260 W. P. E. WELDON. 



coides (= D. capitata), D. metameroides, and D. cau- 

 data. From each of these the Cornish species differs in some 

 respect. From D. vorticoides it is distinguished by the 

 absence of a general coating of cilia on the dorsal surface, and 

 by the presence of definite " segmental" ciliated bands ; from 

 D. metameroides it differs in the entirely superficial nature 

 of the apparent " segmentation," adjacent segments not being 

 separated by infoldings of the body wall. 



I have not been able to consult Levinsen's recent description 

 of D. caudata/ but, so far as I can gather, it resembles D. 

 vorticoides rather than the present form. 



It will be seen from what follows that a further character is 

 presented by the present species, which has not been recognised 

 in others — the possession of a well-marked nervous system. In 

 the absence of any detailed information as to the structure of 

 other forms it would be premature to regard this as a specific 

 character, but even without it there seems to be sufQcient 

 warrant for establishing a new species, which I propose to 

 call D. gigas, from the large size of the sexually mature 

 individuals. 



II. — Anatomy. 



In its general structure D. gigas agrees closely with the 

 D. apatris of Korschelt, differing from it chiefly in the pre- 

 sence of a nervous system and in the histological structure 

 of the ectoderm. The paper of Korschelt^ is so complete, and 

 contains so full an account of the previous observations on the 

 genus, that it is unnecessary to do more than refer the reader 

 to it before passing on to a detailed description of the present 

 species. 



The ectoderm, as has already been seen, varies in charac- 

 ter in different parts of the body. In the head a transverse 

 section (fig. 3) shows a well-marked difference between the 

 dorsal and ventral portions. On the ventral side are seen cells 



' ' Viddensk. Meddel. fra den naturh. JToren. in Kjobenhavn,' 1879-80, 

 ^ ' Zeitschr. f. w. Zoologie,' Bd. xxxvii, Hft. 3. 



