PHYLUM CHORDATA 229 



tion has been more recent than that of any other group of 

 animals, more facts are available concerning the lines 

 specialization has followed. 



In the most primitive vertebrates the chorda is functional 

 throughout life^nd the vertebrae appear only as little carti- 

 lages between the myomeres, or muscle segments. But as 

 the complexity of the skeleton progresses in the vertebrate 

 series, there is a transition from chorda to cartilage, and 

 again to bone. Some vertebrates are as adults only a little 

 beyond the chorda stage/ others, like the sharks, have 

 developed a cartilaginous skeleton, but go no further. The 

 majority of vertebrates, however, pass through three stages 

 in their development: (1) having a chorda, replaced by 

 (2) cartilaginous vertebrae, which in turn give place to (3) 

 bone. Paired limbs % like wise show a rather definite se- 

 quence in their specialization. They are absent in the 

 most primitive vertebrates and appear first as flaps sup- 

 ported by cartilaginous rays. Fins with bony rays are the 

 next step, and these are later replaced by pentadactyl (five- 

 toed) limbs. The pentadactyl limb represents a great ad- 

 vance in racial development when compared with a fin, 

 bi;t it has now been so greatly modified in many groups as 

 to be scarcely recognizable. A bird's wing or a horse's 

 foot show little indication of the ancestral five-toed condi- 

 tion. Most snakes and the blind worms have lost the 

 paired appendages completely. 



The vertebrate circulatory system also affords a good 

 illustration of progressive specialization and development. 

 The most primitive vertebrates have a tubular heart (Fig. 

 105), bent somewhat to be sure, but still nothing but a 

 tube, which pumps blood through six aortic arches to the 

 gills. Very soon the number of arches is reduced to four, 

 then to three* In birds and mammals the arches appear 

 as such only in early development, and in the adult condi- 

 tion all but a single arch on one side of the body have been 

 lost or diverted to other uses. In fishes the heart presents 

 a simple tubular condition; in amphibians and in most rep- 



