THE POND SNAIL. 157 



tions meet, one may sometimes be seen using its shell as a 

 means of offense, clearing its track by striking about with 

 its spire. If the mouth of one that is crawling on the 

 side of glass be watched with a lens, its ribbon-shaped 

 radula, covered with rows of minute, recurved teeth, may 

 sometimes be seen slightly protruded. By means of 

 muscles attached at both ends, the radula is drawn back- 

 ward and forward like a rasp across the objects on which 

 the snail feeds. Cabbage leaves on which the snail has 

 been fed will show areas which have been " scraped " by the 

 radula. All these things and many more may be seen by 

 carefully watching a few live snails in a tumbler of water. 



Development. The eggs of the pond snail are very 

 easily procured, and they offer exceptionally fine advan- 

 tages for the study of those stages of development which 

 immediately succeed fertilization. They are laid in gelat- 

 inous, transparent capsules, half an inch to- an inch in 

 length, flattened, and linear or oblong in outline. After 

 a few snails have been kept a short time in a small vessel 

 of water with their appropriate food, these egg capsules 

 may be looked for on the bottom and sides of the vessel, 

 or closely adherent to the stems or leaves of plants placed 

 in the water. They are so transparent as to be easily 

 overlooked. 



Detach one of these egg capsules by pushing a thin 

 blade between it and its support. Place it in a watch glass 

 with a little water, for examination. 



Examine it with a lens, and draw it. How many eggs 

 does it contain? How are these eggs arranged in the 

 capsule? What is the shape of each egg? 



Note that each egg contains a minute oosperm (or fer- 

 tilized ovum), which is less transparent than other parts. 

 It is the development of this oosperm that especially 

 invites our study. 



