EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 37 



of the bird is homologous to the arm of man. There is another type of 

 homology in which two or more structures in one individual are com- 

 pared. Fundamental similarity of structure between one part of an animal 

 and another part of the same animal is called serial homology. 



An example of serial homology is seen in the arm and leg of man. The 

 segment of each which is attached to the trunk has a single bone as skele- 

 tal support; in the arm this bone is called the humerus (Fig. 3.2); in the 

 leg it is called the femur. In the succeeding segment of the arm and leg 

 there are two bones, called radius and ulna in the arm, tibia and fibula in 

 the leg. Then come a group of wrist and ankle bones, respectively, called 

 carpals in the arm, tarsals in the leg. Next are the bones of the palm of 

 the hand and the sole of the foot, metacarpals and metatarsals, respec- 

 tively. Finally, the bones of fingers and toes are called phalanges in both 

 cases. Evidently, then, our fore and hind limbs are modifications of the 

 same fundamental pattern, modified for grasping and handling in the one 

 case, for locomotion in upright posture in the other. 



Much more elaborate examples of serial homology are afforded by the 

 jointed appendages of invertebrates. Examination of the numerous ap- 

 pendages of a lobster or crayfish reveals that those in difl'erent parts of the 

 body have much resemblance despite the fact that they are modified for a 

 variety of functions (Figs. 3.10 and 3.11). The most conspicuous pair are 

 the "pinchers" or chelae, used by the animal in grasping food and in 

 fighting. Just behind the chelae are the four pairs of walking legs, used in 

 slow locomotion along the bottom of the stream in which the animal lives. 

 Behind the walking legs and attached to the abdomen are several pairs of 

 swimmerets (XIV and XVI in Fig. 3.1 1; not shown in Fig. 3.10). These 

 are small appendages; their name gives a false impression of their im- 

 portance in swimming. In females masses of eggs become attached to 

 them, hanging like tiny bunches of grapes while embryonic development 

 progresses. In males the first pair of swimmerets (XIV in Fig. 3.11) is 

 modified for the transference of sperm cells to the female. A broadly ex- 

 panded, somewhat paddlelike structure will be noted at the end of the 

 abdomen (Fig. 3.10). When the creature wishes to move rapidly it 

 flexes or bends its abdomen powerfully, this terminal structure off'ering re- 

 sistance to the water much as does an oar. As a result the body shoots 

 backward with great speed. The terminal structure employed in this 

 maneuver is composed of a flap (telson. Fig. 3.10) attached to the last 

 segment of the body, augmented by flattened appendages on either side, 

 the uropods (Fig. 3.10; XIX in Fig. 3.11). 



Anterior to the chelae is found a succession of appendages modified for 

 a variety of functions. Some of them, maxillipeds and maxillae (VII and 



