80 



PRINCIPLES OF ANIMAL BIOLOGY 



Certain other organs are repeated in only a few segments, but in general 

 the earthworm's structure is that of a metameric animal. 



In complex animals the metameric arrangement has often become 

 obscured through fusion of somites, loss of organs, and centralization. 

 The primitive arrangement, however, is readily seen in the embryos of 

 such animals. Thus the embryos of the vertebrates generally reveal a 

 well-marked metamerism in certain organs (the muscles, for example), 

 in which this arrangement is later partly or completely lost. Not all 

 metamerism has been lost even in the adult vertebrates, however, for 

 it may be seen in the vertebrae and ribs (Fig. 79), spinal nerves and 

 ganglia (Fig. 117), and branches of the dorsal artery. 



Body Cavities. — Most of the higher animals have a cavity of some sort 

 in their bodies, but these cavities are of several kinds. In Hydra (Fig. 59) 



— VERTEBRA 



KIDNEY 



ONODUCT 

 GONAD 

 MESENTERY 

 PERITONEUM 



COELOM 



ENTERON 

 (INTESTINE) 



Fig. 66. — Relations of body cavities (enteron and coelom). At left, the earthworm; at 



right, cross section of a vertebrate animal. 



there is but one cavity, which is open at one end, the mouth, and closed 

 at the other end. A cavity so constructed is called a coelenteron, though 

 in Hydra, in recognition of its function of digestion and its assumption of 

 some of the tasks of a blood system, it is often named the gastrovascular 

 cavity. Flatworms also have a coelenteron. Undigested food must, in 

 such animals, be ejected through the mouth. 



In most of the metazoa there are two cavities. One is in the digestive 

 tract, the other lies between the digestive organs and the body wall. 

 The digestive cavity in most complex animals is open at both ends and 

 to distinguish it from the closed sac in Hydra is known as the enteron. 

 The space between the digestive organs and the body wall is the coelom. 

 These relations are shown diagrammatically for the earthworm in Fig. 66 

 (left). In vertebrate animals (right) the cavities are in the same relative 

 position; the coelom appears to be filled with many organs. These, 

 however, arc merely pushed into it from the outside. Since some animals 



