22 STYLASTERIDAE 



In its structure Pliobothrus shows several features which indicate, that it stands nearer to the 

 origin of the Stylasteridae than most of the other Stylasterid genera. In the first place, the 

 zooids are not yet collected into distinct cyclosystems but are irregularly scattered over the colony; 

 no regularity in the occurrence of the gasterozooids and dactylozooids can be detected. In the second 

 place, the dactylozooids are less reduced than in the other Stylasteridae, as they still retain their inner 

 cavity; nor do they show the marked division between an expanded basal part attached to the skeleton 

 and a distal tentacle-like part, which is inserted obliquely on the basal part, as in most of the other 

 Stylasteridae. Lastly, the gasterozooids lack a gasterostyle. These features show, when taken together, 

 that Pliobothrus occupies a primitive position. Pliobothrus shows further, that the conservative, somatic 

 parts of the colony are less exposed to the influence of the surroundings, to adaptive tendencies, than the 

 generative parts, the power of which to change plastically in relation to the special biological demands 

 is of vital importance for the existence of the species. In this therefore we also see the reason for 

 the great variety displayed by the hydroid gonophores and in this we have the reason, why the 

 gonophores of nearly allied species may be quite different. Owing to their conservatism in develop- 

 ment the polyps are of the most important phylogenetic in terest. The gonophores, the 

 generative individuals, on the other hand, might almost be said to be a play-ball in 

 the hands of chance biological conditions and thus phylogenetically have much less 

 interest; they are suited to display the subdivision of the genera into biological adaptive groups 

 and are thus more exposed to the influence of convergence than the other parts of the colonies. 



We must therefore not ascribe too great importance to the condition of the gonophores in 

 judging of the affinities of the Stylasteridae in relation to the other Coeleuterata. Their organisation 

 according to the results of all investigations may well be compared with that of the Hydroid gonophores 

 and even if a single genus shows somewhat special features, the}' can by no means be used as evidence 

 for the view, that the Stylasteridae are more distantly related to the Hydroids than the Milleporidae. 



Bringing together the main points of the above discussion the result is, that we in reality 

 cannot consider the Hydrocorallines as anything else but two convergent Hydroid families, which are 

 characterised by their power to develop a skeleton of calcium carbonate and by the dimorphic development 

 of their somatic individuals. The family Milleporidae traces its origin to that of the Corynidae, whilst 

 the Stylasteridae is a highly specialised branch which has been derived from the Bougainvilliidae. Just as 

 the Coryuidae are distinguished from the Bougainvillidae by their capitate tentacles, the Milleporidae 

 with their capitate tentacles are distinct from the Stylasteridae, in which the tentacles are constructed 

 like the thread-shaped tentacle type of the Bougainvilliidae. 



Zoogeographical remarks on the North Atlantic Stylasteridae. 



Few animal groups have been so little investigated in our northern waters as the Hydro- 

 corallines. They do not form any prominent faunistic element, it is true, as they are only represented 

 by four species in the northern Atlantic and of these, as known, only two penetrate into the Norwegian 

 Sea. But by contrast these two species at several places form a very characteristic element in the 

 large biocoenosis of the Lophohelia reefs and form here a complete and extremely interesting parallel 

 to the numerous Stylasterid species of the tropical coral reefs. 



