REGENERATION 



297 



direction, but their size diminishes following the same 

 law. These facts appear significant and without doubt 

 are suggestive of ancestral history. 



If it is true that the radial sj-mmetry of Echinoderms 

 is to be ascribed to a fixed stage in their ancestral his- 

 tory, we are led to suppose that the point of attachment 

 was on the right side of an originally bilateral animal. 

 The life history of Pentacrinus, the larval organ of Aste- 

 roidea, and a great many anatomical and embryological 

 facts support this view. While it is not within the prov- 

 ince of this paper to discuss the relative significance of 

 these matters, the evidence is so overwhelming that the 

 theory is generally accepted. It is also no doubt true 

 that some groups of Echinoderms took to a free-living 

 existence early in their ancestral history, and others re- 

 mained fixed until comparatively a late period. As proof 

 we may cite the embryological evidence that Holotlmrians 

 develop without any attached stage whatever, that the 

 Asteroids develop a larval organ and pass through a 

 Sessile stage for a brief period in their development, 

 while the crinoids usually remain permanently fixed 

 throughout life. At least we can best account on this 

 theory for the deep-seated and fundamental radial sym- 

 metry of some forms; the longer the attachment the 

 more deep-seated would become the type of radial sym- 

 metry. Now if this theory is correct we can use it to ex- 

 plain the conditions described above for Thyone. The 

 ancestors of this form must have broken away from the 

 fixed stage very early, for we find the radial symmetry 

 not well established on the right side of the animal as 

 evidenced by both the position and size of the Polian 

 vesicles. Out of 118 large and medium-sized Polian 

 vesicles, 112 were on the left side, while in a total of 28 

 small or rudimentary Polian bodies, 25 were found on 

 the right side. The arrangement of these organs in 

 Thyone adds one more bit of evidence to support the 

 following statement of Lankester. 



