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 
