178 



HYDROIDA II 



Stechow (1913). As, however, the specimen is unique, and moreover, apparently not fully grown, 

 these points should not be considered as of too great importance in the present state of our know- 

 ledge. For the present, it will be most correct to regard the specimen as representative of a distinct 

 species, bearing in mind, however, that subsequent investigations may possibly show it to be identical 

 with one of those formerly described. 



Branchioceriaiithits reniformis will probably, as the find suggests, prove to belong to the abyssal 

 region. Its occurrence shows that the genus is of circumterrestrial distribution. 



IV. Zoogeographical observations on the Hydroid Fauna 



of the North Atlantic. 



The study of the bathymetrical distribution among hydroids still leaves much to be desired; 

 the data on record are still somewhat scant\', and merely suffice to give an occasional glimpse of the 

 regularity which will doubtless be found to prevail throughout this animal group, just as in others. 

 It has been a generally accepted notion, and is so to some extent even now, that a marine animal 

 species with a wide horizontal distribution will prove of shallower occurrence the farther north it is 

 found in our waters, or rather, the farther it penetrates into the cold areas. This rule, however, is not 

 universally applicable. True, we find that species which must be regarded as more or less arctic are 

 met with in more southerly tracts in the deeper water-layers, but on the other hand, warm atlantic 

 character forms which are found farther south up in the littoral region often exhibit, in the few 

 northern finds made, a tendency to prefer even much greater depths. Evidently then, the biophysical 

 conditions here approach more nearly to those prevailing in the southern home of the species, though 

 we cannot at present find any further explanation of this. At any rate, the question is seen to be a 

 good deal more complicated than was at first supposed, and the available data are unfortunately far 

 from sufficing to give a thorough exposition of the biogeography in hydroids in the northern waters. 



In this connection, it will not be out of place to explain one or two expressions which are 

 constantly recurring when dealing with the different species. I refer to the terms littoral and abyssal 

 region. I have followed the general international acceptance in using "littoral region" for the upper 

 3—400 metres of the sea floor, "abyssal region" for depths be>ond. It is true that certain northern 

 writers have sought to give the terms in question new and sometimes quite different meanings. 

 This, however, gives rise to confusion, and further complicates the still somewhat new field of research 

 known as biogeograpln-. The terms used should be as nearly as possible invariable, and should not 

 require to be defined anew by each writer, as has hitherto been the case. Indeed, it might almost 

 seem well to discard altogether the term littoral region, since it appears to be interpreted in almost 

 as many different senses as there are writers in .Scandinavia. I have here, however, as indicated, 

 employed the expression in the general sense in which it is used outside the works of the scientists 

 referred to. 



