xii PHYLUM MOLLUSCA 781 



poda, of a ladder-like nervous system resembling that of some 

 Turbellaria and of the most worm-like of Arthropods I 'eripatus. 

 The head-kidneys or primitive nephridia of the molluscan and 

 annelid trochophore are very similar, though developmentally 

 unlike, and are probably homologous with certain types of nephridial 

 tubes found in " Worms " from Platyhelminth.es to Chsetopoda. 



If the occurrence of the trochophore be taken as a guide towards 

 the ancestry of the Mollusca, it need not necessarily be regarded 

 as leading back to the Annulata. In fact the absence of segmental 

 repetition of parts in all, with 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 larvae (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 Mollusca 

 were derived from Turbellarian-like ancestors, the conversion of a 

 larva of the type of Miiller's larva into a larval form like the mol- 

 luscan 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 thickened 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 development of specialised respira- 

 tory 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 development of a ccelome. 



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 simplest, but not necessarily the most primitive, members 

 of the phylum are undoubtedly the Protobranchia among Pelecy- 

 poda, and the Aplacophora among Amphineura. The latter take 

 the lowest rank in virtue of the absence of both foot and shell, but 

 the possession by some of a radula indicates 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 



VOL. I 3D 



