90 ZOOLOGY. 



2. Symmetry relates to the possibility of passing one or more planes 

 through the animal and obtaining similar portions on either side of these 

 planes. 



If no such division is possible the organism is described as without 

 symmetry. If each of three mutually perpendicular planes separates the 

 animal into equivalent portions, it may be described as universally sym- 

 metrical. If no plane transverse to the main axis can divide the animal 

 into symmetrical parts, and two or more, which split the animal along 

 the main axis, are capable of doing so, we describe the form as radially 

 symmetrical. When there is only one such plane capable of separating 

 the body into equal parts we have the condition of bilateral symmetry, 

 which represents the highest condition of development, that of active 

 animals. 



4. Antimeres are parts of animals repeated on different sides (two or 

 more) of the main axis of the body. 



5. Metameres are parts repeated in the main axis, i. e., one behind 

 another. The successive metameres may be almost entirely alike (homon- 

 omous), or they may become much differentiated in the performance of 

 diverse functions (hcteronomous). 



6. From the main trunk of animals special appendages often appear. 

 They usually adapt themselves to, and accentuate, the fundamental sym- 

 metry of the organism. They may therefore be asymmetrically placed, 

 or uniformly distributed over the entire surface, or along the radii, or in 

 pairs as in the bilaterally symmetrical forms. There are typically one or 

 more pairs to each metamere, though this number may be much reduced. 

 Paired appendages, in series, are regarded as homologous. 



7. Many animals have the power of restoring by growth parts or seg- 

 ments which have been lost (regeneration). In the lower segmented 

 forms this power is closely associated with the power of increasing the 

 number of new segments in an uninjured animal. In heteronomously 

 segmented animals both these powers are less manifest. 



124. Topics for Investigation. 



1. Determine the nature and degree of symmetry in (i) the sponge 

 of commerce; (2) skeleton of starfish; (3) crayfish or grasshopper. 



2. What is the final criterion by which you determine which is the 

 anterior and which the posterior end of an animal? Justify. 



3. Find among animals of your acquaintance instances of difference 

 between the dorsal and ventral surfaces as to color, form, etc., and see 

 if you can discover any possible advantage resulting therefrom. 



4. What degree of difference have you ever noticed between the right 

 and left halves of the body in various animals? Is perfect bilateral sym- 

 metry ever found? 



5. Can you assign any reason for the location of the sense organs at 

 the anterior end of the body? (Distinguish between cause and advan- 

 tage.) Do you think they occur here because it is anterior, or is it ante- 

 rior because they occur here? 



