XII 



PHYLUM MOLLUSCA 



705 



glands of the foot ; certain exceptional forms such as Vermetus 

 are fixed in the adult condition by the substance of the shell. 

 A few families the Heteropoda and the Pteropoda are specially 

 modified for a pelagic mode of existence, and swim through the 

 water by flapping movements of the lobes of the foot, which act 

 as fins. Gastropods are found at considerable depths up to 

 nearly 3,000 fathoms in the ocean. Many forms, however, are 

 inhabitants of fresh water, while many Pulmonata are terrestrial, 

 and occur even towards the summits of the highest mountains. 



Fossil Gastropoda are known from almost the earliest fossil- 

 bearing rocks, and all the major divisions of the class are repre- 

 sented in formations of paleozoic age. 



The mutual relationships of the various groups of Gastropoda 

 are shown in the following diagram (Fig. 611): 



Heferopoda 



Pulmona^ 



Tech'branchia 



Nudibranchia 



Rhi^idoglossa 



Docoglossa 



Scajahofooda 



FIG. 611. Diagram to illustrate the relationships of the Gastropoda. 



APPENDIX TO THE GASTROPODA. 



A. CLASS IV. SCAPHOPODA. 



The Scaphopoda or Elephant's tusk shells are aberrant marine Molluscs 

 comprising only three genera Dentalium, Siphonodentalium, and Pulsellum. The 

 body is elongated, so as to be 

 almost worm-like, with complete 

 bilateral symmetry. The mantle- 

 folds are almost completely united 

 to form a cylindrical tube en- 

 closed by the shell (Fig. 612), 

 which is in the form of a delicate, 

 curved tube, open at both ends 

 and wider at one end the ante- 

 rior or oral than at the other. 



The foot (Fig. 613, /) is narrow, trilobed at the extremity, capable of being pro- 

 truded through the oral opening of the shell, and used for burrowing in sand. 

 The mouth is situated on a short oral proboscis, and is sometimes surrounded by 

 VOL. I Z Z 



FIG. 612. Dentalium, longitudinal section of 

 shell. (After Keferstein.) 





