324 GENEKAL CONSIDERATIONS ON THE MOLLUSCA. 



were derived later all the varied shapes of shell met with in the 

 different divisions of the Mollnsca. 



The protective dorsal covering which became the shell was, in any 

 case, of great significance for the further development of the Mollusca. 

 Since, starting from the back, the shell had to cover a large part of 

 the body so as to be able to shelter it as completely as possible, the 

 locomotory organ could only develop on the ventral surface, the 

 creeping manner of life leading to the development of the foot, an 

 equally important organ and one highly characteristic of all the 

 Mollusca. 



It has already been mentioned that attempts have been made to 

 derive the foot of the Mollusca from the ventral sucker of the 

 Polyclada, but it appears to us that the exclusive use of the ventral 

 surface as creeping sole, simultaneously with the development of the 

 dorsal shell which necessitated a firmer adhesion to the sub-stratum, 

 alone suffices to explain the greater development of the ventral part 

 of the body into a muscular foot. In a few Annelida and Annelidan 

 larvae, there is a ventral ciliated area extending between the mouth 

 and the anus which evidently assists the animal in creeping. Such 

 a differentiation may in any case be ascribed to the primitive form, 

 and this, together with a strengthening of the ventral musculature, 

 led to the formation of the foot when the primitive form became 

 adapted to a creeping manner of life, an adaptation which, again, 

 was connected with the development of a shell. 



In the Solenogastres, which are elongated and evidently very 

 lowly Molluscs, the foot is only slightly developed and appears 

 as a ciliated ridge lying in the ventral longitudinal groove. This 

 groove, which is represented in Fig. 147 A and B, might be compared 

 with the ciliated area mentioned above as occurring in the Annelida, 

 in some forms becoming depressed in such a way as to produce a 

 ventral ciliated groove. In Ch<ietoderma, there are no signs of either 

 a ventral groove or a foot, and we are much tempted to regard these 

 elongated worm-like creatures (Fig. 147-4) which are totally devoid of 

 shell, as worms rather than as Molluscs, a view which has repeatedly 

 been adopted. In any case, it appears possible that they are transi- 

 tion-forms between the Vermes and the Mollusca, this view being 

 supported by the fact that the formation of the spines in the 

 Amphineura shows great similarity to that of the setae in the 

 Annelida. 



Although there cannot be any doubt that we have, in the Am- 

 phineura, forms which stand very low among the Mollusca, we may 



