50 
Proceedings of the Royal Society of Edinburgh. [Sess* 
IV. — Experiments and Observations on Crustacea: Part I. 
Immersion Experiments on Ligia. By John Tait, M.D., 
D.Sc. (From the Laboratory of Physiology, Edinburgh Univer- 
sity.) Communicated by Professor Sir E. A. Schafer. 
(MS. received July 25, 1916. Read November 6, 1916.) 
Descended from marine ancestors, Ligia oceanica, like other members 
of the sub-order (Oniscoidea or “ slaters ”) to which it belongs, has adopted 
a terrestrial mode of life. The other Oniscoidea have spread inland, 
invading regions far from the sea. Ligia, known as the sea-slater, is 
confined to the coastal fringe, and may possibly form a link between 
marine isopods and the more truly terrestrial forms. 
The author has been led from some early observations on the blood 
of Ligia (5), (7) to study the animal as a whole. Thus, colour-change was 
found to occur in the creature in response to variation in its surround- 
ings (6). Subsequent observations and experiments, the nature of which 
it is impossible to state briefly, have been carried out on Ligia and some 
of its relatives. It is proposed now to deal with these in a series of short 
communications. 
Immersion Experiments. 
One of the first points of interest in connection with the animal is 
the length of time it can survive immersion in salt and in fresh water 
respectively. The bearing of this question is at least threefold. To begin 
with, we know little regarding the ordinary habits of the animal ; it is 
still a moot question whether it voluntarily enters the sea. Secondly, 
the distribution of Ligia along the sea-fringe being discontinuous, every 
suitable locality being nevertheless occupied, one wonders if the animal 
can be safely carried by way of the sea from one habitable zone to another 
across an uninhabitable area, such as a long stretch of sand. Lastly, one 
wishes to know for certain whether the animal has sprung directly from 
a sea-water stock, or whether its ancestors have passed through a fresh- 
water stage in attaining land adaptation. 
Some experiments on immersion of Ligia have already been recorded 
by Hewitt (2), by Tait (5), and, in more detail, by Stewart (4). The 
last-named found that in sea-water one batch of four animals survived 
nine days, their death being ascribed “ to insufficient food rather than to 
the external conditions.” Another batch lived in sea-water for six days 
