66 Proceedings of the Royal Society of Edinburgh. [Sess. 
may assume that the newly moulted part swells by absorption of water; 
if this water cannot come from the outside, it may conceivably be 
abstracted from the other half of the body. 
Many of the land isopods after moulting devour the cast-off cuticle. 
A similar observation has frequently been made in the case of the lobster. 
I have not observed anything similar in the case of Ligia, the exuviated 
cuticle, whether in air-medium or in sea-water, being apparently left 
untouched. Small pieces of the shell of a fowl’s egg left in the tin 
beside the animals were similarly undisturbed. 
To the author the most interesting physiological observation in respect 
of exuviation was the fact that several Ligise, after a long fast in clean 
sea-water, moulted normally, some of them surviving for quite long periods 
thereafter. Out of the set of eight animals recorded in the first communica- 
tion of this series as having been immersed in sea-w T ater, five moulted 
completely and one died during moult. The following table gives the 
number of days after immersion at which moult occurred : — 
Posterior Moult. 
Anterior Moult. 
Total Period of Survival. 
14 days 
18 days 
65 days 
27 „ 
31 „ 
58 „ 
31 „ 
35 „ 
52 „ 
35 ,, 
35 ,, (died during moult) 
46 „ 
? 
>83 „ 
54 ,, 
>83 „ 
From this it appears that after a fast of 54 days (during which there 
was constant expenditure of energy in movement; a Ligia can still moult 
normally. The fact is of special interest in view of ideas that still prevail 
with regard to the subject of moult. Since Reaumur’s classical description 
(1712, 1718) of the moulting process in Astacus, it had been implicitly 
assumed that a crustacean moults because it has grown too large for its 
skin, or, to put it otherwise, that growth is the immediately determining 
cause of the moult. Having suggested the probability that the skin of 
crayfishes does not enlarge in the intervals between moults, Reaumur used 
these words : “ Leur habit devient trop court et trop etroit, il les gene, il 
faut qu’elles le quittent.”* 
Of late years the conceptions with regard to moulting have been under- 
* In this connection the following sentence by a writer in the Encyclopaedia Britannica 
is worth quoting: “As a rule Reaumur avoided theoretical questions, but when he took 
them up his manner of treatment was remarkably clear, chiefly on account of an ingenious 
use of metaphor, often expanding into allegory.” 
i 
