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 



