249 



come from bolh sources. The quesUon whether any (leci)-sea Kchiiiofierms 

 have truly pehigic huvte must, after what we now know, be answered in 

 the affirmative. Lcuiimum diplopora has been directly proved to have pel- 

 agic larva?, and il is almost certain that also Pedicellaster 6-radiatus has 

 pelagic larviv (SlellophA'ni inirabilis); 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^). On the other hand it is evident from tlie facts 

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

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

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

 Auricularid niidihranchiala): this is in good accordance with the fact that 

 the Asterid-larv;e in general require long time for their development, much 

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

 being carried great distances by the currents. 



The transport of the larvae 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-theory)-). It is a well 

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

 to the West Indies (Brazil) 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 Kchinoderms were formerly distributed, or by 

 the transport of thinr larva^ across the Ocean. That the latter alternative 

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

 occurrence of larvse in the open Ocean. Accordingly the Archhelenis-theory 

 is no necessity for understanding the recent distribution of the littoral 

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

 there arc 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 larvae 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 at the surface; we do not know 



') Pelaf/olhiiria nalalrix has large eggs, until 1,2 nini in diameter according to Ludwig 

 (■■.Mbalross'-IIololluirioidea. 1894. p. 119). This large si/e of the eggs probably means that 

 also this form has a direct development, and we have thus the remarkable fact that this 

 typical pelagic llolothurian has not a typical pelagic larva. 



-) H. V. Ihcring. Archhclcnis und .\rcliinotis. Clesamnicltc Heltriigc zu eincr Cie- 

 Mhichlc (Icr Ncol nipischcn Hcgioii. 1907. 



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