12 



THK I'in-:SII-AVATKI^ FISIIKS OF EUROPE. 



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

 and in some fishes contains a g-ood deal of t'at. The spleen is a dark red 

 org^an placed near to the stomach. The intestine terminates in a pore, which 

 frequently includes the outlets for the urinary bladder and the reproductive 

 or<>'ans (Fip;-. 8). 



REL'itourcTivE Ok(;axs. 



The sexes of fishes are always distinct; thoug-h licrniaphrodites are occa- 

 sionally met with in some groups. All European fiesh-water tishes are 

 ovii)arous. The ovaries and milt are frequently mature long before the fish is 

 full-grown, and they usually occcui)y a largt' 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. 0). Some iishes have an external ovi- 

 duct, which is of great length in the Bitterling, R/iodeus amanis. The eggs are 



j^2%V::7f- -'■ 



Fis; 9 — I^TI UN \i \n\iom\ oi ( ^kv 



If, bnuiohia; < r i^ill^ ; c, heart ; ci, iutestiiial caiiul ; o, ovaries ; n, vent ; o', Ofidiict ; <(', uriuary 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 In-eeding season many fishes put on brilliant colours, which are lost 

 when the spawn is deposited, and some, like most Cy))rinoids, develop small 

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

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



(jiuowTii. 

 The i)roi;ortions of the body change with growth, the head usually becom- 



\\\\X. relativelv smaller, and the li^^ 



be( 



ilied. 



(hat 



