220 



METAZOAN PHYLA 



Some snails are terrestrial, in which case they breathe by taking air 

 into the mantle cavity, the surface of which is kept moist by a mucous 

 secretion. Other snails are pulmonate and inhabit fresh water, in which 

 case it is necessary for them to come to the surface of the water at inter- 

 vals to fill the mantle cavity with air. In the case of others, which live 

 at all times submerged, water is taken into the mantle cavity, where 

 respiration may take place through the walls of the cavity or by means of 

 gills. The majority of snails are marine. Many of them reach a con- 

 siderable size and develop shells possessing great beauty of form and 

 color. A snail shell may be made up of many coils in one plane, or it may 



Bucccff , 



opf/'c nerye 



Cere'brcr/ 



Fig. 125. — Somewhat diagrammatic representation of the central portion of the nervous 

 system of Hdix pomatia. {From Lang, " Text-hook of Comparative Anatomy.") 



be extended into a spire. On the other hand, the mouth of the shell may 

 be greatly expanded and the coiling be reduced to one turn or even a part 

 of a turn, as in the abalone. There may be no coils at all, as in the case of 

 the limpets, which have symmetrical conical shells. Still another type of 

 snail is one fitted for pelagic life. This has the foot broadly expanded and 

 of the shape of a pair of wings, giving to the animals the names of butter- 

 fly snails or sea angels. Some snails secrete an operculum which serves as 

 a lid to the opening of the shell, being so placed on the body that when the 

 animal is in the shell the operculum just fills the aperture. 



Another type of snail includes the naked snails or slugs, which may be 

 without any shell at all or may have only a small chitinous disc occupying 

 the same location on the body as the shell and embedded in the mantle. 

 These are in part terrestrial forms and are often seen under objects lying 

 on the ground in fields or gardens. There are also naked marine snails, 



