250 



exactly the depth at which Slellosphtera mirabilis was taken, and we have 

 not yet definitely proved Auricularia nudibranchiata to belong to a deep- 

 sea Holothurian). It seems impossible that the larvae could rise all the way 

 by means of their ciliary movements. One cannot help suggesting that 

 vertical currents may have something to do with the transport of the 

 larvae from the deeper layers towards the surface and the inverse. The 

 existence of vertical currents is a suggestion called forth by various biolog- 

 ical facts. Thus e. g. Prof. Ad. S. Jensen informs me that the distribution 

 of the eggs and the young of Reinhardlius hippoglossoides (Walb.) in the 

 Davis Strait seems to be explicable only on the assumption of vertical 

 currents existing there. While the eggs and newly hatched young larvae, 

 with the yolk sac, are found in depths of 600 1000 Meters, the slightly 

 older young, where the yolk sac has just been absorbed, are found at 30 

 Meters below the surface 1 ). It seems simply impossible that these young 

 fish larva? could have passed that great vertical distance alone by means 

 of their own feeble swimming power. Hjort and Murray in "The depths 

 of the Ocean" (p. 378 380) think that vertical circulation has a great 

 influence upon the distribution of pelagic plants, and upon the whole speak 

 of vertical circulation in the Ocean as an established fact, at least for the 

 upper 200 300 Meters. This, however, will not suffice for explaining the 

 ascent of the larvae from the bottom at much greater depths. This inter- 

 esting problem cannot be solved at our present state of knowledge. Upon 

 the whole, we are hardly in possession of facts enough even for a definite 

 formulation of the problems connected with the deep-sea larva?. We can 

 only see this much that continued researches will be sure to bring im- 

 portant results. 



*) Comp. Rapport til Indenrigsministeriet over Briggcn "Tjalfe"s praktisk-viclenskabe- 

 lige Fiskeriekspedition til Gronlaml. 1909. 



