PROMORPHOLOGY. 89 



metrical animals (Figs. 129, 135, 188). The paired appendages of bilat- 

 eral animals may be attached dorsally, laterally, or ventrally, as deter- 

 mined by the uses they serve. They may be uniformly distributed along 

 the axis, one or more pairs to each metamere, as in some arthropods, or 

 be confined to special segments in certain regions of the body, as in the 

 higher arthropods and the vertebrates. 



120. Practical Exercises. Let the student find illustrations, from the 

 animals with which he is acquainted, of paired appendages with dorsal 

 attachment; with lateral; with ventral. What is the work to be done by 

 each? Does their position appear to be of advantage in the performance 

 of it? Find likewise animals in which the appendages are clustered at 

 the anterior portion of the body; some in which only posterior appendages 

 are found, or at least are better developed than the anterior. Does the 

 arrangement seem in any way related to the habits and surroundings of 

 the animal ? 



121. Specialization of Metameres. In the lowest segmented animals, 

 as worms, the metameres are much alike in external form, in their ap- 

 pendages, and in the contained structures. In the adult insects this be- 

 comes less true, and the various segments are specialized for particular 

 duties. The segments in the head region become very different from the 

 body segments. The same is even more true of the higher vertebrates. 

 This progressive differentiation of a distinct head is one of the most 

 remarkable facts to be noted in animal development. Accompanying the 

 specialization of groups of segments in various parts of the body we often 

 see the complete fusion of such similar segments for the better perform- 

 ance of their common work (as in the head of insects and vertebrates, 

 the thorax in insects and Crustacea). 



122. Formation of New Segments and Regeneration. In many of 

 the animals in which the segments seem of nearly equal value there is a 

 more or less continuous formation of new segments. By this process the 

 organism increases in length and in the number of its segments, and 

 frequently, with the aid of a somewhat similar process, produces two 

 individuals by division, as in many worms. Such a proceeding necessi- 

 tates the formation of a new head or tail in each of the daughters, 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. Nat- 

 urally 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. 



123. Summary. 



i. Promo rphology treats of the ground plan in accordance with which 

 the parts of animals are combined. 



