12 Introduction to Aiiimal Morphology. 



In comparing one persona with another, we call 

 such parts homologous as are of equal morphological 

 value, so that if the two compared forms were descend- 

 ants from a common parent, the homologous parts 

 would be developed from the same parts of the embryo ; 

 thus the wing of a bird and the fore-limb of a horse 

 are homologous. Homologies may be complete or 

 incomplete, according as the organs compared are in 

 whole, or in part only, the equivalents of each other. 

 Thus the humerus of man and that of the frog are 

 completely homologous, while the heart of a fish and 

 that of a man are incompletely so. In comparing 

 different metameres of the same persona, correspond- 

 ing parts are said to be homodynamic (homotypical, 

 or serially homologous) ; thus fore and hind limb are 

 homodynamic parts. In comparing different antimeres 

 of each metamere, the corresponding parts are said to 

 be symmetrical. In each antimere, if organs are re- 

 peated, the repetitions are called homonomous ; thus 

 the fingers and toes of the extremity are homonomous 

 segments. Parts which resemble in function, but 

 having no necessary structural relationship, are said 

 to be analogous ; thus the lungs of a bird and the gills 

 of a fish agree in function, but are in no sense homo-r 

 logous. The wings of insects and of birds are likewise 

 analogues, but not homologues. 



The science which treats of the form, structure, and 

 arrangements of parts in animals, and the laws per- 

 taining thereto, is called Morphology. The science 

 which deals with the functions, actions, and properties 

 of parts is called Physiology. In the following pages 

 the animal kingdom is viewed from a morphological 

 standpoint, 



