44 HYDROIDA 



lamella of the polyp is attached by small chitinous bodies similar to those found in several thecaphores. 

 This psetidohydrotheca is but little visible, or, with intact polyps, when they are extended, almost 

 untraceable, while in contracted polyps it lies round the basal portion of the polyp like a cup richly 

 folded. It might be obvious to regard it as the precursor of the real hydrotheca. Perhaps it might 

 be regarded as a character sufficiently important for the purpose of distinguishing a subfamily: How- 

 ever, the matter has, as yet, been too little examined as to allow the forming of a reasonable opinion; 

 we have, provisionally, to regard it as a generic character. 



Gen. Hydractinia van Beneden. 



The stolons of the reptant colonies, when young, will generally form an open meshwork which 

 afterwards most frequently changes into a continuous chitinous crust. The crust is commonly carrying 

 spines of various appearance, or may grow out into branched formations of colonies. The polyps are 

 cylindrical or capitate with conically pointed oral portion. The tentacles are placed in a whorl below 

 the mouth. There is no erect hydrocaulus covered with perisarc. The polyps are heterogeneously devel- 

 oped into larger nourishing individuals bearing tentacles and into spiralzooids bearing no tentacles; the 

 latter occur at the extremities of the colonies. The gonophores are supported by polyps more or 

 less transformed (blastostyles) or arise directly from the stolons. 



According to this diagnosis, also Podocoryne, Oorhiza and Stylactis, earlier set up as genera 

 are included under Hydractinia. The only reason of distinguishing between Hydractinia and Podo- 

 coryne has been sought in the power of the last-mentioned genus of producing free-swimming medusae. 

 However, Bonne vie (1899) has described a couple of species, Hydractinia Allmani and Hydractinia 

 ornata, having medusoid gonophores less strongly reduced with radial canals, and from Africa has 

 been recorded a species, Hydractinia Michaelseni Broch (1914), whose male gonophores are perfect 

 medusae, which, however, do not seem to break away normally. Also Kiihn (1913) seems to incline 

 fo the opinion that the separation of the two genera is questionable. The intermediate forms of gono- 

 phores mentioned, in fact, forbid the drawing of a certain limit between the genera, and, therefore, the 

 proper thing to be done, is, indeed, to draw in Podocorync under Hydractinia. 



The genus Oorhiza is based on the fact that the gonophores are seated, not, as in Hydrac- 

 tinia, on blastostyles, but, on the contrary, on a short stem rising directly from the stolons. A review 

 of our northern species of Hydractinia, however, shows that, in fact, this criterion is of no vital im- 

 portance. In Hydractinia humilis Bonnevie and Hydractinia Sarsii Steenstrup the gonophores 

 are borne on polyps fully developed of the same size as the sterile nourishing individuals; in Hy- 

 dractinia carnea M. Sars the polyps bearing gonophores are, certainly, fully developed individuals, 

 provided with tentacles; but they have a smaller number of tentacles, and are smaller than the sterile 

 nourishing polyps. In Hydractinia Allmani Bonnevie, Hydractinia ornata Bonnevie, and Hydrac- 

 tinia echinata (Fleming) the reduction is carried still further, the tentacles being reduced or wholly 

 disappearing, so that the bearers of the gonophores are here typical blastostyles. In Hydractinia carica 

 Bergh, finally, the polyp has been reduced to a stem, round the apex of which the gonophores are 



