736 



ZOOLOGY 



SECT. 



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. 658) : 



Plafypoda Heferofjoda 



Rhi^idoglossa 



.Docoglossa 



Tecribranchia 



Nudibranchia 



Scajahopoda 



FIG. 658. Diagram to illustrate the relationships of the Gastropoda. 



APPENDIX TO THE GASTROPODA. 



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 enclosed by the shell (Fig. 659), 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. 660, /) is narrow, trilobed at the 



extremity or provided with a ter- 

 minal disc, capable of being pro- 

 truded through the oral opening 

 of the shell, and used for burrow- 

 ing in sand. The mouth is 

 situated on a short oral pro- 

 boscis, and is sometimes sur- 

 rounded by lobed processes or 

 pinnate palpi. Further back are 

 a pair of tentaculiferous lobes, 



each bearing a large number of filiform tentacles, which are probably respira- 

 tory in function. The mouth leads into a buccal cavity containing an odonto- 

 phore. Connected with the mesenteron is a large bilobed digestive gland (L). 

 The anus is situated ventralty behind the base of the foot. The vascular 

 system is extremely simple, consisting of sinuses without definite walls, and 

 there is no distinct heart, though in the neighbourhood of the rectum there 



FIG. CM'.). Dentalium, longitudinal section of 

 shell. (After Keferstein.) 



