Hogg, on the Water- Snail. 
103 
the animal successively to renew its organs through a series 
of metamorphoseSj which give it permanent conditions not 
only different but even directly contrary to those which it had 
primitively. 
In this one fact are we not furnished with a well marked or 
broad line of demarcation between that of animal and vege- 
table life ? In the development of the animal, the cell-wall 
takes no part in the formative process ; it is but an enveloping 
membrane required for a time, and then thrown off. On the 
contrary, in vegetable life it enters largely into the formative 
process, and ultimate development of all its tissues ; it is ever 
to be found growing with its growth, cell- wall upon cell-wall 
intact, with or without its earliest contents. 
Note. — June Qth, 1854. 
My attempt to arrest the development of some young 
animals is still continued with perfect success. They have 
remained in the same narrow glass-cell, at the stage of growth 
before referred to, viz., about the size the animal usually 
attains during the first two or three weeks of its existence. 
They are now six months old, alive and well, the cilia are re- 
tained around the tentacles in constant activity ; whilst other 
animals of the same brood and age, placed in a situation 
favourable to growth, have attained their full size, and have 
now produced young, which are of the size of their elder 
relations. 
DESCEIPTIO^^ OP PLATE VII. 
Fig. 
1. — A magnified representation of the increase and change of situation - 
occurring to the yolk of egg of Limneus on the fourth day. 
2. — The change observed on the sixth day, showing the transverse fissure 
or divisional line in the mass. 
3. — The formation of the shell proceeding more rapidly, it appears on the 
sixteenth day as the larger portion of the embryonic mass. 
4. — The embryo performing its heliacal windings around the shell. 
5. — The embryo, or young animal, seen soon after it has issued from the 
shell. 
6. — The tentacles, with cilia, seen under a ^-inch object-glass ; the arrows 
indicating the course of the current produced by the cilia. 
7. —The natural size and form of the shell of a full-grown Limneus. 
8. — Parasitic animal found on the body of Limneus, magnified 100 
diameters. 
