Mr. A. Alcock on a case of Commensalism. 207 



XX LI I. — Natural History Xotes from II. M. Indian Marine 

 Survey Steamer ^ Investigator ^^ Lieut.Gordon S. Gunn^ R.N.^ 

 commanding. — Series II., No. 6. A case of Commensalism 

 between a Gymnoblastic Anthomedusoid (Stylactis ininoi) 

 and a Scorpoinoid Fish (Mitious inermis). By A. Alcock, 

 M.B., Surgeon I. M.S., Surgeon-Naturalist to the Survey. 



Contents. 



§ 1. Introductory : some Illustrations of Symbiosis already reported 



from amont^ the (Tymnoblastic Ilydroida. 

 § 2. An Account of a Species of Stylactis always found associated with 



a Minous. 

 § 3. Description of the Stylactis. 

 § 4. Note on the Minoiis, 



§ 1. Introducfory : some Illustrations of Symbiosis already 

 reported from among the Gymnoblastic Ilydroida. 



Many observers have remarked upon the existence of life- 

 associations between Gymnoblastic Hydrozoa and other 

 animals. Such associations may be classed as (1) accidental, 

 (2) commensal, and (-}) parasitic ; and though it is not easy 

 always to be sure into which of these classes any given case 

 shall fall, yet for the purposes of this paper it will be con- 

 venient to consider the three classes separately. 



What may be regarded as instances of accidental associa- 

 tion are too numerous to mention. Such most probably are 

 many of those related or quoted by Professor Allman in his 

 beautiful monograph on the Gymnoblastic Hydroids; of 

 Antigonium pusillum found by Professor Van Beneden 

 attached to crabs (and to various other bodies) : of Dicoryne 

 conferta investing shells of various Gastropod mollusks ; of 

 Perigonimus muscoides^ P. repens^ P.palliatus^ and P. linearis^ 

 all occasionally found on tests of ascidians, on crustaceans, 

 and on shells of living mollusks ; of Eudendrium capillare, 

 sometimes found upon ascidians ; of Ilydractinia echinata and 

 H. polyclinoy sometimes attached to hermit-crabs j and of 

 Ectopleura Dumortieri found on crabs and on Flustra among 

 other objects. Accidental, probably, too are the attachments 

 noted by Professor Van Beneden in his *' Animal Parasites 

 and Messmates " of a Tubularia to a Cephalopod (observed 

 by Gwyn Jeffreys) and of a Tubularia sometimes growing 

 on a living sponge. 



There seems, however, to be something more than a mere 

 chance association in the cases recorded by Professor Allman 

 of Corynitis Agassizii found by ]\I'Grady growing only on 



