PROMORPHOLOGY. 8 7 



zontally placed, with poles alike (right-left axis). Such an animal has 

 only one plane (the sagittal) by which it may be divided into symmetrical 

 halves (Figs. 49, 29, 96). 



117. Antimeres. There is a striking tendency among organisms to 

 repeat or duplicate organs or parts. This we have seen in the occurrence 



FIG. 48. 



m 



ab. o. 



FIG. 48. Diagram of the sea-anemone, illustrating another type of radial symmetry. 

 A, cross section; B, longitudinal section. Lettering as in Fig. 47. c, the chamber be- 

 tween the mesenteries (m). 



Questions on the figure. How does the symmetry of the anemone 

 compare with that of the Medusa? Express the difference clearly in terms 

 of the axes and their poles. Are aa 1 and bb 1 strictly similar axes? Do 

 their planes divide the animal into halves? Are the four halves thus 

 obtained equivalent? In B what difference in the position of the section 

 will account for the differences on the right and left side of the figure? 



of similar rays about the main axis, in radially symmetrical animals like 

 the starfish. Parts thus repeated are known as antimeres. The term is 

 also applied to the right and left or paired halves of bilaterally symmet- 

 rical animals. 



118. Metameres. When the parts or organs are repeated in a linear 

 sequence along the main axis, as in the segments or rings of the Earth- 

 worm, the arrangement is described as segmental or metameric. Metam- 

 erism may be shown both by the internal and external structures. The 

 vast majority of the elongated, bilaterally symmetrical animals are seg- 



