144 



ZOOLOGY 



SECT. IV 



curious genus Hydractinia (Fig. 105, 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 Hydractinia with 



FIG. 104. Bougainviilea ramosa. A, entire colony, natural size; B, portion of the same 

 magnified; C, immature medusa, dr. c. circular canal; ci(. cuticle or perisarc ; tnt. car. 

 enteric cavity; hyd. polype or hydranth ; ky/>. hypostome or maiiubriuni ; med. medusa; mnb. 

 niaimbrium ; rod. c. radial canal ; t. tentacle ; c. velum. (From Parker's Biology, after 

 Alhaan.) 



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 Anthomedusas : the Leptomedusan 



