. The great difference of metamerism in the 

 two cases, the head segments, the relation of the brain 

 commissure to the mouth, and the great difference of the 

 reproductive organs in the two groups, shows that their 

 affinity is remote. On the other hand, Dinophilus and 

 Histriobdella show more relationship with one another in 

 the possession of a distinct head, a nervous system consisting 

 of a metamerically arranged series of ventral ganglia, an 

 alimentary canal essentially similar in both forms, and a 

 close resemblance in the reproductive organs, especially in 

 the male. I agree therefore with Haswell in grouping 

 Histriobdella and Dinophilus in one class separate from 

 Polygordius, Protodrilus, and possibly Ctenodrilus. 

 Regarding this last it is hard to say anything until some- 

 thing has been determined of its life history, as so far it has 



' Nelson (19) claims that Dinophilus shows a remarkable resemblance 

 in its cleavage stages to Polychajts, and thinks that in this respect it cannot 

 be considered a primitive form. 



^ See Pierantoni's (20) figs. 1 a and 1 b. 



