270 FA; BATHE R [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, Psolus phantapus, 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. <Asterias 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. C7ossaster also increases in 
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 borealis 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. ither 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. Théel, for example, has shown that Hehino- 
cyamus needs no more than two months to develop from the egg into a 
sea-urchin crawling on the bottom. Mortensen has” Gbsereal that 
masses of larvae of Asterias rubens and Ophioglypha texturata, floating 
in the Limijord, remained there only a few days. The same author 
remarks that the floating larvae of Echinoderms are found chiefly near 
