174 GASTROPODA. 



Opisthobranchia and points out the great similarity existing between the in- 

 ternal organisation of the Bulloidea and that of the Thecosomata. Between 

 the Gymnosomata and the Thecosomata he finds a great distinction, since he 

 cannot regard the fins in the two divisions as homologous. Since, however, 

 according to him, the Gymnosomata, like the Thecosomata, are to be traced 

 back to Tectibranchia, they have in any case a common root. It appears to 

 us that their development is in favour of a connection between them. Their 

 larval forms agree closely, the resemblance between the long, straight shell of 

 the Gymnosomatous larva and that of the Thecosomata being specially strik- 

 ing. This is a feature which points to a long period of pelagic life of the 

 adult, for the larvae of the Opisthobranchs also live in the sea. We might 

 therefore assume that the Gymnosomata are to be traced back to forms 

 resembling the ancestors of the Thecosomata, which only later underwent 

 the changes now found in their structure and development. We can hardly 

 regard as of much importance the apparent retention of a primitive feature 

 in presence of a small creeping foot in the Gymnosomata, since single 

 primitive characters may be retained in forms which in other respects are 

 highly specialised. It is also by no means certain that this character has 

 not been secondarily acquired. 



We have felt justified in treating the Pteropoda separately from the 

 Opisthobranchia on account of the great deviations found in the structure of 

 the body. In so doing, we do not wish in any way to deny their relation to 

 forms like the Bulloidea and especially Gasteropteron. It is possible that 

 there may be even closer ontogenetic relationship to these forms than is at 

 present known. This would be the case if the ontogeny of a Cephalophoran 

 described by C. Vogt were really found to refer to Gasteropteron, as was con- 

 jectured by Gegenbaur (No. 128). This Veliger larva develops two fin-like 

 structures, and yet, in consequence of various other characteristics, is not 

 comparable to a Pteropod-larva. The conical shell with its transverse lines 

 of growth, further, resembles the shell of the Gymnosomata and would be 

 little suitable to an Opisthobranch. It is thrown off even within the egg- 

 shell. The view that the larva now under consideration belongs to Gasterop- 

 teron has been directly denied by Krohn (No. 58b) who regards another larva 

 as being that of Gasteropteron. We are not acquainted with any more recent 

 accounts of this very interesting larva which may be of great importance in 

 determining the view which should be taken of the Pteropoda. 



E. Pulmonata. 



The transition from the ontogeny of the Opisthobranchia to that 

 of the Pulmonata is afforded by Onchidium, a form which has already 

 been alluded to p. 133. This amphibious form, which lives on the 

 sea-shore, develops embryos with a large bilobed velum. The two 

 lobes are beset with long cilia, while small and delicate cilia are found at 

 the incisions between the lobes. This embryo thus greatly resembles 

 the Veliyer larva of the Opisthobranchia. Although the adult is shell- 

 less, the embryo has a coiled shell like that of a marine Gastropod. 



