322 Variation in Shells. [ ZOE 
will perhaps serve some good purpose by directing closer attention 
to the relations existing between the shell and the animal, if it does 
not settle the question in the minds of all. 
The matter that enters into those parts of the body of an animal, 
which we call secretions, such as bones, teeth, nails and hair, is 
never under the control of the will of the animal, and these organs 
are closely connected with the venous system of the body; conse- 
quently they possess an inherent power of repair, and when fractured 
or broken they grow or knit together by force of this inherent 
power. No mechanical action whatever takes place by the other 
organs in manipulating, forming or moulding their shape— their 
form and size being determined by inherent laws, or by a principle 
that might be compared to that possessed by seeds of plants which 
determines their forms. 
On the other hand, the secreted matter which enters into the 
formation of shells is thrown out on the surface of the body and 
mantle, and then becomes an excreted secretion, bearing the same 
relation to the animal that all excretions or even foreign substances 
do, for it has become separated from and lost that inherent power 
that always remains with bones and other secretions. 
Right here, then, I draw the line between a secretion and a con- 
struction. This excreted matter is now subject only to the control 
of the will of the animal, and it is accordingly used in suspending 
and renewing construction of the shell at its convenience, and in 
repairing fractures. 
The shell possesses no inherent principle which determines its 
form or growth, but on the contrary its growth is effected by irreg- 
ular deposits, and its form is determined by the mechanical manip- 
ulations of this deposited matter by the mantle of the animal. 
“ The shell, as before stated, is formed by the mantle.” Wood- 
ward’s Manual of the Mollusca, page 34. 
‘“‘The edges of the mantle are often developed into fringes at 
certain periods, and form the spines and other irregularities seen on 
the surface of the shell.” Genera of Recent Mollusca, H. & A. 
Adams, page 14. ‘ 
“ The shell being etttaveaculat has no inherent power of repair.” 
Carpenter, Woodward, Tryon. 
The above quotations fully indicate the mechanical action of the 
mantle of the animal in forming and moulding the shell. If the 
