Hogg, on the Water- Snail. 
93 
the course of electro-magnetic spirals. Organic life exhibits the same 
laws in the circulation of the blood, in all cases starting from the left side 
of the animal body ; and in the heliacal windings of the shells of Mollusks, 
which follow a direction determinate for every species. But plants, above 
all, give evidence of a wonderful obedience to such laws, in the direction 
of the spiral vessels, the heliacally winding trunks of trees, winding stems 
and leaves, and probably also in the circulation of their saps." ^ 
Professor Quekett has directed attention to this subject, 
especially with regard to plants, in his ' Histological Lec- 
tures.' To proceed : — 
From the twenty-sixth to the twenty-eighth day the little 
animal was actively engaged in making its way out of the 
egg, in the advanced stage represented at fig. 5, leaving its 
shell behind it in the ova- sac, and immediately attaching itself 
to the side of the glass. The ciliary motion is then better 
seen ; each tentacle being surrounded at the extreme edge by 
a row of cilia kept in motion by bands of muscular fibre : the 
cilia are protruded from beneath the shell, and kept incessantly 
at work, in conjunction with those surrounding the opening to 
the mouth ; thus bringing a constant current of water for the 
aeration of the branchiae, situated above the oral aperture ; 
and at the same time a due supply of nourishment for the 
growth of the little animal. And it is a remarkable fact, that, 
as soon as the gastric teeth are properly matured to enable 
it to cut the vegetable substances growing in the water, the 
cilia being no longer required, then disappear, and drop off, 
from the tentacles. The tentacles and oral fringe of cilia are 
represented magnified, in the drawing, at fig. 6. But if, on 
the other hand, the young animal be kept in fresh water 
alone, without vegetable matter of any kind, it still retains its 
cilia, and attains only to a small size ; it then acquires gastric 
teeth, but of a very imperfect character, which never attains 
to perfection in form or in size. If at the same time it is 
confined to a small narrow cell, it will only grow to such a 
size as will enable it to move about freely ; thus adapting 
itself to the necessities of its existence. 
Dr. Grant, I believe, first pointed out the ciliary motion in 
the embryo of some salt-water species of Gasteropoda. In 
examining the embryos of Buccinum undatum and Purjmra 
lapillus, which are also enclosed in groups within transparent 
sacs, he was struck with an incessant motion of the fluid in 
the sac towards the fore-part of the embryo ; and he then 
noticed that this motion was produced by cilia placed around 
two funnel-shaped projections on the fore-part of the young 
* M. Wichura, " On the Winding of Leaves," translated by Arthur 
Henfrey, F.R.S., ' Scientific Memoirs,' 1853. 
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