268 COELENTERATA HYDROZOA chap. 



thus it remains, the capitate arms standing out rigidly, like the 

 rays of a starfish, until the embryos are ready to escape." * 



Among the Gymnoblastea there are many examples of a 

 curious association of the Hydroid with some other living animals. 

 Thus Hydractinia is very often found on the shells carried by 

 living Hermit crabs, Dicoryne on the shells of various Molluscs, 

 Tubularia has been found on a Cephalopod, and EctopUura (a 

 Corymorphid) on the carapace of a crab. There is but little 

 evidence, however, that in these cases the association is anything 

 more than accidental. The occurrence of the curious species, Lar 

 sabellarum, on the tubes of Sabella, of Campaniclava cleodorae 

 on the living shells of the pelagic Mollusc Cleodora cuspidata, 

 and of a Gorgonia on the tubes of Tubularia parasitica, appear 

 to be cases in which there is some mutual relationship between 

 the two comrades. The genus Stylactis, however, affords some of 

 the most interesting examples of mutualism. Thus Stylactis 

 vermicola is found only on the back of an Aphrodite that lives 

 at the great depth of 2900 fathoms. S. spongicola and S. 

 abyssicola are found associated with certain deep-sea Horny 

 Sponges. S. minoi is spread over the skin of the little rock 

 perch Minous inermis, which is found at depths of from 45 to 

 150 fathoms in the Indian seas. 



In many cases it is difficult to understand what is the ad- 

 vantage of the Hydroid to the animal that carries it, but in this 

 last case Alcock 2 suggests that the Stylactis assists in giving the 

 fish a deceitful resemblance to the incrusted rocks of its environ- 

 ment, in order to allure, or at any rate not to scare, its prey. 

 Whether this is the real explanation or not, the fact that in the 

 Bay of Bengal and in the Laccadive and Malabar seas the fish is 

 never found without this Hydroid, nor the Hydroid without this 

 species of fish, suggests very strongly that there is a mutual 

 advantage in the association. 



Cases of undoubted parasitism are very rare in this order. 

 The remarkable form Hydrichthys mirusf supposed to be a 

 Gymnoblastic Hydroid, but of very uncertain position in the 

 system, appears to be somewhat modified in its structure by 

 its parasitic habits on the fish Seriola zonata. Corydendrium 



1 Hincks, British Hydroid Zoophytes, 1868, p. 74. 

 2 Ann. Mag. Nat. Hist. (6) x. 1892, p. 207. 

 3 Fewkes, Bull. Mus. Comp. Zool. xiii. 1887, p. 224. 



