204 



BONY FISHES 



vir. 9- 



concentrated than its blood. The chloride-secreting cells dispose of 

 the excess salts. It remains to explain the means by which a solution 

 passes against the osmotic gradient from the cavity of the gut to the 

 blood; the membranes here must have some special properties of 

 which we are ignorant. Sodium and chloride enter with the water but 

 magnesium is excluded and may be precipitated in the intestine. 



The genital system is nearly completely separated from the excre- 

 tory in both sexes. The testes (soft roes) are a large pair of sacs opening 



into the base of the urinary 

 ducts. The ovaries (hard roes) 

 are also elongated in the trout 

 and the eggs are shed free into 

 the coelom (Fig. 123) and 

 passed to the exterior by 

 abdominal pores. This condi- 

 tion is unusual among teleosts, 

 in most the ovaries are closed 

 sacs, continuous with the ovi- 

 ducts. 



Fertilization is external and 

 the eggs of the trout are shed 

 in small pits or depressions in 

 the sand; being sticky, they 

 become attached to small 

 stones. The eggs are very 

 yolky and cleavage is therefore only partial, forming a cap of cells, 

 the blastoderm, which eventually differentiates into the embryo. 

 After hatching, the young fish may still carry the yolk sac and 

 obtain food from it for some time, while beginning to eat the small 

 crustaceans and other animals that are its first food. 



10. Races of trout and salmon and their breeding habits 



There is considerable confusion about the various races of trout 

 and their allies the salmon. In both trout and salmon the adult 

 originally spent the great part of its life in the sea but returned to the 

 rivers to breed. Trout and salmon that do this are still abundant on 

 the West Atlantic coasts and ascend all suitable rivers to breed, the 

 process being known as the 'run'. But the trout has produced many 

 races of purely fluviatile animals, living either in lakes (where they often 

 become very large) or rivers, and never returning to the sea to breed. 

 These freshwater races differ in small points from each other and are 



Fig. 123. Ovaries and kidneys of A, typical 



teleostean fish, B, trout and some other fishes 



where the eggs are shed into the coelom. 



(From Norman, after Rey.) 



