VARIATION IN SIZE. 
81 
shown between those sliells inhabiting 100 and 500 cubic centi- 
metres respectively ; for, whereas 
specimens reared in 100 cubic centi- 
metres of water only acquired a length 
of 6 mill, in 65 days, those in 250 
centimetres reached 9 mill., those in 
500 centimetres 12 mill., while the 
individuals in 2000 cubic centimetres 
grew to 18 mill, in the same space 
of time. Ample food and healthy con- 
ditions had been assured to each 
mollnsk, and the experiment therefore 
exhibits the dwarfing effect of too great an abundance of life in any 
circumscribed area apart from scarcity of food. The rate of growth 
Figs. 174, 175, 176, 177. 
Four shells of Liinncea stagnalis^ each 
65 days old and hatched from the same 
mass of ova, but reared in different 
volumes of water to demonstrate the effect 
of the amount of water per individual 
upon the size attained in a definite period 
of time, and illustrating the Volume-curve 
Fig. 173. 
Fig. 174. reared in 100 cubic centimetres 
of water ; Fig. 175, in 250 ; Fig. 176, in 500 ; 
and Fig. 177, in 2,000 (after Semper). 
20 I 
o 
15 ^ 
10 5 ' 
5 § 
rp 
a 
0 
Age of mollusks in days. 
Fig. 178. — Curve of time, showing the average rate of growth of Limncea stagnalis reared 
in from 1,000 to 2,000 cubic centimetres of water (after Semper, modified). 
is also far from uniform, as at first the young Lhnnwa grows at a 
moderate rate, after which follows a period of quickened growth, until 
Figs. 170, 
Four figures of the shell of Limmea stagnalis to 
show the amount and rate of increase in size and 
\ to more strikingly demonstrate the period of most 
rapid growth, averaged from specimens reared in 
/ /\ from 1,000 to 2,000 cubic centimetres of water, 
/ \ and illustrating the Growth and Time- Curve, 
/ Fig. 178. 
( J Fig. 179, 21 days old ; Fig. 180, 42 days old ; 
^ ^ ^ Fig. 181, 60 days old ; and Fig. 182, 85 days old. 
182. 
at length, the older the animal the slower the growth. Maximum size 
can only be attained by favourable conditions at these early and critical 
periods, especially that of greatest increase, as that period once passed 
cannot afterwards be fully compensated for by any subsequently 
favourable conditions, the shell being then constructed on a more 
F 
