THE LARVA OP ASTERTAS VULGARIS. 121 



ferent forms characteristic of the various groups — the larval 

 form distinguished as Auricularia, Bipinnaria, and Pluteus. 

 The fact that all these forms are modifications of a single 

 typical form was long ago pointed out by Johannes Miiller. 

 The recent work by Semon (38) has completed the confir- 

 mation. 



It seems pretty certain that the radial symmetry of the 

 Echinoderms has been derived from bilateral symmetry through 

 the influence of a sedentary mode of life. May we not be jus- 

 tified in supposing that such an animal as the typical Echino- 

 derm larva above described may upon becoming sedentary 

 have been modified in adaptation to its mode of life, even so 

 much as now appears between such a larva and an adult 

 Echinoderm, and that the process of change as now shown in 

 the metamorphosis is in its general character an expression of 

 the course of phylogeuy, but subjected to exceedingly great 

 distortion by the constant tendency towards abbreviation, by 

 the dropping out of details from the ontogeny, and by greater 

 or less shifting of the relative timesof formation of the various 

 organs, particularly in the time of appearance of radial sym- 

 metry, which has been constantly carried forward to earlier 

 appearance in the ontogeny. After assuming the sedentary 

 condition there came in as further adaptive modifications 

 changes in the function of organs. The greatest of these 

 changes concerns the enterocoels; to their earlier function, 

 probably excretory, has been secondarily added that of loco- 

 motion, of relation (feelers, tentacles), and also to some extent 

 of respiration. 



It seems more probable that the ancestral Echinoderm arose 

 by the adaptive modification of a more primitive free-swim- 

 ming form than that a larval form has been acquired for the 

 purpose of distribution. The Echinoderm ancestor was pro- 

 bably a free-swimming animal, in general characters not far 

 removed from the ancestors of the Turbellarians ; a creature 

 with a well-differentiated digestive tract, ciliary locomotor 

 apparatus, excretory system, respiratory surface not localised j 

 coenogenetically modified by the acquirement of transparency, 



