40 THE AMERICAN NATURALIST. (VoL: XXXIX. 
three occasions, the “shells " of which, more or less nearly com- 
plete, were still in the same tube with the * soft-shelled " crabs. - 
When a worm in a tube dies the crabs in the same tube die as 
a result of the failure of food and properly aérated water. Two 
tubes were taken in which this might have occurred as was 
shown by the nearly perfect condition of the extremities of the 
tubes and by the presence of the bodies of three dead crabs 
lodged in the necks of the tubes, which were only recently filled 
with sand. 
Whether or not commensalism is an advantage to Chaetop- 
terus it seems to be a decided benefit to the crabs, Polyonyx 
and Pinnixa, grown specimens of which are za7e/y found outside 
the tubes. The advantage to the crabs is very clearly marked 
by their prolonged breeding season — virtually an example of 
protected industry. 
ZOÓLOGICAL LABORATORY, JOHNS HOPKINS UNIVERSITY. 
ecember, 1904. 
