192 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
daily be placed in a jar, the glaucothoe removed as they appear and 
reared in balanced aquaria, new larvae being continually added and 
the young crabs studied as fast as they attain the adult form. 
Reliance should be mainly placed on sinistral shells. It is a waste 
of material to keep adolescent crabs naked. 
The results from the foregoing study of larvae under various 
conditions with respect to the shells, effectually dispose of any 
probability that the adult crab can be modified by altering the form 
of the shell in which it lives. If in the presumably most plastic 
period of the animal’s life, when there is in addition a slight varia¬ 
bility to work upon, we are unable to alter the structure except in a 
most trivial detail and then but rarely —supposing that the retention 
of the pleopod by the crab in 1900 was due to the form of the shell 
— we cannot expect for an instant that even a long continuance in 
an abnormal shell will modify the adult. 
The occasional discovery of a hermit crab (Milne-Edwards & 
Bouvier, ’ 91 ; Marchal, ’ 91 ) in a left-handed shell has no significance 
in this connection. The supply of shells runs, along shore at least, 
very close to the demand. Residence in a sinistral shell simply 
means that the crab has been dispossessed and is protecting his body 
with the best hollow object available. An endless list of such habi¬ 
tations might be compiled. I have collected adult crabs in the shells 
of Crepidula and Vermetus ; and my glaucothoe used every hollow 
object they came across, from bits of algae that were twisted or rolled 
and broken float-bladders of Fucus or Sargassum, to fragments of 
exuviated crustacean integument and fragments of the fine dirt-tubes 
of small Annelids. There is no case where any departure from the 
normal form has been found in a crab which was using an unusual 
residence. 
Choice of shells. — A consideration of the dextral asvmmetry of 
our hermit crabs with the role played by the shell in the ontog¬ 
eny, naturally leads to the question whether these crabs evince 
any preference for one type of shell over another. The only 
investigator who has attempted to study this experimentally 
(Bouvier, ’ 92 ) has come to the conclusion that they do not show a 
preference for dextral over sinistral shells, lie experimented by 
supplying the crabs with a mixture of shells of different sorts and 
then keeping a record of their movements. Approached from this 
side, the problem is difficult of solution. When a crab seizes on a 
