THE CHORDATES 153 
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. 
145. 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. 
