GASTEROPODA. 



547 



with the tentacles, at e. Upon closely observing it, a 

 fringe of cilia is noticed in motion near the edge of the 

 shell. It is now apparent that the rotatory motion first 

 observed must have been in a great measure due to this ; 

 and the current kept up in the fluid contents of the cell 

 by the ciliary fringes. For days after the young animal 

 has escaped from the egg, this ciliary motion is carried 

 on, not alone by the fringe surrounding the mouth, but 

 by cilia entirely surrounding the tentacles themselves, 

 which whips up the supply of nourish- 

 ment, and at the same time the proper 

 aeration of the blood is effected. Whilst 

 in the ova, it probably is by this motion 

 that the cell-contents are converted into 

 tissues and shell. From the twenty-sixth 

 to the twenty-eighth day, it appears 

 actively engaged near the side ojf the 

 egg, using all its force to break through 

 the cell- wall, which at length it succeeds 

 in doing; leaving the shell in the ova- 

 sac, and immediately attaching itself to 

 the side of the glass-vase, to recommence 

 its ciliary play, and appears in the ad- 

 vanced stage represented at /. It is still 

 some months before it grows to the 

 perfect form represented at fig. 253, 

 Vr here the animal is drawn with its sucker-like foot adhering 

 closely to the side of the glass-vase. One of these snails 

 may deposit from two to three of these ova-sacs a week ; 

 producing, in the course of six weeks or two months, from 

 900 to 1,000 young, thus supplying food for fish. 



The shell itself is deposited in minute cells, which take 

 up a circular position around the axis; on its under-surface 

 a hyaline membrane is secreted. The integument expands, 

 and at various points an internal colouring-matter or pig- 

 ment is deposited. The increase of the membrane goes 

 on until the expanded foot is formed, the outer edge of 

 which is rounded off and turned over by condensed tissue 

 in the form of a twisted wire ; this encloses a net-work of 

 small vessels filled with a fluid in constant and rapid 

 motion. The course of the blood or fluid, as it passes 



N N 2 



Fig. y&.^ 



stagnalis. 



