230 THE SEA SHORE 



place it on a sheet of glass and observe its movements from the 

 other side. We then sec that the foot is in complete contact with 

 the glass, and that a steady but rapid tmdulatory movement is 

 produced by the successive expansions and contractions of the disc, 

 brought about, of course, by the action of muscular fibres. 



A few of the univalves are viviparous that is, they produce 

 their young alive ; but the majority lay eggs. The eggs are often 

 enclosed in horny cases, some of which may be commonly seen 

 washed up on the beach, or attached to rocks and weeds between 

 the tide-marks. The larvae are always enclosed in a shell, though 

 they are sometimes wholly or partially concealed by the mantle. 

 The shell is usually closed by an operculum ; but as the animal 

 advances in age the shell sometimes disappears altogether, or is 

 reduced to a mere shelly plate, as is the 

 case with the land and marine slugs and 

 sea lemons. The young of the water- 

 breathers always swim about freely by 

 means of a pair of ciliated lobes or fins, 

 but these remain only for a brief period, 

 after which the animal settles to the bottom 

 for a more or less sedentary existence. 



The Cephalophora fall naturally into 

 two fairly well-defined groups, which we 

 may describe as the air-breathers and the 



FIG. 159. EGG CASES water-breathers. The former breathe air 



OF THE WHELK direct from the atmosphere through an 

 aperture on the right side of the body, the 



air passing into a pulmonary organ or lung, in the walls of which 

 the bloodvessels ramify, and they include all the land snails and 

 slugs. The latter breathe by gills which are more or less prominent 

 on the sides of the body, and include all the fresh-water snails, as 

 well as the marine species which fall within our special province. 



"We shall first consider the class Pteropo&a, or Wing-footed 

 Molluscs, so called from the wing-like appendages that are attached 

 to the side of the mouth, or to the upper side of the foot, which is 

 either very small or altogether wanting. 



These Pteropods are in many respects lowly organised as com- 

 pared with the higher molluscs ; and as they spend the whole of 

 their existence in the open sea, they can hardly be considered as 

 falling within the scope of the sea-side naturalist's work. Yet 

 since their shells are occasionally drifted on to the shore, and 



