234: COMPARATIVE ZOOLOGY. 



alimentary canal, and a distinct symmetrical nervous sys 

 tern, constitute the subkingdom Echinodermata. 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 snbkingdom Arthropoda. Animals like 

 the Sea-squirts, sack or barrel shaped, with a mantle cav- 

 ity penetratecj 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, 



