HYDROIDA 



43 



polyps are provided with a ring of stinging cells above the base of the tentacle. The stolons form a 

 network of anastomosing tubes. 



The gonophores are enmedusoid with a rudimentary spadix and with the generative cells 

 placed along the four radial canals. The colonies are bisexual. 

 Material: 



Greenland: Egedesminde (on Tellina calcarea). 



The Kara Sea ("Dijmphna"). 



Monobrachium parasitum is indigenous to the middle parts of the literal region far to the north. 

 It is recorded from Spitzbergen, the Kara Sea, the White Sea, and the west coast of Greenland 

 (Text-fig. L). 



Family Bougainvilliidae. 



Hydroids forming colonies with polyps fusiform or capitate, whose oral portion is conically 

 pointed. The stinging cells are small and rodshaped. The tentacles, which are filiform, are placed in 

 a main whorl round the polyp; the stinging cells are equally distributed all over the surface of the 

 polyps or in less distinct transverse belts round them. The polyps are quite naked or surrounded by 

 a jellied, lithe, and pliable pseudohydrotheca below the tentacle whorl. The endoderm is differentiated 

 into an oral portion, consisting of indifferent small-nucleated cells between which occur a large number 

 of mucous gland cells, and the proper gastral portion ; the limit is formed by the tentacle whorl. The 

 colonies have no calcareous skeleton. 



The diagnosis states for the family the same range as was practically already given by Bon- 

 nevie(i899), whom the later authors have generally followed. Kiilin (1913) divides the family into 

 three subfamilies, two of which, Hydractiniinae and Atractylinae, are represented in our northern seas. 

 The main distinguishing mark stated by Ku'hn is that Hydractiniinae are stated to have a vigorous, 

 crustformed skeleton, while Atractylinae, on the other hand, have hydrocauli covered with periderm. 

 However, this character does not seem to be of the importance Ku'hn attached to it. In young 

 colonies the stolons have not coalesced into a crust, and the development of the skeleton is not parti- 

 cularly vigorous. There is even every probability that several species of Stylactis do not at all assume 

 such crustformed skeleton-formations, even when advanced in life. On the other hand, we also know 

 species of Hydractiniinae, in which the hydrocaulus covered with periderm has been reduced to a mere 

 minimum. The character, therefore, must be characterized as a merely gradual one, and can hardly be 

 turned to account as fundamentum divisionis for higher groups. 



A very different interest is attached to the peculiar occurrence of pseudohydrothecae met with 

 in the Bougainvilliidae. I set aside the socalled pseudohydrothecae of Clathrozoon (the subfamily Hy- 

 droceratininae, stated by Kiihn 1913); in fact, this group is not yet so well known that we are able 

 to judge of it entirely, and its "pseudohydrothecae" do not seem to form such a parallel with the for- 

 mations of the thecaphores as those found in certain other Bougainmlliidae , namely in Pcrigo- 

 nimus. In this species the ectoderm of the polyp has secerned a jellied pseudohydrotheca, which has 

 coalesced with the polyp along its distal margin, and to which the basal portion of the supporting 



