CLASSIFICATION OF ANIMALS 17 



shows evidences of metamerism in the arrangement of his 

 ribs, vertebrae, spinal nerves, muscles, etc. The common 

 earthworm is a better example for its body is composed of 

 a chain of very similar metameres. 



4. Symmetry Radial or Bilateral (Fig. 15). Some animals 

 have the chief organs arranged around a central axis. The 

 form is usually disc-like or spherical, perhaps also with pro- 

 jecting arms. In such radially symmetrical animals there 

 are a number of antimeres, or body parts which might be 

 interchanged without destroying the original symmetry, 

 just as one could exchange two quarters of a pie without 



FIG. 15. A jelly-fish and a turtle, showing radial and bilateral symmetry 



respectively. 



spoiling its general shape. Jelly-fishes, corals, and star- 

 fishes are examples. Those which grow fixed to some ob- 

 ject have distal (free) and proximal (attached) ends; those 

 which move freely have no "head" or "tail" end, but 

 possess oral (mouth) and aboral surfaces. In bilaterally 

 symmetrical animals, on the other hand, the body has a 

 front or anterior end, which often is distinguished as a head. 

 The opposite end is the posterior or hind portion; there are 

 right and left sides; a dorsal or back, and a ventral, or belly 

 side. Such animals are symmetrical on two sides of a plane 

 which passes down the middle of the dorsal and ventral 

 sides. The portions on either side of this plane are called 



