212 Mr. A. Alcock on a case of Commensalism between a 



stome, a mouth capable of complete eversion, and long and 

 very numerous tentacles. But beyond negative inference we 

 have positive grounds for believing, not that the polyps live 

 on the fish, but that the polyp-colony aids tlie fish quite as 

 much as the fish aids the polyp-colony in a common compe- 

 tition for food. 



The value of the association to the polyps has already, in 

 the introduction, been suggested, and it only remains to state 

 that their usual position upon the throat and round the gill- 

 opening of the fish seems particularly to enhance the value of 

 the alliance. 



The following considerations lead to the belief that an 

 equivalent benefit is enjoyed by the fish. Many of the 

 kScorptenidaj — especially Scorpcena, Pterois^ kiynancicUum^ 

 and Pelor^ and to a limited degree Minous — have the body 

 and fins capriciously covered with long, wavy, often tufted 

 cutaneous filaments ; and no one who has watched sucii a 

 fish as Pterois voUtans in a reef-pool can doubt that these 

 filaments serve what Mr. E. B. Poulton, in his book on 

 * The Colours of Animals,' calls a " special anticryptic " 

 purpose. That is to say, they assist in giving the fish a 

 deceitful resemblance to the iucrusted rocks of its environ- 

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

 And it appears probable that IStijlactis minoi enables its 

 companion, Minous inermis, in the very same way to assume 

 the same convenient and successful disguise. 



§3, Description of the Stylactis. 

 Stylactis, Allman. 



Stylactis, Allman, Monograph of the Gymnoblastic Hydroids, pt. ii., 

 1872, p. 302. 



Stylactis minoi^ sp. n. 



The polyps, which are of two forms, sterile and proliferous, 

 are all sessile upon a hydrorhiza that consists of a network of 

 close-set ramifying and anastomosing tubes bounded by a 

 flexible, extremely delicate, pellicular perisarc. The sterile 

 polyps are of an elegant caryophyllaceous shape, and termi- 

 nate in a conical liypostome, the base of which is encircled 

 by a single crowded series of long filiform tentacles, to the 

 immber of twenty to twenty-four. In every colony a few 

 large urn-shaped polyps are seen with broadened hypostome 

 and more or less shortened tentacles ; they appear to be 

 merely sterile forms gorged with food. The average length 



