234 
COMPARATIVE ZOOLOGY. 
alimentary canal, and a distinct symmetrical nervous sys¬ 
tem, constitute the subkingdom Echinodermata . 136 Ani¬ 
mals like the Angle-worm, bilaterally symmetrical, one- 
jointed, or composed of joints following each other from 
front to rear, with no jointed limbs, constitute the sub¬ 
kingdom Vermes . Animals like the Snail, with a soft, 
unjointed body, a mantle, a foot, a two or three cham¬ 
bered heart, and a nervous system in the form of a ring 
around the gullet, constitute the subkingdom Mollusca. 
Animals like the Bee, with a jointed body and jointed 
limbs, form the subkingdom Arthropoda. Animals like 
the Sea-squirts, sack or barrel shaped, with a mantle cav¬ 
ity penetrated by an excurrent and an incurrent opening, 
with heart and gills, form the subkingdom Tunicata . An¬ 
imals like the Ox, having a double nervous system, one 
(the sympathetic) lying on the upper side of the aliment¬ 
ary canal, the other and main part (spinal) lying along the 
back, and completely shut off from the other organs by a 
partition of bone or gristle, known as the “ vertebral col¬ 
umn,” and having limbs, never more than four, always on 
the side opposite the great nervous cord, constitute the 
subkingdom 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 Vermes 
are closely related to all the following subkingdoms of 
Invertebrates, most nearly to Mollusks and Tunicates, 
while the latter have affinities with the Vertebrates. The 
Echinoderms and Coelenterates are built on the common 
type of a star; but they differ from each other in the 
presence or absence of distinct alimentary, circulatory, 
and nervous systems. 
