TOPOGRAPHICAL RELATIONS AMONG THE DOCOGLOSSA. 283 



As the pericardium turned to the left and forwards, the k-ft kidney was pushed before it 

 into the roof of the brancliial cavity ; it was also pulled with its excretory aperture to the 

 rif^ht and its Docoglossan position (PL 17. figs. 19, 20, & 24) is thus accounted for. The 

 reno-pericardial canals have already been correctly described by Goodrich and Pelseneer, 

 and it only remains to be said that their elon^^ation, as evidenced by the position of their 

 renal apertures far from the pericardium (fig. 2i), is a consequence of the pushing of the 

 excurrent apertures to the right and of the pericardium to the left. If the space- 

 relations in my sections (fig. 2i) do not exactly correspond with those shown in the 

 drawings of other workers, I would ascribe this to individual variation and variation 

 with age, which undoubtedly occurs. 



The molluscan gonad shows an extreme adaptability of position, and, consideriiig the 

 special pressure on the right, it is natural that it should usually occur on the left among 

 the Docoglossa. 



Part IX. — Affinities of the DocoaLOssA. 



The affinities of the Docoglossa amongst themselves have been discussed by Pelseneer, 

 Dall, Haller, and Thiele. It is now generally admitted that the Lepetidae are specialized 

 forms adapted to a deep-sea life, though, in the retention of the median tooth of the 

 radula and of vestiges of the spiral, they show primitive features. They may be a basal 

 offshoot, but, as it is at any rate certain that the other groups are not descended from 

 them, I have ventured to omit them from the foregoing evolutionary sketch. 



Pelseneer, Dall, and Haller all tliink that the Cyclobranchs have descended from 

 Monobranch-like forms, but Thiele disputes this because : — 



(a) The ctenidium of Acmcea has, according to him, no homology with that of other 

 Gastropods — it is a new development. 



(6) The Cyclobranchs show equal remains of both ctenidia (in the osphradia and 

 neiiihbouring tissue). 



(c) Dall and Haller have described rudiments of mantle-gills among the Monobranchs, 

 and believe these to be very early stages in the evolution of the Cyclobranch 

 gill-wreath. Thiele, however, sees in them disappearing vestiges. 



The conclusion (a) above seems to me to be quite unjustifiable on the grounds given — 

 that the ctenidium is oblique and free except at the basis in Acmcea, attached and free 

 only at the apex in other Gastropods ; that ctenidium and osphradium are not so nearly 

 connected as in Haliolis, &c. ; that the structure of the Acmaean giU is simpler than 

 that of others ; and that the efferent ctcnidial vein joins the great mantle-vein. 



This last is to be expected, as the great mantle-vein is undoubtedly a development of 

 the veins from the mantle which join the ellerent branchial vein even in Ilaliotls. The 

 simplicity of structure is easily understood when we believe that the Acmtean gill is 

 a structure arrested in an early stage of degeneration, and the other two characters do 

 not seem to me to be of morphological importance. I thcrcfoi-e accept the general 

 opinion that the ctenidium of Acmtca is the left ctenidium of the Prostreptoneure, 

 2)robably in a somewhat degenerate condition. Perhaps llaller has made the evolution 

 of Cyclobranchs from Monobranchs ajipear too direct, but if we rather suppose, as 



SECOND SERIES. — ZOOLOGY, VOL. IX. ^O 



