THE PERCH 203 



venosus, a large, deep-red sac which communicates with the 

 auricle. In front of the ventricle is the large bulbus arteriosus, 

 which is the beginning of the aorta. 



Exercise 3. Make a semidiagrammatic sketch of the left side of the 

 fish, showing the opened body cavity and its organs as they appear 

 before they have been disturbed ; label them all carefully. 



The Digestive System. This consists of the mouth, pharynx, 

 oesophagus, stomach with the pyloric appendages, intestine, and 

 liver. 



The Mouth and Pharynx. Cut away the left half of the lower 

 jaw and the gill arches ; the mouth and pharynx are thus fully 

 exposed. They will be seen to form a single large space extending 

 from the opening of the mouth to the oesophagus, the pharynx 

 being the portion of the space which contains the gills. 



The teeth are very small and are present not only in the upper 

 and lower jaws but also on the roof of the mouth and the roof 

 and floor of the pharynx. Examine them carefully with the aid 

 of a hand lens. There are three groups of teeth on the roof of the 

 mouth, a small median patch of vomerine teeth, and a pair of 

 lateral patches of maxillary teeth. Note carefully the position of 

 the teeth on the gill arches. 



Just within the margin of each jaw is a transverse membrane ; 

 probe behind them. These two membranes are the oral valves, 

 which prevent the water from flowing out again through the 

 mouth during respiration. Breathing consists of two actions, 

 the inspiration and the expiration. At the inspiration the oral 

 valves open and the branchiostegal membranes, which form 

 valves at the opening of the gill cover on each side, close ; at the 

 expiration the oral valves close and the branchiostegal valves 

 open, allowing the respiratory water to pass out through the 

 gill clefts. 



Observe carefully the form and arrangement of the gill arches. 

 Note the gill rakers, the short, spiny projections on the gill 

 arches which prevent solid substances from passing out through 

 the gill clefts. Cut out a gill arch and examine the gills on it. 

 Observe that a double row of gill filaments is present. Study 



