BONY-FISH. 25 



INTERNAL STRUCTURE. 



With scalpel and forceps remove a piece of skin from one 

 side of the fish, exposing the underlying muscles. Notice 

 that these are arranged in chevron-like plates, each plate 

 (myotome) extending from back to belly, and being divided 

 into dorsal and ventral portions. Pick among the ventral 

 parts of the muscle-plates. Do you find any ribs? How 

 are they arranged with regard to the myotomes? 



Open the fish by cutting with the scissors from just in 

 front of the vent, forward, in the median line, to the 

 pectoral fins, taking care to cut nothing but the body-wall. 

 Make other incisions transverse to the first, so that the 

 body-wall on either side may be turned out like a flap, 

 thus opening up the body-cavity, or ccelom, containing the 

 viscera. Without further dissection notice the membrane 

 (peritoneum) lining the cavity. Is it silvery or pigmented? 

 In the front part of the cavity is the large reddish or 

 brownish liver; turn this over to the left and expose the 

 stomach, connected apparently with the front wall of the 

 body-cavity. Pass a probe from the mouth through the 

 oesophagus or gullet into the stomach. From the stomach 

 the intestine passes back to the vent. From what part of 

 the stomach does it arise? Is it straight? How is it 

 supported in its position? 



In many fishes worm-like blind tubes (pyloric cceca) 

 arise at the junction of stomach and intestine. Their 

 purpose is to increase the surface secreting the digestive 

 fluids. Do any of these occur in your fish? 



Study the liver more carefully. On its anterior surface 

 see blood-vessels (hepatic veins). Where do they go? On 

 its posterior surface is the thin-walled green or yellow gall- 

 bladder. Can you trace any connection between liver and 

 intestine? 



