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ZOOLOGY 



SECT. IV 



curious genus Hydractinia (Fig. 93, 1} is remarkable for possessing 

 a massive ccenosarc, consisting of a complex arrangement of 

 branches which have undergone fusion so as to form a firm 

 brownish crust on the surfaces of dead gastropod shells inhabited 

 by Hermit-crabs. The constant association of Hydractinia with 



FIG. Pi. Bougainyillea ramosa. A, entire colony, natural size ; B, portion of the same 

 magnified; C, immature medusa; dr. c. circular canal; <<>. cuticle or perisarc ; tut. car. 

 enteric cavity; hi/il. polype or hydrant h ; /<///>. hypostome or manubrium ; -uttd. medusa ; ///'/. 

 uianubrimii ; /<!'. c. radial canal; t. tentacle; v, velum. '(From Parker's Biology, after 

 Allman.) 



Hermit-crabs is a case of commensalism : the hydroid feeds upon 

 minute fragments of the Hermit-crab's food, and is thus its com- 

 mensal or messmate, and the Hermit-crab is protected from its 

 enemies by the presence of the inedible, stinging hydroid. 

 Hydractinia belongs to the Anthomedusse : the Leptomedusan 



