10 THÉEL, PRIAPULIDS AND SIPUNCULIDS OF THE SWEDISH ANTARCTIC EXPEDITION 1901 — 1903. 



Moreover, if we take into consideration the immense space of time which must 

 necessarily be required in order to realize such a migration of, e. g., a worm, a cray- 

 fish, a sea-urchin etc., then we must draw the conclusion that the animals in question 

 must have been obliged to settle in several intervening places, or in short to liave 

 been cosmopolitans in the strict sense of the word. Thus, I do not believe that in 

 recent times a direct exchange of adult shallow-water forms can take place between 

 the two polar seas without settling on suitable intervenient places. 



The third point concerns the distribution of shallow-water animals in their 

 larval stages when they constitute an integrant part of the »plankton» and conse- 

 quently drift about on the surface of the sea or in the deeper water layers. Is there 

 any possibility that shallow-water animals in their early stages of development should 

 be able to stånd being carried along from polar circle to polar circle? When answering 

 this question, it should be borne in mind that those larvae are minute and very sen- 

 sitive organisms, in a high degree dependent on sunlight, abundance of suitable food 

 and almost uniform temperature and salinity. But, above all, we must keep in mind, 

 that their subsistence entirely depends on the depth and the nature of the sea-bottom. 

 For the larva, having reached a more advanced stage of development, must sink to 

 the bottom of the sea in order to settle there and go through their further develop- 

 ment. However, the bottom into which they have sunk must be of such a quality 

 as to offer conditions almost identical with those of their native places. — But this 

 is a mere chance. Certainly more than 90 °/° °f the young are doomed to destruction. 

 The mortality during this period of their life must be enormous. It may be gathered 

 from this fact alone that there is very little chance for a shallow-water species of 

 attaining a wider distribution. 



During more than twenty years I have had the opportunity of studying these 

 matters at the Swedish marine biological station, Kristi neberg, and I have come to 

 the above-mentioned results. The objects of my experiment were shallow-water sea- 

 urchins, especially the common Parechinus miliaris (L.) and Echinocyamus pusiUus 

 O. F. M., the former living on sea-weed, the latter on mud. It must be considered 

 a fact that the greater part of the larvae of these sea-urchins are doomed to destruc- 

 tion even within the fjord regions. Only thanks to the millions of larvae which are 

 brought into the world by a single female sea-urchin, can the existence of the species 

 within the native fjord be secured. Owing to insignificant local currents, always 

 varying in the fjords, by far the greater part of the larvae get drifted to localities 

 with a bottom fatal to them. If they then have attained such a maturity that they 

 must carry on their further life at the bottom, then, according to the natural laws, 

 they must sink and perish. Certainly more than 90 °/o get destroyed in this way. 

 They have not happened to reach a bottom with favourable conditions. What 

 percentage of the remaining larvae has been swalloed up by other marine animals or 

 how many of them have been destroyed in other ways, it is of course not possible 

 to state. The gist of all this is that the larvae have a very härd strugglc for existence. 

 A consequence of the above-mentioned facts is, of course, that the native places 



