98 
Hogg, on the Water-Snail. 
iridescent lustre. Immediately beneath this is placed the 
transparent basement membrane of an even texture and very 
light amber colour, this is the albuminous or animal mem- 
brane ; which with the layer before referred to, and above this, 
appears to me to be traversed by tubes, that no doubt run 
from the inner to the outer portion of the shell substance, and 
probably this net-work of pores have assigned to them similar 
duties to those in the human skin, viz., that of throwing off 
effete particles of matter, &c. 
The very small proportion of animal matter contained in 
this shell is a marked characteristic ; after the removal of the 
calcareous matter by dilute acid, we have the small residuum 
of a grain or two only ; from this cause the shell is very brittle 
at all times. The shell of the fully-formed animal is ovate, 
whorls five or six, elongated and dextral ; thus favouring, as 
before observed, the notion that the circular motion of the 
embryo when in the egg determines the whorl.* 
The mantle of the animal partakes of the same character 
and structure as that of mucous membrane generally, more 
especially that portion of it lining the internal surface of the 
shell ; thence it is reflected over the body, and forms a direct 
communication with the external shell and internal soft parts. 
Its other important use, besides that of depositing carbonate 
of lime, is the secretion of plasma, or a glazing fluid, which 
it spreads over the internal portions of the shell, and with 
which it lubricates the whole of the external parts, thus pre- 
venting any irritation that might arise from a drying up of 
the coarser particles of calcareous matter. Another use I 
have particularly noticed, is that of converting a large part of 
it, beneath the greater whorl of the shell, into an air-bag, or 
receptacle for holding a bladder of air, which must have consi- 
derable influence in rendering the shell buoyant and light, as 
by suddenly discharging it, the animal instantly sinks to the 
bottom. The animal is often seen to rise to the surface of 
the water for the purpose of taking in a supply of fresh air, 
which it does by opening a small valvular aperture, situated 
about the eighth of an inch above the ventral outlet. If the 
animal be removed from the water it immediately squeezes 
out this supply of air, at the same time it presses out the 
water from the body, for the purpose of enabling it to recede 
* For further information and much interesting matter upon this sub- 
ject I must refer to Mr. Bowerbank's researches upon the * Structure of 
Molluscous and Conchiferous Animals,' most accurately and carefully 
illustrated, published in the Transactions of this Society, 1843. Also Dr. 
Carpenter's researches, published in the Reports of the British Association, 
1844 and 1847 ; and his ' Principles of General and Comparative Phy- 
siology.' 
