524 FISHES. 



Except in Elasmobranchs the ova are relatively small, and 

 large numbers are usually laid at once. In Elasmobranchs, 

 the egg is large, and in the oviparous genera it is enclosed 

 in a "mermaid's purse." 



Most sharks and a few Teleosteans are viviparous, the 

 eggs being hatched within the body of the mother, in the 

 lower part of the oviduct in sharks, in the ovary in Teleos- 

 teans. In two of the viviparous sharks (Mustclus Icevis and 

 Carcharias glaucus] there is an interesting union between 

 the yolk sac and the wall of the oviduct, which should be 

 compared with a similar occurrence in two lizards, and with 

 the placenta of most Mammals. 



As to fertilisation, the usual process is that the male 

 deposits spermatozoa or "milt" upon the laid eggs or 

 " spawn," but fertilisation is of course internal when the eggs 

 are enveloped in a firm sheath, or when they are hatched 

 within the mother. 



Most Fishes have a great number of offspring, and parental 

 care is proportionately little. Moreover, the conditions of 

 their life are not suited for the development of that virtue. 

 When it is exhibited, it is usually by the males, witness the 

 sea horse (Hippocampus] and the pipe fish (Syngnathus), 

 which hatch the eggs in external pouches, and u the male of 

 some species of Anus, who carries the ova about with him 

 in his capacious pharynx." The female of Aspredo carries 

 the eggs on the under surface of the body until they are 

 hatched, much in the same way as the Surinam toad bears 

 her progeny on her back, while in Solenostoma a pouch for 

 the eggs is formed by the ventral fins and skin. At least a 

 dozen kinds of fishes make nests, of which the most familiar 

 illustration is that of the male stickleback, who twines grass 

 stems and water weeds together, glueing them by mucus 

 threads exuded as semi -pathological products from the 

 kidneys, which are compressed by the enlarged male organs. 



Fishes have a less definite limit of growth than most other 

 Vertebrates, and it is rare for a fish to exhibit any of the 

 senile changes associated with old age in other Vertebrates. 

 But surroundings and nutrition affect their size and colour 

 very markedly. Some marine forms, such as flounders, may 

 survive being shifted to fresh water, while others, such as 

 salmon and sturgeon, pass from sea to rivers at spawning 



