THE ENDOSKELETON 



135 



ternally. Examples of the former may be seen in salamanders 

 and in many snakes, in which the caudal region, that posterior 

 to the cloacal orifice, may be even more extensive than the 

 remainder of the body; the opposite condition appears in the 

 frog 1 , where the long caudal notochord of the tadpole becomes 

 in the adult consolidated into an unsegmented urostyle, situ- 



Iated between the two elongated ilia and' entirely enclosed by 

 the soft parts. Similar reductions are found in birds, in which 

 the tail skeleton consists of six free and six anchylosed verte- 

 brae, and in the higher anthropoids, in which the 3-5 embryonic 

 vertebrae become in the adult consolidated into a single piece 

 (coccyx). 



There are two distinct sets of ribs developed among Ver- 

 tebrates, having a slightly different history, but subserving the 

 same general purpose, that of protecting the viscera, and of 

 furnishing attachments for the muscles. Since one set is suf- 

 ficient for use in the same animal, both do not occur simul- 

 taneously save in a single instance, but the one set is, with some 

 exceptions, characteristic of fishes, the other of higher forms. 

 The origin of the first of these sets has been already explained 

 in the discussion of vertebrae, where they were described in 

 teleost fishes as expansions of the lower or haemal arches. 

 The other ribs have in their origin no direct connection with 

 the vertebrae, that is, they are not derived from portions of 

 them, but develop from the free edges of the intermuscular 

 septa, the myocommata, where they border upon the visceral 

 cavity. This process of rib formation does not necessarily 

 involve the entire free edge of the septa, but is confined in 

 lower forms to the extreme dorsal region, bordering on the 

 vertebrae, with which they articulate. Thus in selachians, 

 almost the only fish that possess this sort of rib, and in amphib- 

 ians, they are very short, being functionally scarcely more 

 than movable tips for the transverse processes and of no value 

 for the protection of the viscera. They make no attempt to 

 reach around the body, and thus never come into relation with 

 a sternum. This latter, to us the typical relation for ribs to 

 assume, appears first in reptiles, which thus form the prototype 



