THE INVERTEBRATE BODY 



101 



cavity, or coelom, in which are disposed many of the chief organs. 

 Accordingly the Coelenterates, with an enteron but no coelom, 

 are referred to as Acoelomates, and the animals above the Coelen- 

 terates, since they possess the coelom, are known as the Coelo- 

 mates. The difference in structure can best be made clear by 

 comparing the body plan of a higher Invertebrate, such as the 

 common Earthworm, with that of Hydra. (Fig. 313.) 



1. Body Plan 



Whereas the Hydra body is essentially a sac composed 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 



Mouth 



Anus 



limentary 

 canal 



Aortic loop 



Cerebral 

 ganglion 1 



Mouth 



Capillaries 



Dorsal blood vessel 



Anus 



Ventral 

 ganglia 



Ovary \ Sub -intestinal 

 Oviduct blood vessel Nephridia 



Fig. 59. — Diagrams of the body plan of the Earthworm. A and C, longitu- 

 dinal sections; B, transverse section. (From Sedgwick and Wilson.) 



outer tube forming the body wall, and the inner, the wall of the 

 digestive tract, or alimentary canal. The walls of these tubes 

 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, in which is suspended 

 a second tube, the alimentary canal, which opens to the exterior 

 at either end forming the mouth and anus. (Figs. 59, 60.) 



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 



