GENERAL CONSIDERATIONS ON THE MOLLUSCA. 329 1 



is wanting, it has evidently degenerated. To the actual shell, the 

 operculum has been added. The position of this latter is like that of 

 the shell itself, i.e., it lies on the dorsal side of the foot at the point 

 where the latter passes over into the back. The shifting of the anus 

 makes it difficult to establish the origin of the operculum. It has 

 been suggested that it arose through abstriction from the shell, but 

 its independent development and position in the embryo point rather 

 to an independent origin. 



The simple bowl-shaped shell, assumed by us for the primitive 

 form is no longer retained in this form in the Gastropoda, for the 

 shell has become twisted. This condition is connected with the 

 asymmetry of the body which, again, is the result of the one-sided 

 development of the visceral sac, a feature which is specially character- 

 istic of the Gastropoda. This one-sided development brings about 

 displacements of both external and internal organs, and leads to* 

 processes of degeneration (e.g., in the gills, the kidneys, parts of the 

 circulatory and nervous systems) ; these now occur on one side of the 

 body only and thus still further increase its asymmetry.* In forms- 

 which lead a pelagic life, such as the Pteropoda, or in creeping forms 

 that have lost the shell (Onchidium, Opisthobrauchia, Limacidae, etc.) 

 there is a more or less complete return to the symmetrical shape. 



In the form of the gills, the paired character of the kidneys and 

 the auricles, the relations of the coelom and of the nephridia, the 

 Diotocardia are among the forms most nearly resembling the primitive 

 Mollusc, but they appear essentially differentiated from it, as the 

 asymmetry of the body is already found in them. 



The relation of the Gastropoda to the primitive form is easier to* 

 trace than is that of the other great branches of the Molluscan stock 

 (the Solenoconchae, the Lamellibranchia and the Cephalopoda). 



The Solenoconchae may be derived from the primitive form by the 

 extension of the body in a dorsal direction ; the head is much reduced 

 but develops a large number of tentacular filaments. The foot 

 becomes the long burrowing foot, the mantle is found to be influenced 

 by the above-mentioned growth of the body, but on the whole 

 exhibits the usual features, except that it retains an aperture at its 

 dorsal apex. In the same way also we can explain the shape of the 

 shell which resembles a tube open at both ends. According to the 

 most recent investigations as to the structure of Dentalium it seema 

 most probable that this form is related to the Gastropoda, the 



* Cf. on the asymmetry of the body, Chapter XXXII., p. 143. 



