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 Echinoderrns 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, 



