164 



PRACTICAL ZOOLOGY 



called nudibranchs. The periwinkle (Fig. 93, D) is a very com- 

 mon shelled snail on the North Atlantic seashore. It was 

 introduced from Europe, where in many localities it is used as 

 an article of food by the natives. 



The oyster drill (Fig. 93, C) and several other marine snails 

 make a practice of boring through the thick shells of oysters and 

 other bivalves with their radulas and taking out the soft body 

 of the victims through the hole. 



The term nudibranch is applied to certain shell-less marine 

 gastropods. The nudibranchs (Fig. 93, B) resemble the ter- 



L ~_ . r^ , 



A B 



FIG. Q4. A, the squid. 



B, the octopus; A, at rest; B, in motion. 



(After Merculiano.) 



restrial slugs ; they do not breathe air, however, but take oxy- 

 gen from the water by means of naked gills, or through the 

 mantle. 



The shelled marine Gastropoda usually breathe by means of 

 gills. In Sycotypus (Fig. 93, A), for example, there is a trough- 

 like extension of the collar, the siphon, which leads a current of 

 water into the mantle cavity where the gill is situated. 



