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- 
