KESPIRATION, AND OTHEE FUNCTIONS. 7 



of the terrestrial division ; the aquatic species, with 

 some exceptions, being provided with gills. In the 

 air-breathing molluscs, the air is admitted into the 

 pulmonary cavity, not by the mouth but by an aper- 

 ture, which may easily be seen in slugs at the edge 

 of the convexity on the back formed by the mantle, 

 and in snails just within the mouth of the shell ; in 

 both cases on the right' side. 



In regard to locomotion, most univalves crawl 

 upon a large fleshy protuberance which is the homo- 

 logue of a foot and supports the body, and many, 

 even amongst the bivalves, by means of this large 

 foot, are enabled to traverse considerable distances. 

 Not a few of the aquatic univalves are able to swim, 

 or rather creep, upside down upon the under surface 

 of the water. 



The mode of reproduction amongst mollusca 

 varies. Many univalves have distinct sexes, while 

 most of the land snails are hermaphrodite. Some, 

 like the Valvatidce, change their sex after a time, 

 being at first male, and then female. The majority 

 are oviparous, but some are ovoviviparous, and their 

 wonderful fertility may be estimated from a state- 

 ment of Pfeiffer to the effect that the gills of an 



