THE INVERTEBRATE BODY 



107 



hand in hand with change in function, so that although there is no 

 obvious resemblance between the jaws of the Crayfish and the legs 

 employed for swimming, nevertheless a study of their develop- 

 ment shows beyond doubt that they owe their origin to modifica- 

 tions of one primary type. Accordingly the various appendages 

 are said to be homologous, signifying a fundamental similarity 

 of structure based on descent from a common antecedent form. 

 (Figs. 64, 65.) 



On the other hand, organs of fundamentally dissimilar structure, 

 which nevertheless perform the same function, are called analo- 



Dorsal abdominal artery 

 Intestine 



Tergum 



Pleura 



Abdominal muscles 



Ventral 

 nerve cord 



Muscles of 

 appendage 



Protopodite 



Endopodite 

 Exopodite 



Sternum 



Ventral abdominal artery 



Fig. 65. — Crayfish. Transverse section of fifth abdominal segment. 



gous. In Insects the series of head appendages and the legs are 

 homologous with those of the primitive Arthropod type, while the 

 wings are new, unrelated structures and not modifications of the 

 primitive serial appendages of the ancestral form. However, as 

 we shall see later, the wing of a Bird and the arm of Man are 

 homologous, while the wing of an Insect and the wing of a Bird 

 are analogous structures. One of the chief tasks of the branch of 

 biology known as comparative anatomy is to determine the 

 various parts of animals which are homologous, and to study the 

 adaptive changes which are associated with change of function. 

 (Fig. 227.) 



