226 COMPARATIVE ZOOLOGY. 
ing also a radiating body, but a closed alimentary canal, 
and a distinct, symmetrical nervous system, constitute the 
subkingdom Lchinodermata. Animals, like the Snail, 
with a soft, unsymmetrical body, well-developed digestive 
apparatus, and scattered nervous system, form the sub- 
kingdom J/ollusca. Animals, like the Bee, with a sym- 
metrical body composed of numerous segments, a nervous 
system consisting of a double chain of ganglia along the 
lower side of the body, and limbs on the same side as this 
nervous cord, form the subkingdom Articulata. Animals, 
like the Ox, having a double nervous system, one (the gan- 
glionic) lying on the upper side of the alimentary canal, 
the other and main part (spinal) lying along the back, 
and completely shut off from the other organs by a parti- 
tion of bone or gristle, known as the “ vertebral column,” 
and having limbs, never more than four, always on the 
side opposite the great nervous cord, constitute the sub- 
kingdom Vertebrata. 
Comparing these great divisions, we see that the Verte- 
brates differ from all the others chiefly in having a double 
body-cavity and a double nervous system, the latter lying 
above the alimentary canal; while Invertebrates have one 
cavity and one nervous system, the latter being placed 
either below or around the alimentary canal. The Ar- 
ticulate type differs from the Molluscan mainly in being 
jointed. The Echinoderms and Ccelenterates are built on 
the common type of a star; but they differ from each oth- 
er in the presence or absence of distinct alimentary, circu- 
latory, and nervous systems. 
But there are types within types. Thus, there are five 
modifications of the Vertebrate type — Fish, Amphibian, 
Reptile, Bird, and Mammal; and these are again divided 
and subdivided, for Mammals, e. g., differ among them- 
selves. So that in the end we have a constellation of 
groups within groups, founded on peculiar characters of 
