249 



come from both sources. The question whether any deep-sea Echinoderms 

 have truly pelagic larva? must, after what \ve now know, he answered in 

 the affirmative. L<i</(tinim diplopora has been directly proved to have pel- 

 agic larvae, and it is almost certain that also Pedicellasler 6-radialus has 

 pelagic larva? (Sldloplwrd minibilis); judging from the character of the eggs 

 many other deep-sea forms must have pelagic larvae (though a great per- 

 centage of them have large eggs and therefore certainly must have an 

 abbreviated development 1 ). On the other hand it is evident from the facts 

 recorded that larva' of littoral forms may be carried very far from the 

 coasts. In this connection it is important to notice that Bipinnarise are 

 predominant among the larva? found in the Sargasso-Sea (beside the large 

 Anrinildriit niidibrunctnattt): this is in good accordance with the fact that 

 the Asterid-larva' in general require long time for their development, much 

 longer than do the Echinoid-larvae. Thus there will be ample time for their 

 being carried great distances by the currents. 



The transport of the larva? by means of the currents is, of course, of 

 considerable zoogeographical importance. Their eventual transport across 

 the Atlantic has some bearing on the problem of a former land connection 

 between Africa and South America (the Archhelenis-lheory) 2 ). It is a well 

 known fact that quite a good number of littoral Echinoderms are common 

 to the West Indies (Bra/il) and West Africa. This peculiar distribution 

 can be accounted for only in two ways, viz. by the existence of a former 

 land connection (or a series of islands) between the two continents along 

 the shores of which these Echinoderms were formerly distributed, or by 

 the transport of their larva 1 across the Ocean. That the latter alternative 

 is really possible seems undeniable from the facts made known of the 

 occurrence of larva 1 in the open Ocean. Accordingly the Archhelenis-theory 

 is no necessity for understanding the recent distribution of the littoral 

 Echinoderms of the tropical Atlantic. On the other hand, it must be agreed, 

 there are many other facts, especially in the geographical distribution of 

 land- and freshwater animals and plants, which seem to require that theory 

 for an explanation. 



The occurrence of larva? of deep-sea forms at the surface of the ocean 

 is another problem not very easily explained ( due allowance being 

 made for the fact that we have as yet not definitely ascertained a single 

 case of a larva of a deep-sea form found al the surface: we do not know 



') I'l-latjiilluiria nulalri.r has large eggs, until 1,2 nun in diameter according t<> I.udwig 

 ("Albatross"-Holothurioidea. l.S'.M. p. ll'.i). This large si/e of the eggs probably means that 

 also this form has a direct development, and \ve have thus the remarkable fad thai this 

 typical pelagic I lolothurian lias not a typical pelagic lar\a. 



: ) II. v. I tiering. Archhelcnis und Archmolis. (iesammelte Heitriige /.u einer Ge- 

 sehiehle der Neotrnpixchen Region. I '.MIT. 



32 



