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 ratnosa. A, entire colony, natural size; B, portion of the same 

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

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

 manubrium ; rod. c. radial canal ; t. tentacle ; i: 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 



