GADUS 77 



its persistent opening to the outside. In the saccule there are 

 two large calcareous concretions or otoliths ; otherwise the 

 structure of the organ is similar to that of the dogfish. 



The lateral-line canals have already been mentioned, and 

 their course and innervation described. 



Alimentary Canal. — The mouth leads into the pharynx, out 

 of which the five pairs of gill-slits open. There is no open 

 spiracle. The gill-arches between the slits are smaller than 

 those of the dogfish, and do not form a broad septum as in 

 that fish. This is correlated with the fact that they are covered 

 over by the operculum. The gills are supported by two rows 

 of branchial rays on each arch. 



Behind the pharynx, the oesophagus leads to the stomach 

 which bears a number of blind tubes, the pyloric coeca. The 

 liver has a gall-bladder from which the bile-duct runs to the 

 intestine. The latter receives the pancreatic duct from the 

 pancreas, makes a loop forwards and back again and runs to 

 the rectum, which opens at the anus. 



The swim-bladder is to be regarded as a derivative of the 

 alimentary canal, and in many forms it retains its connexion 

 with it by an open duct. This connexion has, however, been 

 lost in Gadus, and the swim-bladder is a closed sac which 

 occupies the dorsal portion of the coelomic cavity, close up 

 against the under side of the vertebral column. Its ventral 

 wall is thick and is covered with the coelomic epithelium ; its 

 dorsal wall is very thin. Inside the bladder is a rete mirabile, 

 a concentration of small blood-vessels forming a gland which 

 secretes oxygen into the bladder. This " red " gland, as it 

 is called, is supplied with blood by the mesenteric artery, like 

 the other viscera. The function of. the bladder is hydrostatic, 

 for by varying the amount of gas which it contains (by passage 

 of gas from the blood to the bladder or vice versa) the fish can 

 accommodate itself to any given depth of water and maintain 

 itself there without muscular exertion. As will be seen later, 

 it probably corresponds to the lung of the air-breathing 

 vertebrates. 



Excretory System. — A pair of mesonephric kidneys extend 

 longitudinally, dorsal to the swim-bladder and below the 



