134 



ZOOLOGY 



SECT. IV 



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

 a massive coenosarc, 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 Hjdractinia with 



FIG. 92. Bougainvillea raxnosa. A, entire colony, natural size ; B, portion of the same 

 magnified ; C, immature medusa ; dr. c., circular canal ; cu. cuticle or perisarc ; ent. car. 

 enteric cavity; hyd. polype or hydranth ; hyp. hypostome or manubrium ; med. medusa; mnb. 

 maiiubrium ;" rud. 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 



