PROMORPHOLOGY 97 



121. Antimeres. — There is a striking tendency among organisms to repeat or 

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



122. Hetameres. — When the parts or organs are repeated in a linear sequence 

 along the main axis, as the segments or rings of the Earthworm, the arrangement 

 is described as segmental or metameric. Metamerism may be shown both by the 

 internal and external structures. The vast majority of the elongated, bilaterally 

 symmetrical animals are segmented. In the higher Vertebrates it is not manifest 

 externally, but is shown in the vertebrae, the nerves, etc. 



123. Appendages. — Nearly all the animals, whatever the fundamental sym- 

 metry may be, have appendages of one kind or another for locomotion, capture of 

 food, protection, respiration, and the like. These outgrowths from the body may 

 be generally distributed over the body surface (as cilia in some Protozoa and free- 

 swimming larvje) ; or radially arranged, — often about the mouth, as in many radi- 

 ally symmetrical animals (Figs. 49, 50, 81) ; or in a right and left series in bilaterally 

 symmetrical animals (Figs. 131, 137, 191). The paired appendages of bilateral 

 animals may be attached dorsally, laterally, or ventrally, as determined 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 vertebrate's. 



124. 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? 



125. Specialization of Metameres. — In the lowest segmented animals, as the 

 simpler worms, the metameres are much alike in external form, in their appendages, 

 and in the contained structures. In the adult insects this becomes 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 per- 

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

 the thorax of insects and Crustacea). 



126. 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 pro- 

 ceedmg necessitates the formation of a new head or tail in each of the daughters, 



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