HYDRACTINIA. 343 



HYDRACTINIA, Van Boieden. 



Name. — A compound of the two generic names Hydra and Actinia, so named from a supposed 

 union of tlie external features of botli tliese genera. 



EcHiNocHORiuM, — Hctssall. 

 Synhydra, — De Quatrcfai/cs. 

 Dysmorpiiosa (?), — Philippi. 



TROPHOSOME. — Hutdkanths daviform, developed at intervals from the free 

 naked surface of tlie hydrophzton ; tentacles filiform, forming a single circlet round 

 the hase of a conical hypostome. 



GONOSOME. — Sporosacs supported on blastostyles, which arise like the 

 hydranths from the free naked surface of the hydrophyton, each carrying round its 

 distal extremity globular clusters of thread-cells, which take the place of the tentacles 

 in the hydranth. 



The generic name of Ilydradinia without a specific designation was given by Van Beneden 

 in 1841 to a hydroid obtained on the coast of Ostend.' The short memoir in whicli this name 

 is thus for the first time used contains no zoological description of the hydroid, beyond what is 

 involved in an account of the gonophores, to which the description is confined, and which, in 

 accordance with the views generally entertained at that period, are regarded as eggs. A figure, 

 however, is given of one of the hydranths, as well as of a blastostyle loaded with its gonophores, 

 and there is therefore little difficulty in identifying the animal to which the name of Ilydradinia 

 was intended to apply. 



Nearly at the same time Dr. Hassall obtained in Dublin Bay specimens of a production 

 with which zoologists had been familiar, as forming a horny spinous crust on the surface of empty 

 univalve shells, and which Fleming, De Blainville, and Johnston, had regarded as the horny ectocyst 

 of a polyzoon. Hassall, however, from an examination of living specimens, became aware of the 

 incorrectness of this interpretation, and, recognising its hydroid affinities, assigned to it the name 

 of Eddnocorium claviyerum." We now know that the Echinocoriiim of Hassall is the animal to 

 which Van Beneden had just before given the name of Hydradinia. 



In 1842 Dr. A. Philippi' noticed under the name of Dysmorpiiosa conddcola a hydroid 

 from the Bay of Naples. He tells us nothing of the gonosome, and his description, so far as it 

 goes, will apply in all points to the Hydradinia of Van Beneden ; but it is also just as applicable 



' ' Bui. de I'Acad. Roy. de Bru.x.,' t. viii, 1841. 

 - ' Ann. Nat. Hist.,' 1841, vol. vii. 

 ' ' Wiegman's Archiv,' 1843. 



