466 BOUNDARIES OF THE SEA 



statement, that the tropical West and East Pacific coasts ha\e no 

 echinoid species in common (Ekman, 1953; Hyman, 1955). 



The holothurian diagram, based on only six species, is disre- 

 garded, and we turn to the prosobranchiate gastropods. As seen 

 from Fig. 1, the pelagic life is very short, no species so far examined 

 having a longer stay in the plankton than 9 weeks. The shells are 

 heavy, the veliger larvae retract within their shell at the slightest 

 disturbance and then sink toward the bottom, which slows down 

 transportation. Mid-water metamorphosis also seems to be 

 unknown. Thus, for the usual type of prosobranch veligers, the 

 chances for larval transportation are limited to only very short 

 distances. However, we know also of groups of pronounced long- 

 distance larvae among prosobranchs. They are not indicated in 

 the figure simply because, thus far, they have never been reared 

 through metamorphosis under controlled conditions. 



Recent studies have shown that such genera as Cypraea, 

 Lamellaria, Tonna, Cassis, Charonia, Cymatium, and Bursa have 

 larvae hatching as tiny veligers, which live long and grow vigor- 

 ously during their pelagic phase and often do not leave the 

 plankton until their shells are 4 to 5 mm long. They have 4 or 

 more (up to 12) enormous velar lobes (Fig. 2), and several of them 

 have been found midway across the oceans. The eggs are laid by 

 the parents in huge quantities, and the younger larvae show 

 another feature, making them fit for a long planktonic life, i.e., 

 long spines on the shell which may protect them against enemies 

 and help them to reduce significantly loss during their stay in the 

 plankton (see DawydofT, 1940; Lebour, 1945, p. 483, Fig. 34). The 

 length of pelagic life in these genera is not known ; I should guess 

 that it might last for some 6 months. They are true long-distance 

 larvae, able to cross the oceans, and just within these genera we 

 know of a long series of circumtropical species, clearly proving the 

 efticiency of larval transport. 



The lamellibranchs normally have a very short pelagic life. The 

 longest larval life is 86 days for Mytilus crassitesta from Japan, 

 80 per cent of them metamorphose and settle earlier than 5 weeks 

 after hatching. Like the gastropod veligers, their shells are heavy; 

 the velum is withdrawn when they are disturbed, which makes 

 them sink toward the bottom and counteracts transportation. 



