374 Transactions. 



Art. XXXIX. — Vascular System of Siphonaria obliquata Sowerby. 



By A. J. Cottrell, M.A., M.Sc. (N.Z.). 

 Communicated by Professor Benham. 



[Bead before the (Hugo Institute, 3rd October, 1911.] 



In a previous paper (Trans. N.Z. Inst., vol. 43, 1911, p. 582) 1 described 

 the general anatomy of Siphonaria obliquata, the common limpet-like 

 marine Gastropod, whose affinities are still undecided ; for, while some 

 authorities incline to the view that it is a modified Opisthobranch, others, 

 and perhaps the majority, regard it as a Pulmonate. I now supplement 

 that article by giving here an account of the vascular system, which was 

 omitted purposely from that paper. 



The adaptation of Siphonaria to a life along the sea-shore, where at 

 times it is below water, at other times above, is well illustrated by its 

 '* dipnoan " character, having, that is, both a lung and a gill, each with 

 its own independent blood-supply. If, as seems the more probable, it is 

 a Pulmonate which has left its terrestrial home and ventured back into 

 the original habitat of the Gastropod molluscs, we must look on the gill 

 as a new structure, analogous to those of such forms as the Prosobranch, 

 Patella, (limpets), and the Opisthobranch, Pleurophyllidia , which has arisen 

 in relation to this new mode of life — that is, it is an " adaptive " gill, as 

 in these forms. Its structure is not that of a typical ctenidium, as I 

 pointed out in my previous article, though it has some resemblance to 

 certain gills amongst the Opisthobranchs. We already know a few Pul- 

 monates which have taken to an aquatic life in which a gill is present. 

 Thus, in Isidora, Protancylus, and others there is a single-folded lamina ; 

 but in Siphonaria, as I have described it in my previous article (p. 585), 

 there is a series of independent laminae arranged in a semicircle round the 

 mantle-cavity, each lamina bearing secondary laminae, so that it is much 

 more complex than the gill in the above Pulmonates. 



Lang regards these pulmonate gills as probably ctenidia, though Pel- 

 seneer and others hold them to be " adaptive " gills. On the other hand, 

 supposing that Siphonaria is an Opisthobranch which has come able 

 to remain out of water for a considerable portion of each day, and to 

 breathe air during this period, we must suppose that it is the lung that 

 is the new thing ; and although, so far as 1 am aware, no Opisthobranch 

 has been found to have a lung, yet amongst the Prosobranchs several 

 genera, belonging to different families, have developed a network of blood- 

 vessels on the mantle-roof ; so that the cavity acts as a lung, and the 

 genera can be arranged in series, in which the ctenidium gradually decreases 

 in size and importance till it remains as a mere vestige (as in Cerithidea). 



Hence, whatever view is taken as to the affinities of Siphonaria, it 

 remains an extremely interesting form to biologists, having either an 

 adaptive lung or an adaptive gill. 



The Heart. 



The heart lies in the pericardium on the left side of the dorsal surface 

 of the body, just in front of the middle of the length. The position of 

 the heart, as seen in the animal after the shell has been removed, is shown 

 in fig. 7 of plate 28 of my former article. The walls of the pericardium 



