GASTEROPODA. 263 



is usually found attached to the surface of stones, pieces of weed, or 

 other matters, under the water ; and generally connected together in 

 long ribbons or delicate ova-sacs of a curious and beautiful form. The 

 mass of eggs deposited by the Doris resembles a frill of lace of great 

 beauty. In the Aplysia the spawn resembles long strings of vermicelli, 

 of varying tints throughout the different parts of the thread. In the 

 LimncKus stagnalis it is deposited in small sacs, containing from fifty to 

 sixty ova; one of which is represented at a, fig. 115. If examined 

 soon after they are deposited, the vesicles appear to be filled with a 

 perfectly clear fluid ; at the end of twenty-four hours a very minute 

 yellow spot may be detected adhering to one side of the cell-wall. In 

 about forty-eight hours afterwards, this small spot is seen to have a 

 smaller central spot of a rather deeper colour ; this is the nucleolus. 

 On the fourth day the spot or nucleus may be observed to have changed 

 it position, and enlarged to double the size : a magnified view is shown 

 at b ; upon viewing it closely, a transverse fissure or depression may 

 be seen, which on the eighth day most distinctly divides the small mass 

 into shell and soft part of the future animal, as at c. It then becomes 

 detached from the side of the cell, and moves with a rotatory motion 

 around the whole of the cell-interior ; the direction of this motion is 

 from the right to the left, and is always increased when sunlight, and 

 consequently heat, is thrown upon it. The increase is gradual up to 

 the sixteenth day, when the greater half appears to be the shell por- 

 tion, as seen magnified at d. The spiral axis can now be traced and 

 seen by oblique light from its darker colour ; offering a striking differ- 

 ence in appearance to the soft parts. On the eighteenth day, these 

 changes are more distinctly visible, and the ova crowd down to the 

 mouth of the ova-sac; when, by using a higher magnifier, a minute 

 black speck, the future eye, may be seen protruded with the tentacles, 

 as at e. Upon closely observing it, a fringe of cilia will be seen in 

 motion near the edge of the shell : it now became apparent that the 

 rotatory motion first observed must have been in a great measure due 

 to this ; a current, no doubt, being 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 brings the whole supply of nourishment required ; 

 at the same time, and by the same means, the proper aeration of the 

 blood is effected. Whilst in the ova, it is, no doubt, by this motion 

 that the cell-contents are converted into the various tissues of the body 

 and shell. From the twenty-sixth to the twenty- eighth day, it appears 



