266 MARINE INVERTEBRATES 
In some genera both hands are alike; in others the left one is 
the larger. In the case of hermits which live in tooth-shells, the 
right hand is cylindrical in form, fitting the circular opening of 
the shell. The other extreme is found in the genus Cancellus, 
where both the chelipeds and the first pair of ambulatory feet 
are ingeniously shaped, so that when closed they form a round 
operculum, or door, which closes the way into the compact sponge 
which this crab has for its carcincecium, or house. It is interest- 
ing to note these differences and observe the perfection with 
which the feet are formed and jointed to suit the openings of 
different shells. Some genera show other adaptations to their 
twisted houses, the eye-stalks being uneven in length and the 
ambulatory feet unequal on the opposite sides. Besides this, 
some species are hairy and cover themselves with dirt for further 
protection. The exposed claws of some hermits have a special 
armature of spines, which make a fringe of points around the 
opening of the shell when the crab is withdrawn. 
As the hermit grows he is occasionally obliged to find a new 
or larger shell, and there are amusing anecdotes of the troubles 
he experiences at these times while house-hunting. Very often 
he tries several shells before he finds one to fit. There is a tradi- 
tion, not, however, well authenticated, that if the shell chosen 
happens to be occupied by its natural owner, the crab tears out 
and devours the unfortunate occupant. If the desired shell is 
occupied by another hermit inferior in strength to himself, he 
proceeds to take possession by violence. He then examines care- 
fully the empty shell, inserting his legs and feelers, and if he 
decides to oecupy it he withdraws his abdomen from the old 
shell and darts it so quickly into the new one that the act of 
transference is difficult to follow. After walking about with the 
new shell it sometimes proves to be unsatisfactory and further 
search becomes necessary. After itis well domiciled the crab never 
ventures outside the shell until it is obliged to change again on 
account of growth, or because some stronger crab dispossesses him. 
The shells of hermit-crabs serve frequently as the home of other 
animals which live with them a commensal life. The hydroid 
Hydractinia polyclina often covers the exterior of such shells with a 
brown, velvety growth. Some sea-anemones also are commensals 
