1902] The Holothurioidea of Northern Norway. 27 



more: Mesotfmria ijitestinalis, Cucumaria hispida and Psohcs squa- 

 matus. But these statements are questionable (cf. p. 22). 



I have in many cases held a different opinion to Ludwig 

 (1900), regarding the classification of a species as purely boreal 

 (subarctic), or purely arctic or both arctic and boreal. Cuctwicnia 

 mimda, C. calciqera, C. hispida, Psolus fahricli, Ps. squamatus, 

 Trochostoma borecde, Anhjroderma jeffreysii and Chiridota lævis are 

 said by Ludwig to be both subarctic and arctic; while I con- 

 sider Cucumaria hisjnda and Fsohis squamahis to be purely boreal, 

 and the remaining six to be purely arctic. (I take, however, now 

 no account of the existence of Trochostoma and Aiikijroderma in 

 the deep sea). I will now in a few words give my reasons for 

 these differences. 



Ludwig (1. c. p. 169) makes the Arctic circle the limit for the 

 arctic fauna. Still he has reckoned to the purely arctic species 

 even those, which gO a little south of this limit, even including a 

 species, which — albeit wrongly — is said to exist in Skager Rack, 

 57° N. (Mijriotrochus rinlii). In the same manner, he regards as 

 purely boreal (subarctic) even such species as go a little into the 

 Arctic region. 



Topographically speaking, it is of course the case that the 

 Arctic circle limits the Arctic region. But in the case of zoo- 

 geography, it is a w^ell known rule to define the limits of a region 

 on quite different principles, whether it be for Jand or sea fauna 

 (cf. with regard to the latter Ortmann, 1896). One Avould get 

 very extraordinary results if one, for instance, were to make the 

 Southern polar circle the limit for the antarctic fauna. In reality 

 it is not much hetter when the Forthern hemisphere is con- 

 cerned. 



On the European coast the limit for the arctic sea fauna is moved 

 many degrees towards the north, in consequence of the influence 

 of the warm Gulf Stream. On the east coast of North America 

 there is, as we know, a cold arctic stream, which carrying with it 

 masses of ice, in a high degree lowers the temperature of the sea. 

 The border of drift-ice, which nowhere touches the Norwegian coast, 

 goes during the winter on the east coast of North America to 38° 

 N. and reaches even during the summer down to the Newfound- 

 land Bank. In consequence hereof the sea fauna has quite a different 

 character, than at corresponding latitudes on the European side. 

 The arctic coast fauna is prevalent as far as to Massachussets, where 



