270 FA. BATHER [october 



the Sound, is another Arctic form that, on the coast of Norway, increases 

 in number towards the north ; it has been found in the Kattegat only 

 at Samso. Another holothurian, Psoitis phcmtapus, does, it is true, 

 occur in various parts of the Kattegat, but is more usual in its south- 

 west corner, and is common in the mid-region of the Sound. The same 

 is the case with Cribrella and Solaster endeca, although these star-fishes 

 are not quite so common in the Sound. Astcrias muelleri has only 

 been observed a few times in the Kattegat, and then in its southern 

 portions ; but it is not rare in the Sound. Crossaster also increases iii 

 number towards the south. Again, a common brittle-star of the Sound, 

 Ophiopholis aculeata, is rare in the Kattegat until its south-west portions 

 are reached. In fact, as shown by C. G. J. Petersen, all the Arctic 

 Echinoderms of the Kattegat are concentrated towards the south-west. 

 Many similar examples are seen among the Mollusca, e.g. Modiolaria 

 nigra, Modiola and Bela trevelyana. Astarte borecdis is exceedingly 

 rare in the Kattegat proper, and is also rare in southern Norway, but is 

 common towards the Belt and in the Sound. Chiton albus is found 

 only in the southern Kattegat, the Belt, and the Sound, C. marmorens 

 begins to be common below Samso, and so on. It would take too long- 

 to go through all the other classes of animals ; one can just allude to 

 such purely Arctic forms as Lithodes and Mysis oculata, which are found 

 in the Sound, but not at all, or very rarely, in the intervening seas. It 

 is clear enough that a large number of Arctic forms occur in the Sound 

 (as also in the Belt) far removed from their natural area. 



How is the existence and origin of this Arctic element to be 

 explained ? There are two possibilities. Either it has wandered in 

 recently and is constantly recruited, or it has persisted here from a by- 

 gone age when conditions differed from those of to-day and were of an 

 Arctic nature, like those which the forms in question now find in their 

 proper home. 



The first hypothesis seems at first to be supported by the existence 

 of marine currents which every year, about February and March, bring 

 water from Greenland to the Skagerack and the Kattegat. The fauna 

 of the Sound and the southern Kattegat might therefore be recruited by 

 larval forms floated across from Arctic regions in these currents. But 

 to this view there are various objections. It is not likely that a larval 

 form should float in the water long enough to complete the journey 

 from Greenland to the southern Kattegat, since this occupies about 

 half a year. The time required by the various forms to pass through 

 their pelagic larval stages is not known for every case, but it can hardly 

 be so long as half a year. Theel, for example, has shown that Echino- 

 cyamus needs no more than two months to develop from the egg into a 

 sea-urchin crawling on the bottom. Mortensen has observed that 

 masses of larvae of Asterias rubens and Ophioglypha texturata, floating 

 in the Limfjord, remained there only a few days. The same author 

 remarks that the floating larvae of Echinoderms are found chiefly near 



