EVOLUTION OF THE ECHINODERMS — FELL 487 



plates are a post-cliinianasterid development, not part of the calyx. 

 It is highly probable that the origin of asterozoans from crinoidlike 

 ancestors came about through a dislocation of the main radial gradi- 

 ent of each arm, so that proximal brachial elements were deflected 

 adorally, to give rise to the jaw. The change in feeding habits which 

 the acquisition of jaws made possible would lead to the change in 

 orientation, though some asterozoans still retain the ancient attitude, 

 in which the mouth is directed upward, and the arms sweep through 

 the overlying water. 



MAJOR GRADIENT PATTERNS IN ECHINODERMS 



Figure 17 illustrates diagrammatically the two major patterns of 

 dominant gradients which may be recognized in echinoderms. The 

 upper diagrams show the essentially meridional pattern established 

 in young echinoids and young holothurians. During metamorphosis, 

 the hydrocoel encircles the gut, and then sends out five meridional 

 water tubes which encircle the body. Thereafter the whole skeleton 

 and nervous system, as well as the hydrocoel itself, differentiate under 

 the same meridional gradients, and new meridional ones are estab- 

 lished in the interradii. The lower diagrams show the contrasted 

 system based on dominant radial gradients. This is found in young 

 crinoids, asteroids, ophiuroids, and somasteroids. Instead of growing 

 along meridians, the water tubes are thrust outward in the horizontal 

 plane, as five spokes radiating from the mouth, carrying the body wall 

 and coelom with them, as five finger-shaped processes, which become 

 the arms. The whole skeletal, nervous, and hydrovascular system 

 thereafter differentiates under the influence of these five major radial 

 gradients, with pinnate gradients arising as a secondary lateral growth 

 from the main radial gradients, these pinnate gradients suffering all 

 manner of slipping and dislocation as already inferred, eventually 

 giving rise to radial longitudinal fields in late asteroids. The calyx 

 alone retains the ancient meridional system. 



Thus, I would now argue that the clue to the phylogeny of echino- 

 derms lies in the postmetamorphic stages, which alone are directly 

 comparable with, for example, the young stages of Crustacea. A 

 newly metamorphosed sea star bears the same relation to its adult as 

 say a nauplius or metanauplius does to its adult. On the other hand a 

 pluteus or an auricularia is a totally different kind of creature, with 

 different symmetries, orientation, and habits. The phylogenetic per- 

 plexities which led to such sterility in echinoderm studies came in 

 when secondarily evolved larval forms were mistakenly taken to be 

 ancestral patterns of development. This supposition did not elucidate 

 one single fact of echinoderm phylogeny, and only led to the complete 

 divorce of echinoderm paleontology from echinoderm embryology. 



