57 
1916-17.] Experiments and Observations on Crustacea. 
rocks near the edge of the sea, attempting to escape pursuit, frequently 
ran into the water. More conclusive still, numerous examples were 
discovered half-way down the beach under stones just left bare by the 
receding tide. It is clear that immersion in salt water holds no terrors 
for Ligia. 
The present set of experiments, however, equally brings into prom- 
inence the reverse side of the question. If Ligia can live in apparent 
comfort for nearly three months in sea- water, why does it prefer a land 
habitation ? Some immediate cause must keep the animal to the landward 
rather than to the seaward side on the beach. In its general bearing this 
problem is common to all cases in which a terrestrial begins to spring from 
an aquatic stock ; we should not forget that evolution has its physiological 
as well as its morphological side. In absence of experimental data the 
most plausible hypothesis for all such cases is the respiratory one. Atmo- 
spheric air with its 21 volumes per cent, of oxygen offers great advantages 
as a respiratory medium over water with its maximum of, say, 8 cm. oxygen 
per litre — in this connection see Winterstein (8). The most plausible general 
hypothesis is, however, but a feeble substitute for experiment. 
- An equally obscure problem, possibly cognate to the present work, is 
the determination of the obstacles, immediate and remote, that restrain 
Ligia from spreading inland. A vitally important agent (acting sometimes 
immediately, but as a rule remotely) for all sea-derived animals is rain, 
with its solvent or washing properties. 
To be able to endure long periods of fasting is a common feature among 
Crustacea. Bethe (1), after commenting on the voracity of the common 
shore-crab, remarks : “ It can also subsist for weeks without food.” Lobsters 
are occasionally kept by fishermen and dealers for long periods without 
food. The writer has kept four specimens of the amphipocl Orchestia in 
clean sea-water for 26 days, at the end of which period the animals were 
to all appearance as vigorous as before. The capacity of Ligia for enduring 
hunger should perhaps be kept in mind in pondering over the fact that its 
normal haunts often appear bare of any visible food material. 
From the point of view of general physiology and of special crustacean 
physiology the most interesting facts elicited in the present inquiry are : 
firstly, the extraction of salts from Ligia and the oedema caused by distilled 
water (this line of work, however, is not entirely new); and, secondly, the 
increase in resistance to distilled water of recently moulted as compared 
with long since moulted animals. As these questions remain under in- 
vestigation, discussion will be postponed to a later occasion. 
