CATEGORIES OF CLASSIFICATION. 37 



Leuckart proposed to subdivide the Radiates in- 

 to two groups : the Coelenterata, including Polyps 

 and Acalephs or Jelly-Fishes, and Echino- 

 derms, including Star-Fishes, Sea-Urchins, and 

 Holothurians. His reason for this distinction 

 is the fact, that in the latter the organs or 

 cavities of the body have walls of their own, 

 distinct from the body-wall ; whereas in the 

 former they are formed by internal folds of the 

 outer wall of the body, as in the Polyps, or are 

 hollowed out of the substance of the body, as 

 in Jelly-Fishes. This implies no difference in 

 the plan, but merely a difference in the execu- 

 tion of the plan. Both are equally radiate in 

 their structure ; and when Leuckart separated 

 them as distinct primary types, he mistook a 

 difference in the material expression of the 

 plan for a difference in the plan itself. 



So some naturalists have distinguished Worms 

 from the other Articulates as a separate prime 

 division. But the structural plan of this type is 

 a cylinder divided by transverse constrictions or 

 joints ; and whether those joints are uniformly 

 arranged from one end of the body to the other, 

 as in the Worms, or whether the front joints 

 are soldered together so as to form two regions 

 of the body, as in Crustacea, or divided so as 

 to form three regions of the body, as in winged 

 insects, does not in the least affect the typical 



