xii PHYLUM MOLLUSCA 805 



the exception of Nautilus, would seem to indicate the derivation 

 of the phylum from a group in which metamerism had not arisen. 

 It will be readily recognised that the gap between the typical 

 trochophore and certain forms of Turbellarian larvas (Miiller's larva) 

 is not a very wide one, and might be covered by adaptation of the 

 larval Flat- worm to a freer pelagic life. If we were to suppose that 

 the most primitive MoUusca were derived from Turbellarian-like 

 ancestors, the conversion of a larva of the type of Miiller's larva 

 into a larval form like the moUu.scan trochophore would also have 

 to be postulated. This might involve a common platyhelminth origin 

 ■for Annulata and Mollusca, with subsequent extreme divergence — 

 a divergence in which the respective trochophores would take part, 

 though in a limited degree. The chief changes which the adult 

 animal would have to undergo in order to assume the character 

 of a primitive Mollusc on this supposition, would be — (1) The 

 development of some kind of protective layer of hard material, 

 perhaps composed at first of spicules in a thickend integument, on 

 the dorsal surface — the rudiment of the shell ; (2) The greater 

 development of the muscular layers of the body-wall on the ventral 

 side to give rise to a more efficient and specialised creeping organ 

 than was possessed by the Turbellarian ancestor ; (3) The develop- 

 ment of specialised respiratory organs in the form of ctenidia — a 

 change rendered necessary by the great reduction in the available 

 respiratory area brought about by the development of the shell ; 

 (4) The formation of an anus and proctodseum ; and (5) the develop- 

 ment of a coelome. 



With regard to the relationships of the various classes of 

 Mollusca, the following points are some of the most important to 

 be borne in mind. 



The lowest members of the phylum are undoubtedly the Proto- 

 branchia among Pelecypoda, and the Aplacophora among Amphi- 

 neura. The latter take the lowest rank in virtue of the absence of 

 both foot and shell, but the possession by some of a radula indi- 

 cates a comparatively high degree of specialisation. On the other 

 hand, while there is no indication of an odontophore, even in a 

 rudimentary condition, in the Pelecypoda, the foot and shell are 

 well developed even in Nucula and its allies. There is no actual 

 evidence to show that the foot and shell have been lost by degenera- 

 tion in the Aplacophora or the odontophore in Pelecypoda ; and it 

 would appear, therefore, that the two groups are to be derived 

 independently from some primitive form. 



The facts that the pelecypod shell, at its first appearance, is 

 univalve, and that the foot of the Protobranchia is of the creeping 

 type and their ctenidia plume-like, suggest the derivation of the 

 class from a form resembling a simple type of Gastropod with no 

 odontophore and with undisturbed bilateral symmetry. The Amphi- 

 neura are also bilaterally symmetrical, with paired ctenidia, kidneys. 



