504 FISHES. 



Nervous System. 



The brain has very small cerebral hemispheres and large 

 optic lobes. The ear has a peculiar connection with the 

 swim bladder. External to each of the well developed eyes 

 are two immovable transparent folds of skin, with a vertical 

 slit between. 



Alunentary System. 



The mouth has a narrowed gape. The upper jaw moves 

 downwards and forwards when the mouth is opened. Small 

 visible teeth are borne on the tongue and on the vomer, 

 but those on both jaws are inconspicuous. The food — 

 chiefly small crustaceans — is probably in part crushed by 

 the gill rakers, which also prevent it passing out by the gill 

 clefts. From the posterior end of the gullet a caecum or 

 crop is given off A narrow communication leads from 

 beneath this crop to a thick-walled, gizzard-like, muscular 

 organ directed forwards. This in turn has an opening into 

 the intestine, which runs straight to the anus. About twenty 

 digestive c^ca open into the beginning of the intestine. 

 The swim bladder has a silvery exterior, and lies close under 

 the back bone. The herring differs from most Physostomi 

 as regards the connection between the swim bladder and 

 gut, for the bladder does not communicate with the gullet 

 but with the caecum by means of a narrow, twisted canal. 

 Anteriorly on each side the swim or air bladder gives off a 

 thin duct which, passing through the walls of the skull, 

 divides into two branches, each ending in a dilatation close 

 to the ear. Posteriorly the swim bladder has a duct opening 

 externally on the left side of the anus. 



Respiratory System, &-r. 



The gill filaments are fixed in a double row on the outer 

 edge of each branchial arch. The specially wide opening 

 behind the gill cover permits of a free current of water for 

 respiration. The heart, kidneys, &c., are much the same 

 as in the haddock. 



Reproductive System. 



The testes or milt of the male and the ovaries or roe of 

 the female, lie on each side of the abdominal cavity. In 



