Proceedings of the General Meetings for Scientific Business of the Zoological Society of London 1894

Page 772

668 MR. WALTER GARSTANG ON COLPODASPIS PUSILLA. [Nov. 20, narrow stalk-a feature which it shares with most Prosobranchs. G w y n Jeffreys even informed him that he was inclined to consider Colpodaspis as the young of Cyprcea europcea-a view which now, at any rate, can no longer be entertained. In spite of our ignorance of the anatomy of Colpodaspis w e may, however, as a result of the above observations, be certain that Colpodaspis is a true Opisthobranch. It resembles various Cepha-laspidea in the pleuropodial expansions of its foot (cf. Haminea), in the posterior appendage of the mantle (Haminea, Philine), in its inflated shell (Haminea, Utriculus), and in its radula (Philine). O n the other hand it resembles the Notaspidea, and differs from the above types of Cephalaspidea, in the great extent of the mantle and in the "form of the head and tentacles. In the latter point it again resembles the Anaspidea, for in the young Aplysia, as I have often observed, there is only one pair of tentacles (the anterior one) for a considerable period, and these are grooved just as in Colpodaspis and Pleurobranchus. These various points of resem­blance are all explicable if we regard Colpodaspis as a very primitive type of Tectibranchiate mollusk, belonging indeed to the Cephal­aspidea, but retaining in an unspecialized condition an unusual number of those primitive characters which the common ancestors of the Cephalaspidea and Notaspidea alike possessed. It supplies an indubitable connecting-link between these two great subdivisions of the Tectibranchia; but it belongs to the group Cephalaspidea, iu spite of the inappropriateness of the name, owing to its acquisi­tion of pleuropodial expansions and a posterior pallial appendage- two associated features which are especially characteristic of this group. The question still remains open whether or not the creature described by Sars and myself has assumed its adult features. Fischer1 has suggested that Colobocephalus costellatus and Colpo­daspis pusilla are possibly only young stages of Philine or of neighbouring genera of Tectibranchs, owing to the radula in these two types resembhng very closely the radula of certain species of Philine (velutinoides, lima, angulata). This theory, however, is, in m y opinion, altogether untenable in the case of Colobocephalus, which, beyond the radula, presents no particularly Cephalaspidean, or even Opisthobranchiate, features. The probability, on the other hand, that the PhiiinidaB have been derived phylogenetically from a Colpodaspis-\ike ancestor is sufficiently great to render Fischer's view in this case worthy of consideration. The white colour of the body and the early enclosure of the shell by the mantle support this view; but the fact that all the specimens so far taken, which have been captured at such different times of the year as June, August, aud February, have been practically identical in structure, and have shown no special approach towards the adult organization of Philine, seems to m e to render the view improbable. The possession of a similar radula by so different a creature as 1 ' Manuel de Conchyliologie,' 1887, p. 564.