MM ZOOGENESIS W§i 



water vascular system, and with the gonads not con- 

 nected with the coelome in the adults. The young 

 larvas of all echinoderms are always bilaterally sym- 

 metrical instead of radially symmetrical, and are 

 almost always free-swimming. 



The echinoderms are divided into five classes, the 

 sea-urchins, holothurians or sea-cucumbers, starfishes, 

 brittle-stars, and crinoids (sea-lilies and feather-stars) 

 and their allies. These five classes are entirely and 

 widely distinct from each other and do not intergrade. 

 So far as we know there never have been any inter- 

 grades between them. From time to time various 

 attempts have been made to construct evolutionary 

 trees which shall account for the peculiarities of these 

 several groups by reducing them to a hypothetical 

 ancestor. But these attempts have simply served to 

 emphasize the complete distinctness of the creatures. 



The development of the feather-stars differs very 

 considerably from that of the forms included in the 

 other classes. The divergence begins with the com- 

 plete formation of the gastrula, and rapidly increases. 

 Almost immediately afterwards the embryonic stages 

 of the other classes begin to diverge, and as develop- 

 ment goes on they diverge more and more widely. 



So in the echinoderms the only relationship be- 

 tween the five included classes is found in the embry- 

 onic stages immediately following the gastrula. As 

 it is impossible to assume that these stages could ever 

 have led an independent existence as adult animals, so 

 it is impossible to assume that any adults ever existed 

 which were intermediate between these five classes as 



[lOl] 



