THOMPSON: METAMORPHOSES OF HERMIT CRAB. 203 
of the shell here plays only a very subordinate part. Adolescent 
crabs which are kept without shells show an increase of mortality 
similar to that among glaucothoe. 
The anatomical modifications that appear during the glaucothoe 
stage are, with but one exception, uninfluenced by either the 
presence, absence, or form of the shell. The exception is found in 
the retention of rudimentary pleopods on the right side of the body 
in the sixth stage, though typically at this period appendages should 
be absent from this side. The percentages of crabs that vary from 
the typical arrangement and the number of the variations displayed, 
are larger among larvae which have been reared without shells. 
The form of the shell is important to a small degree, and sixth-stage 
larvae reared from the glaucothoe in sinistral shells are slightly more 
variable than individuals reared with dextral shells. 
In a single instance a crab reared in a sinistral shell reconstructed 
or retained one perfect pleopod on the right side of the abdomen. 
But this crab was otherwise so abnormal that there is reason to 
doubt whether the variation was connected with the special environ¬ 
ment. Outside of this case, there is no evidence that the anatomy 
can be modified by a longer or shorter residence in shells of peculiar 
shape. Typically the retained pleopod rudiments are lost in the 
moult that closes the sixth stage. 
There seems to be no reason to infer that hermit crabs learn the 
shape or size of a shell, except by trial and use. But on the other 
hand there is evidence for the assertion that they show a preference 
for dextral shells over those of other forms, based on the fact that if 
a dextral shell is dropped into an aquarium it will, in the long run, 
be found and used, even when the bottom is encumbered with shells 
of other kinds. 
It is not possible at present to determine whether the asymmetry 
of the Pagurids is primarily attributable to life in spiral shells. 
The relationships of the genera are too imperfectly understood. 
Nevertheless, the asymmetry in its structural details is very closely 
adapted to'the conditions imposed by this mode of life, which raises 
a strong presumption in favor of the view that the asymmetry was, 
from the first, a result of life in dextrally spiral shells. If the dis¬ 
placement in ontogeny, of the right liver to the left of the intestine 
points to a similar displacement in phylogeny the latter might 
well be the outcome of a compression of the right side of the body 
