50 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



but no body cavity, have no separate nervous and vascu- 

 lar regions, and the parts of the body radiate from a 

 center. Such form a branch called Ccelenterata. Ani- 

 mals like the starfish, having also a radiating body, but 

 a closed alimentary canal, and a distinct symmetrical 

 nervous system, constitute the branch Echinodermata* 

 Animals like the angleworm, bilaterally symmetrical, 

 one-jointed, or composed of joints following each other 

 from front to rear, with no jointed limbs, constitute the 

 branch Annulata. 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 branch Mollusca. Ani- 

 mals like the bee, with a jointed body and jointed limbs, 

 form the branch Arthropoda. Anifnals like the ox, hav- 

 ing a double nervous system, one (the sympathetic) 

 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 partition 

 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 branch 

 Vertebrata. 



Comparing these great divisions, we see that the ver- 

 tebrates 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 inverte- 

 brates have one cavity and one nervous system, the 

 latter being placed mainly below the alimentary canal. 



But there are types within types. Thus, there are 

 five modifications of the vertebrate type fish, amphib- 

 ian, reptile, bird, and mammal ; and these are again 

 divided and subdivided, for mammals, e.g., differ among 

 themselves. So that in the end we have a constellation 

 of groups within groups, founded on peculiar characters 



