STRUCTURE AXD MECHANISM OF FISHES 



11 



slits. In order to understand this jaw-gill-arch complex we must go back 

 to the more primitive fishes, especially the sharks, in which we find that 

 the gill-slits in the side of the throat are supported by a jointed frame- 

 work of gristly, or cartilaginous, rods and connecting pieces arranged as 

 in Fig. 5. Several rows of attached muscle slips arranged in circular 

 and oblique series cause the alternate expansion and contraction of 

 this framework, so that the water is pumped into the throat and 



EYE SOCKET .CONNECTING PIECE BETWEEN 



BRAIN CASE /SKULL AND JAWS 



TONGUE'BONE 



Fig. 5. SKULL AND ATTACHED PARTS OF A SMALL SHARK 

 (From Jordan, after Parker and Haswell). 



ejected through the gill-slits. In typical sharks each of the five 

 gill-slits opens freely on the side of the head, but in all the higher 

 fishes such as the bass these side slits are covered on the outside by a 

 large bony and fleshy flap called the operculum (Fig. 2) which opens 

 to the rear and is hinged in front to the back part of the upper jaw. By 

 means of suitable muscles the opercular flap can be alternately opened 

 or closed, so that it acts as an escape valve for the water in the gill 

 chamber. In some cases, as in the Moray, the opercular flap becomes 

 pouch-like and leathery and acts like a rubber pump, or syringe. 



The gills themselves consist of extremely numerous long plaits of 

 thin membrane, which cffer in a small space a large exposure to the 

 oxygen-bearing water. The oxygen gas passes freely through the mem- 

 brane and is taken up by the blood in the very minute tubes of the gills. 

 This blood comes from the body where its supply of oxygen had been ex- 

 hausted. It is pumped to the gills by the heart, entering the lower part 



