LARVAL TRANSPORT BY OCEAN CURRENTS 465 



observation by stating: "there seems no reason to doubt that a 

 new complete and ultimately metamorphosing larva may be the 

 result. Thus we would here have a true case of metagenesis, 

 otherwise totally unknown in Echinoderms." 



I should not be surprised at all, if such a metagenetic larval 

 development would prove to be a common mode of reproduction 

 in some tropical ophiurans and perhaps also in some sea stars. 

 Who knows? Nobody has ever looked for it! If Mortensen is right, 

 the ophiopluteus must be regarded as an asexual polyp, producing 

 sexual bottom stages and, what is of special interest for our 

 problem, the larval life may be prolonged to an unknown extent. 

 But even if such a "budding" from the tropical brittle star larvae 

 might be regarded as an exception, not the rule, it might very well 

 give some of the billions of brittle star larvae drifting away in the 

 water masses a chance to cross the wide ocean basins. This is 

 actually what it seems to have done. Hyman (1955) mentions 

 seven species of ophiuroids common to the tropical Pacific-Panama 

 region and to the Indo-West Pacific and (or) Oceania, and she 

 adds: "Ophiuroids thus appear to have been more successful than 

 other echinoderm groups in migrating from the Indo-Pacific to 

 the western shores of tropical America." 



Also without assuming this new and sensational mode of 

 dispersal, larval asteroids and larval ophiurans have significantly 

 better chances for long-distance transports than indicated in the 

 diagram. They are known to metamorphose very often in mid- 

 water, which might prolong their pelagic life to a considerable 

 degree — how much, we do not know. 



The echinoids often have a fairly long pelagic life. Arbacia 

 punctulata has a larval life of up to 97 days and Evechiniis chloro- 

 ticus up to 116 days. In echinoids, the larval body is, however, 

 completely destroyed by metamorphosis (verbal information from 

 Professor L. von Ubisch), so that a "budding" like that suggested 

 for ophiurans and asteroids cannot help to prolong larval life. 

 Mid-water metamorphosis and postponement of metamorphosis 

 until the right substratum is encountered is, however, a common 

 occurrence among echinoids and may considerably prolong their 

 pelagic life. Nevertheless, true long-distance larvae will hardly be 

 found within this group, and it is, in good agreement with this 



