50 CLASSIFICATION AND CREATION. 
tubular as in the Polyps. To carry the com- 
parison further, widen the partitions between the 
chambers of the Polyps, and the chambers are then 
reduced to narrow tubes, which completes the ho- 
mology. In Echinoderms the difference consists, 
as we have seen, in the fact that the various cav- 
ities of the body, instead of being simply scooped 
out of its substance, have walls of their own ; 
these walled cavities being enclosed as intestines 
by the outer wall of the body. I shall return to 
this subject again, when. I explain the homolo- 
gies of Radiates more in detail, but have thought 
it well to allude to it here in connection with 
this more general sketch of their structure. 
I have mentioned only three classes of Radi- 
ates. Cuvier had five in his classification ; for 
he had placed among them the Intestinal Worms 
and the Infusoria or Animalcules. The Intes- 
tinal Worms are much better known now than 
they were in his day. Their anatomy and em- 
bryology have been traced, and it has been shown 
that the essential features of these parasites are 
the same as those of all Articulates, their whole 
body being divided into successive movable joints 
or rings. Cuvier was misled by the circular ar- 
rangement of certain parts around the mouth, 
and by the presence of a wreath of feelers around 
the head of some of these Worms, resembling the 
tentacles of many Radiates. This is, however, 
