THE ANIMAL BODY 121 



posed many of the chief organs and organ systems. Accord- 

 ingly the Coelentrates, since they lack the coelom, are often 

 referred to as ACOELOMATBS, and the animals above the 

 Coelenterates, since they possess the coelom, are known as 

 the COELOMATES. The difference in structure can best be 

 made clear by comparing the body plan of a higher Inverte- 

 brate, such as the common Earthworm, with that of Hydra. 



C. EARTHWORM 



Whereas the Hydra body is essentially a single tube com- 

 posed of two layers of cells surrounding the enteric cavity, 

 the body of the Earthworm is built on the plan of a tube 

 within a tube the outer tube forming the body wall, and 

 the inner, the wall of the ALIMENTARY CANAL. The walls of 

 these tubes become continuous, or merge into each other at 

 both ends, and thus together they enclose a space, the coelom. 

 Or, to state it another way: the outer tube, or body wall, 

 surrounds a space, the coelom: through the coelom runs a 

 second tube, the alimentary canal, which opens to the ex- 

 terior at either end forming the mouth and anus. (Fig. 66.) 



The coelom of the Earthworm is divided by a large number 

 of transverse partitions, called SEPTA, which extend from the 

 inner surface of the body wall to the outer surface of the 

 alimentary canal. The result is that the worm's body cavity 

 is not a continuous space running from one end of the ani- 

 mal to the other, but consists of a linear series of chambers 

 through the center of which runs the alimentary canal. The 

 limits of these chambers are indicated on the outside of the 

 worm by a series of grooves which encircle the body wall. 

 In short, the body is made up of a series of essentially similar 

 units known as METAMERES, and thus affords a simple exam- 

 ple of METAMERISM, which is a characteristic of all the higher 

 animals. (Fig. 67.) 



