Something about Crabs. 895 
This association of forms, which is known as commensalism—a 
term, a free translation of which would be fellow-boarders—receives 
numerous exemplifications among the crabs. One form, first 
described from the Pacific ocean by the late Dr. Stimpson, settles 
down upon a piece of growing coral, which then proceeds to build 
a protecting nest about it, leaving only openings for the ingress 
and egress of the water bringing food and oxygen to the prisoner. 
Even more remarkable is the case of the shrimp, which so far has 
only been found enclosed within the glassy network of that beau- 
tiful sponge, the “ Venus-flower-basket.” So strange was this 
association that when it was first noticed it was thought that those 
dextrous fingers of the east, which furnish those wondrous ivory 
carvings and which fashion those curiosity-exciting mermaids, had 
placed the shrimps within the sponge, and then had closed the 
opening so skilfully that the detection was impossible. It would 
now appear that this is not a counterfeit to be be laid at their 
doors. 
Another strange case of commensalism is furnished by some small 
crabs which are more nearly related to the fiddler with which we 
started, and of which the little oyster crab is an example. These 
crabs live inside the valves of oysters, clams and other molluscs, 
and in olden ‘times quite a pretty myth grew up about them. The 
delicate Pinna of the Mediterranean (a bivalve mollusc somewhat 
resembling our familiar salt-water mussel) was eyeless, and in order 
to escape the jaws of some wandering fish it had to be told when 
to close its shell. This position of watchman was filled by the 
little crab which was constantly on the alert, and which in return 
for its services was protected by the mollusc. This little crab was 
called Pinnotheres—guardian of the Pinna—and the same name is 
embodied in the scientific nomenclature of to-day. Relatives of the 
oyster crab seek other homes. On our Carolina coast is one which 
lives in the strong tubes built by one of the worms, while in the 
Pacific are found others which take up their residence in the pos- 
terior portion of the alimentary tract of certain sea-~cucumbers or 
holothurians, 
The fiddler-crabs are largely air-breathers. In fact, so long as 
they are in a moist location, where they are in no danger of drying 
up, they do not need to actually enter the water for weeks or months 
atatime. In this respect, however, they are equalled, if not excelled, 
