METAZOA MOLLUSCA. 231 



face for air, but whether this is a necessity for maintain- 

 ing life is not yet settled. The model shows the long 

 respiratory tube containing bubbles of air which the snail 

 extends from the respiratory opening to the surface of the 

 water. The tentacles are flattened, as seen in No. 498. 

 When ponds dry up, these snails bury themselves in the 

 mud and in this situation can live for a considerable time. 



Planorbis corneus Linn. (No. 499), is a good example 

 of a shell coiled on a horizontal plane. The whorls are 

 few in number and the aperture is entire and flaring. 

 Polygyratia polygyrata Bonn. (No. 500), is a still better 

 example of a horizontal spiral consisting of eight closely 

 coiled whorls with the ninth unfinished. The aperture 

 like that of Planorbis is entire. 



Campeloma (No. 501, C. decisa Say) and Melania (No. 

 502, M. hastata Lea), are exceptional Gastropods, inas- 

 much as both are viviparous. Vivipara (= Paludina) 

 carries its young about with it for three or four weeks 

 after birth. 



The land Gastropods are represented by a number of 

 species. Helicostyla leai Pfr. (No. 503), is a smooth 

 beautiful white spiral of three or four whorls. The aper- 

 ture is entire, and there is no umbilicus. The eggs of 

 this genus are protected by a limy shell so that they re- 

 semble those of birds. 



Helix (No. 504) is one of the commonest land snails. 

 The youngest shells (see No. 504) are so thin and fragile 

 that they can be easily crushed between the fingers. The 

 margin is sharp and lipless. When older the shell is 

 stouter with a thick white lip (No. 504). The last speci- 

 men in the upper right hand corner is reduced, being 

 small in size but provided with the thickened lip. The 

 adult animal (No. 505, H. hortensis O. F. Mull. ; No. 506, 

 H. pomatia Linn., see topmost shelf), is represented with 

 foot extended, and with the shell containing the spiral 

 portion of the body in normal position when the snail is 

 crawling. The head has two pairs of tentacles, and the 



