:,58 DISCOVERY REPORTS 



appear to be probable. What actually happens to make the South American barrier more 

 effective than that of South Africa is, I imagine, that, since the currents are opposed, 

 any mixing that may take place is nullified by the fact that the cold current from the 

 south encounters and passes under the Brazil current, continuing its journey northwards 

 beneath the surface. In this way, any floating bodies reaching the northern current will 

 be restored once again to the warm surface waters of the South Atlantic. 



While it is true that many details have yet to be considered and the hypothesis tested 

 in other parts of the globe, it is reasonable to infer from the evidence afforded by the 

 sponges common to the West Indies and Brazil, on the one hand, and to Australia, 

 on the other, that while temperature does exercise considerable influence, their dis- 

 tribution will probably be found also to be correlated largely with ocean currents. 



Theoretically, there is one great objection to this idea; that is the short duration of the 

 larval period, and it was this objection that doubtless gave rise to the view hitherto held 

 that extension of the range of distribution in sponges must always be explained by a 

 slow creeping action round the margins of the land masses or along the sea-bottom, and 

 that where a shallow- water species is represented in two areas widely separated by water, 

 the land masses of these areas must at some time in the earth's history have been closely 

 adjacent or actually connected. The free-swimming larval period of a sponge is normally 

 24 hours or less. Under abnormal conditions, as in marine aquaria, it may be extended 

 to 2 or even 3 weeks, but this is exceptional and, owing to technical difficulties, it is not 

 known whether such a larva will develop into a normal adult or not. The probability is 

 that it does not. For all practical purposes, therefore, we must assume that the larval 

 period is short, and that trans-oceanic migration on the part of the larva is either nor- 

 mally impossible or, if possible, that successful metamorphosis sufficient to establish 

 the species cannot be accomplished at the end of the journey. On the other hand, it is 

 not impossible that transport is effected after metamorphosis, and that young recently- 

 metamorphosed individuals may be carried by floating objects. In this respect it is 

 pertinent that the young sponge grows very little indeed for several weeks after meta- 

 morphosis, remaining usually as a thin inconspicuous incrustation, and in this condition 

 it would not only escape observation but could be carried on floating fragments of such 

 size and of such a nature as to be overlooked. 



It is interesting to note that the species involved in this apparent migration from 

 Australia to the West Indies belong to two groups only, the order Euceratosa and to the 

 family Clavulidae of the order Tetraxonida. Whether it is that the larvae of these two 

 groups have certain habits and characteristics which fit them for such migration, or 

 whether the adult individuals are more adaptable and can become established in areas 

 where individuals of other species, carried thither by the same migratory sequence, will 

 perish, is a matter for future research. The important point is that some species of 

 sponges appear to be capable of transportation over long stretches of oceanic waters by 

 the medium of the prevailing currents. 



The distance from Australia to the West Indies is very great, and from the presence 

 of species of commercial sponges on the Barrier Reef of Australia identical with, or 



