38 THE VOYAGE OF H.M.S. CHALLENGER. 



to the tip of the head, which is more or less truncated, that the cephalic slits are compara- 

 tively short, extending on to the anterior surface of the head, where they do not, however, 

 coalesce, but leave a small interval in which the proboscidian aperture is situated. 



This is all that can be said of the external appearance. 



The internal structure further confirms the supposition which the external characters 

 point to, viz., that these two specimens belong to the same species. Thus, certain chief 

 points in the series of transverse sections made through both the specimens coincide to a 

 degree that may be judged of by the following table : — 



The superior brain commissure is situated in section No. 



The inferior brain commissure is situated in sections No. 



Canal of the posterior brain lobe is situated in section No. 



The mouth begins in section No. 



The mouth ends in section No. 



First appearance of nephridia in section No. . 



In Specimen a. In Specimen h. 

 30 34 



35-39 38-44 

 43 51 



55 60 



84 103 



97 112 



In specimen a (May 8, 1873) there are noticed four transverse deferent ducts to the 

 longitudinal nephriclial duct on the left side (in sections 138, 175, 217 and 278), and four 

 on the right side (sections 147, 191, 217 and 278). 



In specimen b (May 20, 1873) there is a less regular arrangement, some of the 

 deferent ducts being double, i.e., two at the same level or in the same section. Once in 

 this specimen this is so arranged that in the one section (No. 197) there are four deferent 

 ducts cut nearly throughout their whole length. In the portion of the oesophageal region 

 sectionised, which, however, does not embrace the whole nephridial region, I count in this 

 specimen on the left side six deferent ductules (sections 112, 144, 156, 159, 197, 207), and 

 to the right also six, which are only partly opposite to the left ductules (sections 142, 168, 

 178, 180, 197, 208). In judging of this apparent discrepancy it should not be lost sight 

 of that the short distance separating the two ductules 156 and 159 on the left is still 

 symmetrically repeated on the right side in 178 and 180, though somewhat further 

 backwards. 



A commissure uniting the two vagus nerves after they have left the brain and before 

 they have yet reached the oesophagus was distinctly noticed. 



The histological details of the integument fully correspond to the type of Cerebra- 

 tulus corrugatus, which will hereafter be more fully described and figured (cf. PI. XIII. 

 fig. 6), and which is diagrammatically represented in fig. 9 of PI. XL as differing from 

 figs. 10 and 11. 



One of the specimens showed a very curious pathological degeneration, which I will 

 however, only touch upon very briefly. The muscular tissue on a restricted but ring- 

 shaped region not far behind the head, and only as far as the circular and longitudinal 

 muscular coats are concerned, is replaced by (or gradually passes into) a homogeneous 



