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Proceedings of Roycd Society of Edinburgh. [sess. 
of this species is, as Gosse has pointed out (1860, pp. 89-92), 
extremely variable. Specimens of this species collected by one 
of us in the Faeroes are both smaller and more intensely pigmented 
than others from the Scottish coast. Specimens kept in captivity 
show little tendency to increase in size, hut become decidedly 
paler in colour. These old captives are lighter in colour than 
individuals which have been more recently taken from a rock pool. 
All the individuals of this species which we have observed are 
sensitive to changes of light and of temperature, becoming and 
remaining semi-contracted during cold weather and at night, hut 
expanding to their fullest in warm, bright weather. The old ones 
are much more strongly affected by unfavourable conditions than 
those which are more than thirty years younger, and also are 
longer in recovering when conditions become again favourable. 
When the aquaria are examined in early morning or in fine warm 
weather succeeding a period of cold, it is found that the old 
specimens remain contracted for some time after their children 
and grandchildren are fully expanded. 
The most notable difference between the old (fifty years) and 
the younger (fourteen years) individuals of Sayartia troglodytes is, 
as would he expected, in point of fertility. In 1903 the sixteen 
old ones did not produce altogether more than half a dozen young ; 
indeed, it is doubtful whether they bred at all, as the few young 
found beside them may not have been their progeny. During 
the same period their children and grandchildren reproduced in 
large numbers (hundreds), though, as mentioned above (p. 297), 
only a few of these survived. 
Sagartia troglodytes , in these aquaria at any rate, apparently 
takes three years to reach maturity. 
In the early part of 1904 the aquaria were somewhat 
neglected, the water was aerated less frequently and not changed 
for over three months, and the animals remained unfed for a longer 
period than usual. Probably as a result of these less favourable 
conditions only a few young, much fewer than usual, were produced, 
even by the younger specimens of Sagartia ; these younger ones 
were abnormally thin and transparent, and were not extruded until 
early in April. The sixteen original specimens produced no 
offspring whatever in the spring of this year (1904). 
