gS ZOOLOGY 



by a segment which, in the mother, was a body segment. When such an animal 

 is artificially cut in two, each half may reproduce segments like those which have 

 been removed from it. This is known as regeneration. Naturally this ceases to be 

 possible in animals in which the segments become more highly specialized; yet 

 even in the highest animals some power of replacing lost tissue or even lost organs 

 remains (as in healing of wounds, formation of a new tail by lizards, etc.). It is 

 recognized as a general law that in making these repairs or healings the newly 

 formed material tends to restore the symmetry possessed at the outset. 



127. Summary. — 



1. Promorphology treats of the ground plan in accordance with which the parts 

 of animals are combined. 



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. 



3. 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 symmetrical. 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 (homonomous), or they may 

 become much differentiated in the performance of diverse functions Qieteronomous). 



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

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

 metrical 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 segments 

 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. 



128. Topics for Investigation. 



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

 merce; (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 symmetry ever found ? 



