172 ANIMAL FORMS 
or exhaustion after the breeding season is passed. The 
eggs, like those of the chubs, suckers, sunfishes, and cat- 
fishes, are usually buried in shallow holes in the sand, and 
the males of most fishes keep a faithful watch over the 
young until they are able to live in safety. In some of 
the sticklebacks and several marine species elaborate nests 
are composed of grass or seaweeds; some of the catfishes 
carry the eggs until they hatch in their mouths or else in 
folds of spongy skin on the under side of the body; in the 
pipefishes and sea-horses a slender sac along the lower sur- 
face of the male acts as a brood-pouch, in which the female 
places the eggs to remain until developed; and some fishes, 
such as the surf-fishes and a number of the sharks, bring 
forth their young alive. On the other hand, the young of 
many of the herrings, salmon, cod, perch, and numerous 
other fishes are abandoned at their birth, and fall a prey to 
many animals, even their parents often included. 
In the former cases, where the young are protected, only 
a relatively few eggs are produced: where they are aban- 
doned the female often lays many millions. In every case 
the number of eggs is in direct relation to the chances the 
young have of reaching maturity, a few out of each brood 
surviving to perpetuate the race. 
165. Development and past history.—The eggs of the 
higher bony fishes are usually small (one-tenth to one-third 
of an inch in diameter), and the young when they hatch 
are accordingly little; in the sharks the eggs are larger, 
the size of a hen’s egg or even larger, and the young when 
born are relatively large and powerful. These differences, 
however, do not greatly affect the early development, for 
in every case the head and then the trunk soon become 
formed, gills arise, the nervous system appears, which is 
invariably supported by a skeleton in the form of a gristly 
rod—the notochord. In the lower forms of fishes this per- 
sists throughout life; but in the sharks and skates it be- 
comes replaced in the adult by another and higher type of 
