no PROCEEDINGS OF THE ACADEMY OF [Feb., 



at intervals into compact groups to form five pairs of well-defined 

 ganglia, each of which is united by a transverse commissure. Each 

 pair of ganglia corresponds to a trunk segment, although situated 

 slightl}' posterior to the middle of the segment, as indicated on the 

 exterior by the constrictions and ciliated rings. 



In the same year with Weldon's ]5aper (1886) there appeared one on 

 D. gyrociliatus by Repiachoff. This investigator described and 

 figured the brain of this species and the anterior portions of the two 

 circumoesophageal commissures. The brain is, as in the other species, 

 composed of a central mass of nerve fibres (neuropil) invested on all 

 except its posterior surface with ganglion cells. These latter are 

 closely united to the ectoderm, as in D. gigas, whereas in D. tceniatus 

 they are distinctly marked off from it. 



Schimkewitsch (1895) has, in his account of the structure of D. 

 vorticoides , found in the White Sea, given the most complete account 

 of the nervous system of Dinophilus, as well as of other organs. The 

 nervous system of this species is, as a whole, essentially like that of 

 D. tceniatus, but differs from it in two important particulars. The 

 ganglion cells do not invest the lateral nerve cords throughout their 

 length, but are completely segregated into ganglia, the first pair of 

 which, together with their transverse commissure, lies entirely outside 

 of the hypodermis. If the nervous system be used as a criterion, then 

 D. vorticoides has reached the highest stage of development attained 

 by any member of the group thus far investigated. Judged in this 

 aspect, D. gigas should be placed at the foot of the scale; D. conklini 

 second, D. tceniatus third, and lastly D. vorticoides. 



Harmer (18S9a) has made a somewhat detailed comparison between 

 the nervous system of Dinophilus and that of the archiannelids. 

 Dinophilus agrees with all of the members of that somewhat hetero- 

 geneous group in the general plan of the nervous system and in its close 

 relation to the hypodermis (ectoderm). With Protodrilus (Hatschek, 

 1880) and with Histriobdella (Histriodrilus , Foettinger, 1885) it further 

 agrees in the separation of the lateral halves of the ventral nerve 

 strand. However, in a new species of Histriobdella described by Has- 

 well (1900) the lateral halves of the ventral nerve cord are completely 

 fused. Goodrich (1901), in discussing the systematic position of 

 Saccocirrus, has drawn attention to the well-known fact that the 

 epithelial position in this form is shared by chsetopod annelids belong- 

 ing to widely separated groups, as, for example, the Eunicidw (Spengel, 

 1881), Syllidce (Malaquin, 1893), Polynoidw, Chcetopteridw, Maldcmidce 

 (Macintosh, 1885), and Myzostomidce (Graff, 1877). The same 



