12 THE FKKSII-WATKK FISIIKS OF EUROPE. 



species. 1 The liver may be simple or lobed ; it usually includes a gall-bladder, 

 and in some fishes contains a good deal of fat. The spleen is a dark red 

 organ placed near to the stomach. The intestine terminates in a pore, which 

 frequently includes the outlets for the urinary bladder and the reproductive 

 organs (Fig. 8). 



REPRODUCTIVE ORGANS. 



The sexes of fishes are always distinct ; though hermaphrodites are occa- 

 sionally met with in some groups. All European fresh-water fishes are 

 oviparous. The ovaries and milt are frequently mature long before the fish is 

 full-grown, and they usually occcupy a large portion of the abdominal cavity. 

 In the Smelt there is an internal oviduct, but commonly the eggs are dis- 

 charged into the abdominal cavity (Fig. 9). Some fishes have an external ovi- 

 duct, which is of great length in the Bitterling, Rkodeus amarus. The eggs are 



Fig. 9. INTERNAL ANATOMY OF CARP. 



br, branchiae cr gills ; c, heart ; ci, intestinal canal ; o, ovaries ; a, vent ; o', oviduct ; a', urinary outlet. 



commonly globular, but oval in the Lampreys. They vary much in size in 

 the same individual in different periods of life and in different species. In 

 the Eels they are microscopic; the Sticklebacks have large eggs for their size, 

 but among the largest eggs of our fresh- water fishes are those of the Salmon. 

 Sometimes the eggs are deposited on plants ; the Bitterling is said to deposit 

 them in the shells of living bivalve Mollusca, while other fishes, like the 

 Salmon, cover the eggs with gravel, and Sticklebacks construct nests for their 

 better protection. Among the Cyprinoid fishes hybrids are constantly met 

 with, and they are probably not uncommon in the Salmon tribe. 



At the breeding season many fishes put on brilliant colours, which are lost 

 when the spawn is deposited, and some, like most Cyprinoids, develop small 

 tubercles upon the scales in both sexes, or in the male only. The Eels, though 

 commonly accounted fresh-water fishes, spawn in the sea. 



GROWTH. 



The proportions of the body change with growth, the head usually becom- 

 ing relatively smaller, and the fins also become modified, so that the caudal, 



