346 HYDRACTINIA ECHINATA. 



Colour. — Various shades of brownish red. 



Development of Gonophores. — March to November. 



Habitat. — Investing the surface of dead univalve shells, chiefly such as are inhabited by 

 hermit crabs, but occasionally occurring on other submerged bodies. 



Bathy metrical distribution. — Littoral to deep-water zone. 



Localities. — Atlantic shores of France and Belgium, Van Beneden, De Quatrefages. Gene- 

 rally distributed round the shores of the British Isles. 



Hydractinia echinata is the only well-established European species of Hydractinia. It is 

 an abundant hydroid in the British seas, and may be easily kept alive in the confinement of our 

 tanks. Among some hundreds of specimens which have come under my observation, I have 

 never met with it except upon some gasteropodous shell, and then always with the shell 

 inhabited by a hermit crab ; or, if empty, affording evidence by abrasions of its surface that it 

 had at one time been similarly tenanted. A''an Beneden, however, has found it attached to 

 pieces of timber. 



In the singular structure of its hydrophyton, and in the polymorphism of its zooids, it 

 possesses a deep significance for the student of hydroid morphology. 



The spiral zooids constitute a vei-y striking feature in a coXouy oi Hydractinia echinata. They 

 occur close to the margin of the colony, and may be easily watched in a living specimen, when 

 they will be seen in a constant state of activity, rolling and unrolling themselves, and bending 

 backwards and forwards on the slightest irritation. Besides these Wright has described certain 

 long, contractile, non-spiral tenticula-like filaments, which also spring from the ccenosarcal base 

 near its margin. These have been also described by Hincks, but they are certainly not constant 

 in their occurrence, and I regard them as an abnormally modified form of some of the other 

 zooids. 



Wright's description of sporosacs, borne directly by the basal expansion without the inter- 

 vention of blastostyles, I regard as founded on an error of observation, Dr. Wright having been, 

 as I believe, deceived by the shortness of the blastostyle which carried them. 



I have made Hydractinia echinata the subject of one of the special studies of hydroid 

 anatomy given above (Part I, p. 220), where these and other morphological features of the 

 hydroid are discussed at length. 



Gegenbaur figures under the name of Hydractinia a hydroid in which the sporosacs are 

 borne on ordinary hydranths instead of on blastostyles.' I believe that there is here some con- 

 fusion between Hydractinia and Stylactis. 



^ Gegenbaur, ' Gruudziige der vergleich. Anatomie,' p. 90. 



