192 INVERTEBRATE ZOOLOGY 



Class Scaphopoda 



An external tubular shell, open at both ends and more or less 

 curved, covers the body of the scaphopods. The shape of the 

 shell is responsible for the common name tooth shell so fre- 

 quently apphed to members of this class. Jaws and radula are 

 present but in the paired liver and general arrangement of the 

 nervous system the Scaphopoda resemble the Acephala. Gills 

 are wanting and the rudimentary heart possesses only a ventricle. 

 The foot, which is rather long and conical, extends from the larger 

 opening of the shell and bears two lateral lobes. Dentalium is 

 a modern genus of this class all members of which are marine. 



Class Gastropoda 



The snails, limpets, slugs, and sea hares are examples of the 

 Gastropoda. Representatives of this class usually have head, 

 visceral mass, foot, and mantle, though in some instances one or 

 more of these may be wanting. While most gastropods live in 

 the oceans, many have become adapted to life in fresh water and 

 some to terrestrial existence. A shell, when present, is composed 

 of a single piece. In some snails, either a limy or a horny disc 

 called the operculum serves for closing the opening when the 

 animal is withdrawn into the shell but is not considered as 

 comparable to a second valve. The foot is usually flattened so 

 that it presents a large ventral surface upon which the animal 

 crawls, but in many of the pteropods the foot is modified as a fin 

 used for swimming. The head, which is located anterior to the 

 foot, bears hollow tentacles, eyes either at the base or tips of the 

 tentacles, and a mouth. The mantle covers the dorsal surface 

 of the body and bounds a spacious mantle cavity which contains 

 the gills in the water-breathing forms. A siphon, through which 

 water enters and leaves the branchial chamber, is frequently 

 formed by an outgrowth from the edge of the mantle. The shape 

 of the shell is often greatly influenced by the presence of the 

 siphon. In the pulmonate snails, the mantle cavity is largely 

 replaced by a highly vascular sac or lung into which air is admit- 

 ted through a single opening, the spiracle. 



Gills occur in all marine snails and in some fresh-water forms. 

 Lungs seem to have been developed in connection with the change 

 to terrestrial habits, for land snails are the most characteristic 

 pulmonates. Aquatic pulmonates such as the common genera 



