THE PHYLUM MOLLUSCA 



ribbon is protracted, drawn upward against the food mass, and retracted. 

 The particles of food rasped away by the teeth are then ingested. Some 

 carnivorous snails use the radula to drill holes in the shells of bivalves on 

 which they feed. 



The marine gastropods also include an interesting group known as Nudi- 

 branchia, which, as indicated by the name, have exposed gills. In these 

 snails the adult is without a shell (Fig. 13.13). The gills of nudibranchs are 

 not homologous with those of other gastropods but are secondarily developed 

 projections from the dorsal surface of the body. The young nudibranch has a 

 spirally coiled shell which is lost during growth to maturity, as the dorsal 

 part of the visceral mass assumes a bilateral symmetry. Internally, however, 

 the anatomy of the adult animal retains a coiled condition. Nudibranchs 

 are often large, up to 6 inches in length, and some exhibit spectacularly 

 brilliant coloration. Two other small divisions of the marine gastropods, 

 the Pteropoda and Heteropoda, contain many species that are without 

 external shells. These animals are pelagic, swimming actively in the open 

 sea by means of a highly modified foot. 



In all the marine gastropods the primary organs of gaseous exchange 

 are the gills, with the mantle often serving as an accessory in this function. 

 Although eminently suitable in an aquatic environment, gills are impractical 

 on land. The terrestrial gastropods, doubtless descended from aquatic 

 ancestors, have lost the gills of their forebears and have replaced them with 

 lungs. Air is drawn into the mantle cavity, and the mantle itself, which 

 is richly vascularized in these forms, .serves as the site of gas exchange. 

 Many fresh-water snails "breathe" in this fashion and are presumably 

 descended from terrestrial forms, just as aquatic mammals such as the seal 

 are presumed to have evolved from terrestrial ancestors. The land slugs, 

 of which Anolimax is a representative genus, are a modified type of terrestrial 

 snail in which the shell persists as a vestige embedded in the dorsal wall of 

 the body (Fig. 13.14). Terrestrial and fresh-water air-breathing snails, 



Fig. 13.13. A nudibranch gastropod, Eolis coronata. The numerous dorsal projections are the 

 ccrata, or gills. (Redrawn from J. Alder and A. Hancock, 1846, Monograph of British .Nudi- 

 branch Mollusca, Part 2.) 



385 



