THE STRUCTURE AND ACTIVITIES OF A FISH. 175 
flexible axis for the body, and separates the neural 
from the visceral cavities. 
In the young of all vertebrates a rod of cartilage 
called the notochord runs through the body between 
the two cavities just mentioned. In some of the lower 
vertebrates this rod persists throughout life, but in nearly 
all cases it disappears with the hardening of the centra 
of the vertebrae. The soft substance extending through 
the centra of the vertebrae of the fish probably repre¬ 
sents in that animal the remnant of a notochord. In 
all adult vertebrates higher than fishes there is left no 
trace of this structure. 
The neural cavity, as its name implies, contains the 
main nerve of the body, the spinal cord. This cord is 
enlarged at its anterior end in almost all vertebrates, 
forming a brain. In all higher vertebrates and in by 
far the greater part of those lower in organization, this 
brain is enclosed in a bony box called the skull. 
Paired organs of sense are connected with the brain. 
The visceral cavity contains the organs of digestion, 
the heart, and organs of reproduction. In higher 
vertebrates it also contains the lungs. 
In the great majority of vertebrates there are two 
pairs of limbs. These are, in the higher vertebrates, 
usually attached to the main axis of the body by con¬ 
necting bones called respectively the pectoral girdle 
and the pelvic girdle. Even in vertebrates which have 
no arms or legs rudiments of these girdles may some¬ 
times be found. 
Fishes. The fishes (Pisces) include animals which 
live in water and breathe by gills. The body is usually 
covered with scales, and the form is adapted for loco¬ 
motion through the water. The paired fins are chiefly 
used for balancing the body, and the caudal fin is the 
principal organ of locomotion. Although nasal sacs 
and nostrils are present, they do not connect with the 
mouth or throat. There are no external ears. In 
most fishes an air-bladder is present, often connecting 
