Hogg, on the Water-Snail. 
95 
Mj own observations upon the Limneus, in many important 
particulars, coincide with those of Mr. Bowerbank, made in 
1843, and published in the Transactions of this Society, upon 
the Structure of Shells of Mollusca, &c. Mr. Bowerbank thus 
explains the development of the shells of these animals : — 
*' Let us suppose the rudiment of the future shell to have been the 
result of the excretion of some mucus or lymph (properly, albumen) ; it 
would then be nothing more than a very thin transparent membrane, with 
a determinate figure dependent upon the figure of its species. In this 
membrane organizing cytoblasts and cells are produced and multiplied in 
rapid succession, until, by their increase and opposition, a cellular struc- 
ture is formed in it. On their first appearance the cells are transparent 
and globular, but pushed on by the law of growth, which regulates their 
development, they very soon begin to secrete, from their inner surfaces, 
carbonate of lime. The cells being filled with it, a solid structure is the 
result of their close aggregation ; the pattern being modified only by the 
form and degree of condensation of the calcigerous cells, in which it has 
been secreted. * * * * :jf. 
A layer or stratum of shell being thus formed, another is produced from 
its inner surface by the same means, and then others, until the normal 
set is completed : the whole being kept together as one by the living 
tissues." 
Mr. Bowerbank believes that the truth of this mode of 
formation is proved, not only by the structures he has dis- 
covered, but also by the phenomena which occur in its repa- 
ration of injuries ; for he says : — 
" This reparation is not made by a coat of calcareous matter, spread 
over the wound by the collar or mantle of the animal, as has been main- 
tained, but by an effusion of coagulable lymph, in which cytoblasts are 
produced in the first instance, and quickly succeeded by a cellular struc- 
ture, in which the earthy basis of the shell is secreted, and by which the 
scar is filled up, or the fracture cemented together." 
This I have repeatedly verified, and always found that after 
an injury to the shell of either an embryonic, or more perfectly- 
formed animal, in a few hours subsequently the process of 
repair has been commenced by a deposition of cells, less in 
size, and somewhat more irregular in form than the first. 
Upon breaking off an eighth of an inch from the edge of the 
shell of a full-grown animal, I observed that it first threw out 
a series of exudations of plasma, or albuminous matter; which, 
after some days, became hardened by a calcareous deposit, 
corresponding in appearance to the lines of growth of the old 
shell, but only to the extent required to convert the edge into 
a smooth and strong margin of about one-half the breadth 
broken off; and, ultimately, new lines of growth were 
thrown out beyond the edge of the mantle ; this I clearly 
ascertained by scraping it with a fine knife. In reference to 
this part of my inquiry I may be pardoned for directing 
