THE CHORDATES 163 



The tail disappears, and with it the notochord and the 

 greater part of the nervous system. The sense-organs van- 

 ish, the pharynx becomes remodeled, and numerous other 

 changes occur, leaving the animal in its adult condition, 

 with little in its motionless, sac-like body to remind one of 

 a vertebrate. 



151. The vertebrates. Since the remainder of this vol- 

 ume is concerned with the vertebrates it will be well at the 

 outset to gain some knowledge of their more important 

 characteristics. One of the most apparent is the presence 

 of a jointed vertebral column, composed of cartilage or 

 bone, which supports the nervous system. To it are also 

 usually attached several pairs of ribs, two pairs of limbs, 

 either fins, legs, or wings, and in front it terminates in a 

 more or less highly developed skull. In the space par^ 

 tially enclosed by the ribs, the body-cavity, a digestive sys- 

 tem is located, which consists of the stomach and intestine, 

 together with the attached liver and pancreas. The cir- 

 culatory system is also highly organized, and consists of a 

 muscular heart, arteries, and veins which ramify through- 

 out the body. Breathing, in the aquatic animals, is car- 

 ried on by means of gills, and in the air-breathing forms 

 by means of lungs, which, like the gills, effect the removal 

 of carbonic-acid gas and the absorption of oxygen. The 

 nervous system, consisting of the brain situated in the 

 head and the spinal cord extending through the body 

 above the back-bone, even in the lower vertebrates, is far 

 more complex than in the invertebrates. The sense-organs 

 also attain to a high degree of acuteness, and in connec- 

 tion with the highly organized nervous system enable these 

 forms to lead far more varied and complex lives than in 

 any of the animals heretofore considered. 



