XX] MAIN DIVISIONS 489 



moved still further forwards till the pelvic girdle is attached to the 

 front lower end of the cleithra, and so the pelvic fins are actually 

 in front of the pectoral. This position is known as the jugular. 

 Next to the condition of the pelvic fins is the condition of the 

 air-bladder. The function of this organ is to enable the fish to 

 adjust its weight to the density of the water in which it lives. 

 In the more primitive families the air-bladder is connected with 

 the pharynx by a duct opening in the mid-dorsal line and the fish 

 swallows air into its bladder. In the majority of Teleostei the 

 air-bladder is shut off altogether from the alimentary canal and 

 secretes its contained gases from the blood : a thickening in its 

 wall richly supplied with blood-vessels, being termed the gas-gland. 

 In the more primitive families the maxilla bears teeth and forms 

 part of the gape behind the premaxilla, but in the vast majority 

 of Teleostei the maxilla is a toothless bone embedded in the cheek 

 and passing up behind the premaxilla. When the mouth is opened 

 the lower end of the maxilla rotates forward and pushes out the 

 premaxilla which is then said to be protrusible. In a large number 

 of the more modified Teleostei certain of the lepidotrichia covering 

 the dorsal, pectoral and pelvic fins are fused together to form hard 

 pungent spines, and in very large groups of families, the pelvic fin 

 possesses one such spine followed by five articulated rays. This 

 arrangement is almost as constant as five fingers on the hands of 

 men and monkeys. 



The great sub- order CLUPEIFORMES is characterised by ab- 

 dominal pelvic fins, an air-bladder with a duct and the maxilla 

 forming part of the gape. The coracoid has an anterior fork 

 termed the mesocoracoid. This group includes amongst other 

 families the Clupeidae or Herring family, and the Salmonidae 

 (Salmon and Trout and White-fish). 



The Clupeidae have a single dorsal fin and the air-bladder 

 sends processes forward which extend into the auditory capsules 

 and come close to the internal auditory sac, and thus this sense 

 organ is directly affected by changes of pressure in the air-bladder. 

 The genus Clupea includes the Herring, Sprat and Pilchard as well 

 as the Shad, which is a more deep bodied form. The Anchovy dis- 

 tinguished by its projecting snout is Engraulis. 



The Herring is probably the most important food-fish. It lives 

 in comparatively deep water at a moderate distance from the coast, 

 but comes into shallow water to spawn, its eggs being attached to 

 stones. It is usually caught when on this spawning migration. 



