94 
Hogg, on the Water-Snail. 
animal, which form the borders of a cavity, in which he per- 
ceived a constant revolution of floating particles. He also 
observed these circles of cilia in the young of the species of 
Trochus^ Nerita, &c., in which the embryo was seen revolving 
round its axis. He met with the same appearance in the naked 
Gasteropoda, as the Doris, Eolis, &c. The embryo of these 
revolve round its centre, and swims rapidly forward by means 
of its cilia, when it escapes from the ovum. Dr. Grant 
assigns various uses to these motions, but does not connect 
them with respiration or nourishment, although there can be 
little doubt that they are so. 
In some six weeks, or two months, the flattened form of 
the shell becomes gradually changed into that of the conical 
form of the full-grown animal Cfig. 7).* 
That this little creature is hermaphrodite, like the common 
snail, is proved by my having only this solitary animal in my 
vase ; and yet nearly all the eggs deposited by it arrived at 
maturity. Like the common snail it is also copulative, as I 
have seen two animals mutually pass a thin tongue-like organ 
into a fissure between the body and upper surface of the pos- 
terior portion of the foot. 
I observed in the few eggs that did not come to maturity 
that the yolk onh' slightly increased in size, and then remained 
in that state until all the others were hatched, when the ova- 
sac became the prey of other animals. 
This one snail deposited two and three of these ova-sacs 
in the course of the week ; and in two months I calculated 
that upwards of 800 young would result therefrom ; thus 
it will be seen, that the number of eggs deposited by each 
individual is very great ; fully explaining the rapidity with 
which this class of animals increases, either on land or in 
the water. 
The shell, as we have before seen, is begun at a very early 
stage in the formative process. It is first observed to have 
the shape of minute ovoid cells, which are deposited side by 
side around the axis, or central cell ; and this may be 
described as a cytoblast, enclosing a certain quantity of colour- 
ing matter, just sufficient to give it a distinctive appearance, 
from the previously-formed basement membrane. The sides 
of one cell being in close contact with those of other cells, a 
gradual compression, or elongation, takes place, and we have, 
finally, resulting divisional ribs, hardened by the deposition 
of calcareous matter into a shelly covering. Subsequently all 
trace of the earliest cells and cytoblasts are lost. 
* In warm weather the eggs arrive at maturity in a much shorter time, 
especially when exposed to the light and warmth of the sun. 
