n6 THE LIFE OF THE MOLLUSC A 



an inverted position on the under side of the surface 

 film of the water. 



The float of the ocean-roving Ianthina (Plate XXX., 

 Fig. 2) is another product of the pedal glands in 

 which air bubbles are entangled with the mucus. 



All Gastropods, however, do not glide onwards 

 with the steady movement of the Snail. Melampus 

 and Pedipes have the front portion of the foot (" pro- 

 podium ") marked off by a groove across the ventral 

 face, and the animals, when crawling, first advance 

 this propodium and then drag the rest after it. In 

 the Periwinkle (Littorina) and some of the Pomatiidse 

 ( = Cyclostomatidae) the foot is cleft longitudinally, 

 and the animal walks by lifting and advancing the 

 alternate halves, like a man with his ankles tied 

 together (Plate II., Fig. 5). 



The propodium is strongly developed and reflected 

 over the head in Natica (Plate II., Fig. 2), Sigaretus, 

 Oliva, Harpa, and others that force their way through 

 sand on the sea-bottom, and thus apparently acts 

 after the manner of a snow-plough. The expansion 

 of the foot in the Naticidse is assisted by the 

 presence of aquiferous spaces, which are completely 

 separated from the circulatory apparatus. 



In certain of the Rhachiglossa and some of the 

 Opisthobranchs, on the other hand, the margins on 

 either side of the lower surface of the foot (pleuro- 

 podia) are greatly extended (Plate II., Figs. 4, 6 

 and 8), and the animal is enabled to crawl on the 



