254 ELEMENTARY ZOOLOGY 



to the stems or leaves of plants placed in the water. 

 They are so transparent as to be easily overlooked." 

 Young snails may be reared from these eggs. 



There are other snails common in ponds, also called, 

 like the pulmonate forms, pond-snails, which have gills 

 and no lung-sac. These pond-snails belong to a different 

 order of molluscs, and live on the bottom of the pond, 

 crawling about in the soft mud and feeding on animal 

 instead of vegetable food. 



The shells of the various kinds of snails vary much. 

 In many of the land-snails the spiral is not spire-shaped 

 or conical, but is flat. In some the whorls of the spiral 

 run from left to right (dextral) when the shell is looked 

 at with apex held toward one, while in others the whorls 

 run from right to left (sinistral). 



Of the hosts of marine Gastropods we can notice only 

 a few kinds. The nudibranchs (fig. 109) are a group of 



FlG. 109. Three Pacific Coast nudibranchs; Doris tuberculata (in lower 

 left-hand corner), Echinodoris sp. (upper one), and Triopha modes ta 

 (at right). (From living specimens in a tide-pool on the Bay of Mon- 

 terey, California.) 



beautiful forms in which the shell is wholly wanting and f 

 the mantle is usually absent. The gills are thus exposed 

 and are usually in the shape of delicate freely projecting 



