BARRIERS AND MEANS OF DISPERSAL 59 



the fewer are the effective barriers. By contrast, the suspended plank- 

 ton is dependent on passive dispersal by currents. 



Passive dispersal of marine animals is almost confined to the 

 surface forms. The great oceanic currents can be detected to depths 

 of 130 to 150 meters, and wave motion, to somewhat greater depths. 

 Movements of the water, of this type, operate to distribute the weak 

 swimmers and especially the plankton, which is composed of plants 

 and animals which do not swim independently of the motion of the 

 water. Much plankton is thus transported by the Gulf Stream from 

 the warmer parts of the Atlantic to the neighborhood of Spitsbergen. 

 Animals of this kind may escape the currents by downward vertical 

 movement, or by rising, may become subject to its motion. 



The oceanic currents are of especial importance to the distribution 

 of sessile marine forms which have a free-swimming larval period. 

 Littoral animals may be carried in this way to places where the depth 

 is too great for them to develop further, and so be destroyed. The 

 eggs and larvae of many animals of the depths rise to the surface, 

 undergoing their development in the light. Many crustaceans (Eu- 

 phausidae, Penaeidae) and deep-sea fishes (Muraenidae and Scope- 

 lidae) afford examples. 7 This procedure is naturally effective for the 

 distribution of these forms because the larvae come within the influ- 

 ence of the currents. Among the brachiopods, only Discina atlantica 

 has a world-wide distribution. This is explained on one hand by the 

 rise of its free-swimming larvae to the surface and on the other by the 

 fact that this species is distinctively a form of the greater depths, 

 so that the mature larvae find suitable habitat conditions almost 

 wherever they sink to the bottom. 8 



The distances traversed depend upon the rate of movement of the 

 current in question and the duration of the free larval period, which 

 varies greatly among different forms. The larva of the brachiopod 

 Terebratulina septentrionalis attaches itself after 10 to 12 days; the 

 zoea stage of the decapods continues for 25 to 30 days; and echino- 

 derm larvae may drift for long periods (20 to 60 days) before 

 metamorphosing. The larval life of the trochophore of annelids and 

 mollusks seems to be much shorter (4-5 days), as does that of the 

 planula larva of corals and sea anemones and the Miillerian larva of 

 turbellarians. 9 Almost all the Crustacea of the group Stomatopoda, 

 whose adults are slow-moving bottom-dwellers, are widely distributed 

 because they have a long larval life with exceptional powers of sus- 

 pension; Gonodaetylus chiragra occurs in all oceans, and Squilla 

 cruposa on the coasts of North America and Africa. 10 



Even the adult stage of sessile forms may be transported by cur- 



