BRANCH ARJHROPODA: CRUSTACEANS '55 



the original inhabitant. Some hermit crabs always have 

 attached to the shell certain kinds of sea-anemones. It 

 is believed that both crab and sea-anemone derive advan- 

 tage from this arrangement. The sea-anemone, which 

 otherwise cannot move, is carried from place to place by 

 the crab and so may get a larger supply of food, while 

 the crab is protected from its enemies, the predaceous 

 fishes, by the stinging threads of the sea-anemone, and 

 also perhaps by the concealment of the shell its presence 

 affords. This living together by two kinds of animals to 

 their mutual advantage is called commensalism. ^r sym- 

 biosis (see Chapter XXXj. The hermit crabs are not true 

 crabs, but are more nearly related to the crayfishes and 

 shrimps than to the true broad-bodied, short-tailed crabs. 



Barnacles. TECHNICAL NOTE. Specimens of barnacles may 

 be got readily from the tide rocks or from piles in a harbor. In- 

 terior schools should have, if possible, specimens preserved in 

 alcohol or formalin for examination. The " shells " of acorn (ses- 

 sile) barnacles may often be found on oyster shells (get at restau- 

 rants). 



/ Crustaceans which at first glance are hardly recogniz- 

 able as such are the stafked^or sessile barnacles (fig. 37) 

 which live fixed in great numbers on the rocks between 

 the tide lines, or on the piles supporting wharves, or on 

 the bottom of ships or even on the body-wall of whales 

 and other ocean animals. In the stalked forms the stalk 

 is a flexible stem or peduncle covered with a blackish 

 finely-wrinkled skin bearing at its free end the greatly 

 modified body of the barnacle. This body is enclosed in 

 a sort of bivalved shell or carapace formed by a fold of 

 the skin and stiffened by five calcareous plates. Within 

 this curious shell is the compact, rather worm-like body- 

 mass, showing little or no indication of segmentation. 

 The legs, of which there are usually six pairs, are much 

 modified, being long, feathery, and divtcTecT nearly to the 



