THOMPSON: METAMORPHOSES OF HERMIT CRAB. 193 
new shell, it turns it over and over, thrusts the chelipeds within 
the chamber and then probably enters it. As far as I can judge, 
the preliminary exploration merely tells the crab that the “shell ” is 
hollow, empty, and clean, and I cannot agree with those who would 
see in it a measuring or comparing of the shell. Apparently the 
crab does not perceive either the type or the size of the shell until it 
has inserted its abdomen into the chamber and tested the shell by 
moving about in it, etc., deciding by actual trial whether the new 
will prove better than the old house. It almost invariably keeps a 
firm hold on its former habitation, so as to be able to return to it if 
the new house proves ineligible. This method of selection by trial 
leads to numerous changes that are inconclusive and only serve to 
confuse an observer. 
On the other hand, there seems to me to be evidence which points 
to a preference for shells of dextral type. If a single right-handed 
shell is dropped into an aquarium where the crabs are in straight 
tubes or sinistral shells, it will in the long run be found and used — 
unless its size be utterly inadequate — even though the bottom is 
encumbered with empty shells of other sorts. Similarly there is 
a tendency to abandon sinistral shells for straight tubes. This does 
not indicate any deliberate choice. The crabs are constantly 
changing shells and if one obtains the dextral shell he does not as 
readily exchange it as he would a shell of other type. Hence that 
shell will be found in use when the aquarium is examined. 
In general, for both larvae and adults, the need seems to be, first 
for a covering and then for one of suitable size and shape. The 
animal is twisted dextrally. Therefore, other things being equal, 
the desire for a well fitting house will tend to bring the crab into 
a right-handed shell. But there are evidently a number of unknown 
factors which enter into a choice, especially a choice between shells 
of the same type. And there are many instances where crabs leave 
shells that, as far as appearances go, are vastly more suitable than 
the ones they substitute for them. 
The larvae show the tendency to take dextral shells more strongly 
than do the adult crabs. The experiments show that young 
provided with shells of unusual form are more restless than those 
provided with dextral shells. The number that were found swim¬ 
ming after the first twenty-four hours, which means that they were 
either deferring their entrance into a shell or had come out again 
