38 VERTEBRATE ZOOLOGY 



at different seasons of the year. They are often very large and 

 may occupy a large portion of the abdominal cavity. They will be 

 seen just dorsal to the intestine, extending from the hinder end of 

 the cavity forward. 



In the pericardial cavity, beneath (dorsal to) the thick muscles 

 between the gills, will be seen the heart. It consists of the median 

 ventricle, a large muscular organ, at the sides of which appears 

 the deep-red auricle ; at the back of (dorsal to) this organ is the 

 sinus 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, but do not injure the heart ; 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 



