144 



ZOOLOGY 



curious genus Hydradinia (Fig. 105, 2) 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 



Pio.104.— Bougainviilea ramosa. A, entire crtony, natural size; B, portion of the same 

 magnified ; C, immature medusa, cir. c. circular canal ; cu, cuticle or perisarc ; ent. cav. 

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

 manubrium ; rad. c. radial canal ; (. tentacle ; c. velum. (From Parker's Biology, after 

 AUman.) 



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 



