756 



ZOOLOGY 



a reversed position, adhering to frothy mucus secreted by the 

 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 in the ocean at considerable 

 depths — up to nearly 3,000 fathoms. 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 Palaeozoic age. 



The mutual relationships of the various groups of Gastropoda 

 are shown in the following diagram (Fig. 648): — 



Platypoda Heteroboda 



Rhibidoglossa 



Docoglossa 



Pulmonata 



Tecfibranchia 



Nudibranchia 



Scabho^oda 



Fig. (J48. — Diagram to illustrate the relationships of the Gastropoda. 



APPENDIX TO THE GASTROPODA. 

 A. CLASS IV. -SCAPHODA. 



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. 649), 

 which is in the form of a delicate, 

 curved tube, open at both ends 

 and wider at the anterior or 



oral end than at the other. The foot (Fig. 650,/) is narrow, trilobed at the 

 extremity or provided with a terminal disc, capable of being protruded through 

 the oral opening of the. shell, and used for burrowing in sand. The mouth is 



Fio. 649. — Dentalium, longitudinal section of 

 shell. (After Keferstein.) 



