96 
Hogg, on the Water-Snail. 
attention to the very interesting observations of Professor 
Paget 
" Tlaat the reparative power in each perfect species, whether it he 
higher or lower in the scale, is in an inverse proportion to the amount of 
change through which it has passed in its development from the embryonic 
to the perfect state. And the deduction to be drawn is, that the powers 
for development from the embryo are identical with those exercised for 
the restoration from injuries : in other words, that the powers are the same 
by which perfection is first achieved, and by which, when lost, it is 
recovered. Indeed, it would almost seem as if the species that have least 
means of escape or defence from mutilation were those on which the most 
ample power of repair has been bestowed ; an admirable instance, if it be 
only true, of the beneficence that has provided for the welfare of even the 
least of the living world, with as much care as if they were the sole 
objects of the Divine regard." 
Dr. Carpenter differs in some particulars from Mr. Bower- 
bank, more especially with reference to the vascularity of 
the shelly which, I believe, he entirely denies, and somewhat 
inclines to the more generally received opinion of Reaumur ; 
who, after careful examinations of the shells of Gasteropoda, 
came to the following conclusions : — 
" That these calcareous defences are mere excretions from the surface 
of the body, absolutely extra-vital and extra-vascular, their growth being 
carried on by the addition of calcareous particles deposited in consecutive 
layers. The dermis, or vascular portion of the integument, is the secreting 
organ, which furnishes the earthy matter, pouring it out apparently from 
any part of the surface of the body, although the thicker portion, distin- 
guished by the appellation of the mantle, is more especially adapted to its 
production. The calcareous matter is never deposited in the areolse of the 
dermis itself, but exudes from the surface, suspended in the mucus which 
is copiously poured out from the muciparous pores, and gradually hardened 
by exposure ; this calciferous fluid forms a layer of shell, coating the inner 
surface of the pre-existent layers to increase the size of the original shell, 
or else in furnishing at particular points for the reparation of injuries 
which accident may have occasioned." * 
Now, if it be a mere excretion from the surface of a mem- 
brane, and neither vital nor vascular, how does Reaumur 
account for the deposit of the calcigerous cells, and subse- 
quent formation into shell, so early seen in the embryo ; and 
that long before these cells can become consolidated by 
exposure to air? Mr. Bowerbank has seen, as well as myself, 
that at a very early stage of embryonic life, calcareous matter 
is deposited, and hardened into shell ; and this can be readily 
proved, by simply breaking up the egg, and submitting a 
portion of the contents to the action of a drop of very dilute 
acetic acid, when the carbonate of lime will be very quickly 
* ' Article Gasteropoda.' By Professor Rymcr Jones. * Cyclopajdia of 
Anatomy and Physiology.' 
