FORMS OF ANIMALS 53 



the case of which different metameres bear different types of appendages, 

 such as feelers, or antennae, moiithparts, and legs. 



82. Appendages. — If the body possesses such structures as have 

 just been named or others which might be added, these are termed 

 appendages. An appendage may be 

 defined as a projecting part, capable 

 of movement and performing some 

 active function. Immovable horns, 

 spines, hairs, and scales are not 

 termed appendages, but movable 

 spines, tentacles, legs, wings, and 

 tails could all be recognized as such. 



83. Homology and Analogy. — 

 Whenever parts of the body, whether 

 in the same or in different animals, 

 are similar in plan of structure, they 

 are termed homologous and the con- 

 dition is referred to as homology. 

 This usually involves similarity in 

 origin and in mode of development. 

 Whenever parts which are structur- 

 ally or morphologically different in Fig. 14. — Outline of the wings of a 



1 c ,1 r i- aU„ , butterfly and the veins in them, for compar- 



plan perform the same function, they .^^^ ^j^ ^^ ^.^ ^3 ^^ ^1^,^,^^, ^,„^io^gy. 

 are analogous, and the phenomenon 



is referred to as analogy. Thus the foreleg of a horse, the arm of a man, 

 the wing of a bat, and the wing of a bird are all homologous parts (Fig. 

 13). This homology concerns not only the division into segments which 

 correspond to upper arm, forearm, wrist, and hand but also involves the 

 skeleton, muscles, blood vessels, and nerves. The wing of a bird and the 

 wing of a butterfly, however, possess nothing in common structurally but 

 perform the same function, and the case is recognized as one of analogy. 

 The wing of a butterfly (Fig. 14) is formed by an outfolding of the surface 

 layers of the body wall, mostly of the skin, becomes jointed at the point 

 of attachment, and is moved by muscles within the body. Since all 

 likeness may be expressed in varying degrees, homology and analogy can 

 be spoken of as being more or less complete or perfect. 



