REPRODUCTION 47 



stones. In many nest-building fishes the nest, after deposit of the eggs, 

 is guarded for a time by one of the parents, usually the male. The eggs 

 of the stickleback are placed in a rude nest which the male builds of 

 weeds glued together by a substance secreted by the kidneys. After the 

 eggs are laid the male stands guard over the nest. In the sea-horse 

 (Hippocampus, Fig. 34) and the pipe-fish (Syngnathus) the male acts as 

 nurse by carrying the developing eggs in a "brood pouch" situated on the 

 ventral surface of the body or tail, an arrange- 

 ment suggestive of the marsupial pouch of the 

 female kangaroo. In some of the cat-fishes the 

 eggs, during development, are carried in the 

 mouth of one of the parents, usually the male. 



Thick tough shells such as enclose the large 

 eggs of oviparous sharks and skates must be 

 highly protective. The curhng tendrils (Fig. 30), 



, . 1 , , J i_^i BROOD-POUCH- 



which such egg-cases commonly possess doubtless 

 serve to attach the egg in a favorable locality. 

 In the somewhat shark-like marine fish, Callo- 

 rhynchus (one of the Holocephali), the egg-case 

 may attain a length of 25 cm. Its external 

 appearance is suggestive of a strip of kelp. Fig. 34. — Sea-Horse 



Most efficient of all is the protection afforded (Hippocampus); male. 



^ with brood-pouch. (Re- 



by viviparity, exceptional among fishes, but drawn after Bouienger in 

 occurring in many sharks and skates and in a J}}^ Cambridge Natural 

 few Teleostei. 



The eggs of most oviparous fishes are so scantily endowed with yolk 

 as to be capable of only a small amount of growth until additional food 

 can in some way be obtained from the environment. Such eggs develop 

 rapidly and soon attain functional differentiation of the organs of all 

 systems except the reproductive, but with relatively little growth because 

 of the lack of building material. The resulting miniature fish at once 

 becomes actively free-living and self-supporting. It then enters upon a 

 long period of life concerned mainly with feeding and growth. The 

 bulk of the prospective adult may be hundreds of thousands or millions 

 of times that of the newly hatched young. During the brief develop- 

 mental period the embryo is directly exposed to the external water so 

 that, until specialized respiratory organs (gills) are developed, the neces- 

 sary oxygen must be obtained by absorption through such surfaces of the 

 embryo as are in contact with the water. 



In many fishes the young differs from the adult in such respects as 

 possession of adhesive discs or long spines or other organs of merely 

 temporary significance, or in form of body or fins, or in gill structure and 

 various other features. These differences may be so marked as to compel 



