78 MOLLUSCA, CRUSTACEA AND REITILIA FOR 



as it would seem, of the motion of numerous cilia, or 

 minute hairs attached to the under surface of the body, 

 making what is called the foot. This mode of progression 

 can be seen better in an Aquarium than in a pond or 

 stream, for they seem to be more addicted to it in con- 

 finement than in their natural habitat — whether because of 

 the absence of breezes ruffling the surface or not, I can- 

 not say. This animal lays its eggs in clusters of from 

 fifteen to sixty, contained unsymmetrically in a transparent 

 sac, whilst the L. columella attaches its eggs in groups of 

 about a dozen, arranged in rows around the stalk of some 

 water-plant, and also contained in a transparent investing 

 membrane. If these eggs are examined under a magnifying 

 glass, we shall see that they have a small yellow spot in 

 some part of them : this is the yolk ; after a time we 

 shall see that this spot increases in size, and assumes more 

 and more the form of the parent animal. At the same 

 time, the young snail, having a slight shell investing it, 

 begins to rotate about in its egg-shell, until at last, when 

 it is perfectly formed, it breaks through one end of the 

 shell and makes its escape a perfect animal, and at once 

 takes upon itself the habits of its parents. I have had 

 hundreds of these juveniles floating, shell downwards, upon 

 the surface of tho water of a tank that I kept on pur- 

 pose for rearing them in (for if we attempt to do so in 

 an Aquarium,, the eggs, *as well as the young snails, will 

 be rapidly eaten by the fish ; and iu this way, if we keep 

 enough snails, we can feed our fish with food that they 

 relish). The movement of the ovum in the egg is always, 

 in the case of the right-handed shells, from right to left, 



