60 COMPARATIVE ANATOMY CHAP. 



In Scissurella, Pleurotomaria, and Emarginula, there is a median indentation in 

 the anterior edge of the shell, which corresponds with an incision in the mantle 

 edge. This is the case in the young Fissurella, but, during further development, 

 the edge of the shell grows across the incision, so that in the adult animal the 

 aperture lies near the apex. Beneath it is the anus, placed high up in the mantle 

 cavity. If such a cleft were to arise at both the anterior and posterior edges, and 

 to become very deep, a double shell would result comparable with the bivalve shell 

 of the Lamellibranchia. It is in fact probable that this notching of the shell edge 

 is of great phylogenetic significance. 



In Haliotis we have a row of perforations of the shell, the process of formation 

 of the perforation in Fissurella being often repeated ; the older apertures are always, 

 however, closed by shell substance, and the younger only remain open as long as 

 they lie immediately over the respiratory cavity. 



In very many Prosobraiichia (the Siphoniata of earlier writers), there is, at the 

 columnar edge or lip of the shell, a notch which gives passage to a channel -like fold 

 of the mantle margin. This channel keeps up communication between the mantle 

 cavity and exterior, even when the shell is closed by the operculum. Instead of a 



A B 



FIG. 60. A, Dextrally twisted ; B, sinistrally twisted shell of Helix pomatia. 



notch, a more or less long process or beak may enclose a corresponding process of the 

 mantle, the siphon. The latter may become a tube by th'e apposition of its edges. 



It has already been mentioned that the shells of most Gastropods are 

 dextrally twisted. There are, however, a few families, genera, or species in which 

 the shell has a sinistral twist ; and in some species where the twist is dextral, a few 

 individuals with sinistral twist may occur, and vice versa. It is a curious fact that 

 some species, in which the shell has a sinistral twist, show the asymmetry of the 

 dextral twist in the soft body, whereas, in others, the asymmetry of the soft body 

 corresponds with the twist of the shell. We shall return to this point. 



For details as to the growth of the shell, and the capacity of the animal to 

 dissolve the shell already formed, both of which are points full of interest, we must 

 refer to special works on Conchology, as also for detailed descriptions of forms of the 

 shell and opercula, and differences due to age. 



Progressive reduction of the shell occurs in each of the three divisions of 

 the Gastropoda. In the Prosobranchia, this has only been observed in marine, 

 free-swimming Heteropods and in Titiscania ; in the Pulmonata, it is much more 

 common ; and in the Opisthobranchia, so frequent that nearly all the members of 

 this division have more or less rudimentary shells. Many adult Opisthobranchia 

 have even lost every trace of a shell (Pteropoda gymnosomata, Nudibranchia, and 

 most Ascoglossa), although, in their earliest stages at least, they possessed a coiled 



