ECHINODERMATA 459 



the ontogeny of all five groups, and from this argues for a 

 common ancestor having that form. This theory encounters 

 the difficulty that the five groups, if they had assumed such 

 an independent development, would scarcely show so great 

 a resemblance in their organization as they in fact do. It 

 seems to us more justifiable to search for the ancestral forms 

 of the Echinodermata among the existing material which is 

 oifered to us by palaeontology. In this, however, the other 

 difficulties arise that the material is not complete, since 

 delicate forms have not been preserved, and that it allows 

 only the external shape to be recognized. 



At all events, it will be the stalked forms among which 

 we are to search for the ancestors of the Echinodermata; 

 for at any rate it was the influence of the attached mode of 

 life which in the Echinoderms, as in other groups of animals, 

 called forth the radial structure. Such forms as the Gystidce 

 — which are in part stalked and in part without stalks, and 

 of which one part obviously led an attached life, while the 

 other part, on the contrary, led a free existence — seem best 

 fitted to stand as the ancestral forms of the Echinodermata 

 (comp, also Neumayr, ISTo. 43). Their shape is spherical, 

 not yet being prolonged into arms. In many of them the 

 plates are irregularly arranged, and then no trace of a radial 

 arrangement is noticeable. On the other hand, five radial 

 furrows may extend out from the mouth, similar to the 

 ambulacral furrows on the disc of a Crinoid or Asteroid. 

 However, it is the relationships which exist between the 

 Cystidae and the other groups of Echinoderms that seem 

 to be especially important. The Cystidte, by means of tran- 

 sitional forms, are said to stand in relation to the Crinoids, 

 the Asteroids, and the Echinoids (Nedmayr, No. 43). Since, 

 however, the Echinoids have been placed nearer to the Holo- 

 thurioids by P. und F. Sarasin, and since evident relation- 

 ships between the Cystidae themselves and the Holothuri- 

 oidea have been discovered by the same authors, this latter 

 group can also be compared with the rest of the Echino- 

 dermata. To us it seems probable that the Echinoderms 

 established their radial structure by an attached mode of 

 life long continued, and only later returned again to a free 



