116 THE LIFE OF THE MOLLUSCA 
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 Pomatiide 
(=Cyclostomatidz) 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 Naticide 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 
