330 ZOOLOGY 



first gill arch, which swings into position in the course 

 of development. Thus it may be said that our posses- 

 sion of a lower jaw depends on the fact that our ances- 

 tors were aquatic. The exact number of bones in a 

 skull depends upon the amount of fusion and modifica- 

 tion which takes place in development. Thus in the 

 upper jaw of man there is no separate piece (the pre- 

 maxilla) in front, except at an early age. This part 

 completely fuses with the main body of the jaw; but 

 in various other animals it is permanently distinct. 

 Pectoral and 4. The pectoral and pelvic girdles serve for the attach- 

 gkdies ment of the anterior and posterior limbs respectively. 



In the lowest fishes (sharks and rays) the girdles are 

 represented by cartilaginous structures of simple form, 

 but in higher vertebrates they are more complex, and 

 are represented by several different bones. In man we 

 recognize a scapula or shoulder blade, and clavicle or 

 collar bone, forming the pectoral girdle. The clavicle 

 is not present in all animals ; thus in the ungiilates or 

 hoofed animals it is absent. This absence is evidently 

 due to the loss of the structure, and it has been reported 

 that traces of it may be found in the embryo of the 

 sheep. On the other hand, the scapula is compound, 

 consisting primitively of more than one bone. Near 

 the concave surface for the articulation of the first bone 

 of the arm is a process which seemed to the anatomists 

 of olden times (who possessed a very lively imagina- 

 tion !) to resemble the head and beak of a crow (cor ax). 

 Hence they called it the coracoid process. Later on it 

 was discovered that in various vertebrates (e.g., birds) 

 the coracoid process is represented by a large and im- 

 portant bone, to which the name coracoid bone was 

 given. This coracoid bone represents in fact the ven- 

 tral portion of the primitive pectoral arch, joining the 



