560 The Animal Kingdom 



other major invertebrate group. It is now clear that all these theories are 

 incorrect and that the origin of the vertebrates is still lost. Perhaps it is 

 to be looked for in some dipleurula-like larva that swam in the early 

 Cambrian seas. Certainly such a soft-bodied form would leave no trace 

 and the best that can be done is to continue to speculate. 



Characteristics of the Phylum. — Most of the characteristics of 

 the members of this phylum are not unique, for they may be seen among 

 the more highly developed of the invertebrates. Among these charac- 

 teristics are the possession of bilateral symmetry, metamerism, three 

 germ layers, a well-developed enterocoelous coelom, a complete digestive 

 tract, and an organ grade of construction. Like many of the inverte- 

 brates, all the chordates possess a closed circulatory system with a cen- 

 tral pumping mechanism, the heart; and in most forms the sexes are 

 separate. 



Three unique characteristics set the members of this phylum off from 

 all other animal groups. These three characteristics are the possession 

 of a dorsal supporting rod, the notochord, the pharyngeal gill slits, and a 

 dorsal tubular nervous system. 



The notochord is a dorsal cartilaginous rod from which the name of 

 the phylum is derived. This notochord is found at least in the embryo 

 of all chordates and in many it remains throughout life. Among the 

 higher vertebrates, the centra of the vertebral column gradually squeeze 

 it out until the cartilage remains only as pads between the vertebrae. 



Gill slits in the region of the pharynx again are seen at least in 

 the embryos of all chordates. In aquatic forms such as the fish, the gill 

 slits remain functional throughout life ; among terrestrial forms, the slits 

 normally close off during development. In the embryo, the gill slits ap- 

 pear as outpocketings of the pharyngeal lining and inpocketings of the 

 body surface. Breaks in the wall form the gill slits. In aquatic forms, 

 gill filaments develop in relationship to these slits, and water passing 

 through the mouth emerges through the gill slits and in so doing, passes 

 over the gill filaments where respiratory exchange takes place. With 

 the advent of lung breathing terrestrial vertebrates, the gill slits are no 

 longer necessary and appear only in the embryo. 



The dorsal tubular nervous system is the third unique characteristic 

 of the chordates. This system persists throughout the lifetime of all 

 but a few forms. In many, the anterior portion is differentiated into the 

 brain. Embryologically the central nervous system rises as folds of 

 the dorsal ectoderm which join and then sink below the surface. This 



