458 EMBRYOLOGY 



organs (H. P. Carpenter, Bury), but these relations are 

 quite uncertain still. Even the conception of the homology 

 of the plates founded by Lov^N and championed by Car- 

 penter, especially those which in the different groups of 

 Echinoderms lie about the apical pole, is not to be considered 

 as assured. 



All Echinoderms have a radial structure ; the larvae, on the 

 contrary, are bilaterally symmetrical as regards both their 

 internal and their external organization. It has been shown 

 by different examples how the radial structure arises onto- 

 genetically from the bilateral ; but the question now presents 

 itself, How is the shape of the Echinoderms to be explained 

 phylogenetically ? A reply to this involves another question, 

 namely, whether the different groups of Echinoderms are 

 derived one from the other, and in that event which of them 

 stands the highest and which the lowest. Recently the 

 Holothurians, and especially the apodal Holothurians 

 (Synapta) , have been looked upon as the lowest forms, and 

 transitions have been sought from them to the Crinoids on 

 the one hand and to the Echinoids on the other, because certain 

 Holothurian characters were found on the one hand in the 

 (fossil) Cystidce, on the other hand in the soft-shelled Echino- 

 ihuridce (P. und F. Sarasin, No. 47). Additional transitions 

 to the Asteroids and Ophiuroids are also demonstrable. 

 This theory, to be sure, traces the Echinoderms back to 

 simple forms, but gives no explanation of the origin of the 

 radial structure. Even the apodal Holothurians are still 

 radially constructed, and it does not seem at all impossible 

 that the simplicity of their structure is only a phenomenon 

 of degeneration. The Echinoderms, however, as we may con- 

 clude from their ontogeny, are to be referred to bilateral 

 forms as their source. 



Another theory is that which traces back the different 

 divisions separately to a common ancestral form known as the 

 Pentactcea (Semon, No. 55). This ancestral form corresponds 

 to that stage in the ontogeny in which the larva had already 

 passed from the bilateral to the radial form by the closing 

 of the water-vascular ring, and by the putting forth of 

 five evaginations. Semon tinds such a Pentactula stage in 



