SYSTEMS OF THE HERRING. 439 



fin ; the body is more compressed ; the ventral edge is 

 covered by a " series of sharply keeled bony shields." 



Nervous System. — The brain has very small cerebral hemi- 

 spheres and large optic lobes. The ear has a peculiar con- 

 nection with the swim-bladder. External to each of the 

 well-developed eyes are two immovable transparent folds of 

 skin, with a vertical slit between. 



Alimentary 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 incon- 

 spicuous. The food — chiefly small crustaceans — is probably 

 in part crushed by the gill rakers, which also prevent it 

 passing out by the gill clefts. In the region of the gill arches 

 a median opening leads into the gullet, which passes into a 

 conical caecum or crop. A narrow communication leads 

 from beneath this crop to a thick-walled, gizzard-like, mus- 

 cular organ directed forwards. This in turn has an opening 

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

 twenty digestive cseca 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 crop by means of a narrow, twisted canal. Ante- 

 riorly 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 ex- 

 ternally on the left side of the anus. 



Respiratory System. — 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. 



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 each sex there is a single external 

 opening behind the anus. The number of mature eggs 

 spawned at one time by the female has been variously esti- 

 mated at from 10,000 to 30,000. In British waters there is 

 a spring as well as an autumn spawning — probably however 



