IX MOLLUSCA 369 



MoUusca do not lend themselves to such easy generalization. The 

 Pelecypoda and Scaphopoda burrow, the Gastropoda crawl, and the 

 Cephalopoda propel themselves by projecting squirts of water through 

 the funnel. 



But if we take into account Drew's statement (1899) that Yoldia 

 — surely one of the most primitive of Pelecypoda, — when just meta- 

 morphosed, glides over the mud by means of its ciha, we might be 

 inclined to conclude that the primitive habits of the original MoUusca 

 consisted in crawling, or gliding over the surface, in contradistinction 

 to the burrowing mode of life adopted by primitive Annehda. It is 

 highly probable that the most primitive living Cephalopod, Nautilus, 

 in which the constituent folds which 'make up the funnel are not 

 united, can flatten out this organ and crawl. 



But if we are entitled to conclude that the Trochophore larva 

 represents the common ancestor of Annelida and MoUusca, we must 

 regard the various Veliger larvae as representing an anticipation of 

 adult conditions ; a telescoping of development, in all respects similar 

 to that shown by the post-trochophoral stages of development in 

 Annelids. 



The Veliger of Gastropoda, with its spirally twisted shell, can 

 hardly represent an ancestral stage ; because, as we have seen, the 

 unequal growth of the mantle edge which causes the twisting is most 

 plausibly explained by the overbalancing of a tall visceral hump, such 

 as would surely occur in an animal crawling over uneven ground, 

 not in one which was free-swimming. The Veligers of Pelecypoda 

 and Scaphopoda exhibit in the free-swimming stage the distinguish- 

 ing adult characters of their respective groups. As has been mentioned 

 several times already, this reflection of adult characters into success- 

 ively early stages of life-history is a phenomenon which meets us 

 everywhere in embryology, and it is one of the most suggestive 

 features in the whole process of development. 



Turning now to the development of Cephalopoda, it is at first 

 sight difficult to find any points of contact whatever between their 

 development and that of other MoUusca. Thus, the gills are amongst 

 the earliest organs to be formed in Cephalopoda, whilst they are the 

 latest in Gastropoda and Pelecypoda. All trace of the Trochophore 

 stage has been eUminated from Cephalopod ontogeny, and there is 

 nothing corresponding to the vehger stage. Even the early history 

 of the shell must be greatly hastened through, for the shell-sac and 

 sheU gland are two different things. Lankester (1890) has pointed 

 out that, in the later stages of the development of the snail {Helix), 

 a large ventral protrusion of the foot filled with yolk is produced. 

 This he rightly compares to the external yolk-sac of the Cephalopod 

 embryo, for this certainly represents a median protrusion of the foot, 

 since the rest of the foot is formed all round it. We have here, how- 

 ever, a case of analogous development, not of real homology, for the 

 heavily yolked egg of the asymmetrical snail has not been derived 

 from the heavily yolked egg of the Cephalopod, or vice versa. 



■VOL. I 2 b 



