Note on Pelagic Organisms and Evolution. 243 



coming to describe a folded rather than a purely equatorial course. Evolution 

 in such a direction is exemplified by numerous larval forms — e.g. 

 by the Auricularia, in which the ciliated band merely meanders about 

 on the surface of the body ; by, the Bipinnaria, in which it extends out 

 on to short, thick lobes of the body ; and by the Actinotrocha larva, or 

 the Echinopluteus, or Ophiopluteus, in which it is carried out on long 

 slender processes such as reach their maximum development in forms 

 like the Spatangoid pluteus or the Ophiopluteus of OpMothrix fragilis. 



We should expect larvse in which the ciliated band meanders about on 

 the surface of the body to serve as the starting-point for a further series of 

 evolutionary changes. For in such meandering bands those parts of the 

 bands are placed advantageously, so far as causing movement is concerned, 

 which are placed most nearly perpendicularly to the axis of movement. Such 

 parts of the band may therefore be expected to undergo exaggeration, to 

 become more strongly developed than parts of the band placed less 

 advantageously. Indications of such a process are seen in the tendency of a 

 few of the Ophioplutei to form " epaulettes," and in the Auricularia while 

 metamorphosing into the barrel-shaped larva. 



These few examples are perhaps sufficient to serve the purpose of this 

 note — to accentuate the importance, before making use of any pelagic 

 organism for purposes of phylogenetic speculation, of assuming a 

 particularly critical attitude in considering to what extent its characteristics 

 may be merely adaptive in their nature. 



{Issued separately, 26th January 1912.) 



VOL. XVIII. 



