226 HOMOLOGIES. 



their representatives in all succeeding times, and 

 are still very numerous in the present epoch. 



To show the correspondence of the Holothu- 

 rians with the typical formula of the whole class 

 of Echinoderms, I will return to the Sea-Urchins, 

 since they are more nearly allied with that Order 

 than with any of the other groups. We have seen 

 that the Sea-Urchins approach most nearly to the 

 sphere, and that in them the oral region and the 

 sides predominate so greatly over the ab-oral 

 region, that the latter is reduced to a small area 

 on the summit of the sphere. In order to trans- 

 form the Sea-Urchin into a Holothurian, we have 

 only to stretch it out from end to end till it be- 

 comes a cylinder, with the oral region or mouth 

 at one extremity, and the ab-oral region, which, 

 in the Holothurian, is reduced to its minimum, 



Holothurian. 



at the other. The zones of the Sea-Urchin now 

 extend as parallel rows on the Holothurian, run- 

 ning from one end to the other of the long cylin- 

 drical body. On account of their form, some 

 of them have been taken for Worms, and so clas- 



