VARIATION IN SIZE. 
S2 
diniinntive scale. A specimen that does not exceed 20 mill, in length 
during the first season is necessarily undersized when adult. 
Temperature exercises a very powerful influence upon the size 
attained, as the young L'lnuKca only begins to assimilate food and 
therefore grow, when the 
waiter attains a temperature 
of about 0,-5° Fahr., and 
although a much lower tem- 
perature does not seem to 
be vitally injurious to the 
mollusk, its effects are e.x- 
hibited in the decreased 
power of assimilation and 
consequent checks to, or 
inhibition of growth. 
Semper has experimentally 
demonstrated the effect of cohl in retarding the growth of specimens 
in every other respect (juite as favourably circumstanced as those 
attaining a much larger size. 
The mean temperature of a dis- 
trict may be apparently favourable 
to a species, but this average may 
be tbe result of two extremes, and 
these extremes of temperature, 
if occurring during the growth 
periotl have a baneful influence 
and unfavourably affect the size attained by the animal. 
During tbe winter mouths, however, most of our land and fresh- 
water mollusks are more or less complete hibernants, but on the 
termination of this winter dormancy there quickly follow's in the 
young a^ period of rapid assimilation and growth, and unfavourable 
inlluences at tins season necessarily retard the growth and induce 
dwarfed shells, as no after favourable circumstances can fully com- 
pensate for retardation of growth in early life. 
'I'hus we are enabled to understand and .appreciate the periodic 
variations in size and the cbaracters in correlation with it, so often 
notice<l to occur, more especially amongst the fre.shwater mollusks, 
and to consiiler them with great probability to be the biological 
Four fii^ures of the shell of L. stagnalis 
show'iiis the retUiced sizeaitaiiied owin^ totlie 
temperature of the inhabited water becoming 
lowered to nearly oa Fahr., and illustrating 
the Volume ami Growth Curve, Fig. 183. 
Fig. 181, reared in 100 cubic centimetres ot 
water ; Fig. ISa, in 2”)0 ; Fig. 180, in 500 ; and 
Fig. 187, in 2,000 (compare Figs. 177 & 187). 
Fi(',. 183. — Volume-curve of Linuutui stai:^naris, 
totally altered by a sudden fall of temperature arresting 
the growth. The various vessels containing the un- 
eiiual bodies of water in which the Liinno<p lived, 
stood by a window where the sun .shone for about two 
hours in the afternoon, elevating the water in the 
smaller ves.sels to a favourable temperature, but being 
insulTicient to do so with the larger ones. Hence the 
Lintmva in the largest vessel containing 2,00(1 cubic 
centimetres of water, which should have been under 
ordiiiarily favourable circumstances very much larger, 
were actually smaller than those in many of the 
smaller vessels (after Semper). 
