LOBSTER OE CEATFISH 157 



for the openings where the blood-vessels coming 

 from the gills enter the pericardial sinus. Lying 

 within the sinus will be found 



h. The heart. — "What is its shape ? Its size as com- 

 pared with the sinus ? Look for the fine thread- 

 like muscles, the alary muscles, estending from 

 the corners of the heai't to the wall of the sinus. 

 How many such muscles do you find? On the 

 upper surface look for the two dorsal cardiac 

 apertures. What is their position ? Shape ? 

 Size ? Can you detect the lip-like valves guard- 

 ing the openings ? Remove the roof with scissors. 

 What is the structure of the wall of the heart ? Is 

 the cavity single, or is it divided into chambers, 

 as auricles and ventricles ? On the floor of the 

 heart look for the ventral apertures. How 

 many do you find ? What is their position? Do 

 they have valves ? Compare in all respects with 

 the dorsal apertures. On the sides of the heart 

 look for lateral apertures. How many are 

 there? What is their position? Compare with 

 the other apertures. Cut away one side of the 

 heart down to the level of an aperture, and try 

 to see how the valves work. Do they prevent 

 the exit or the entrance of the blood at these 

 apertures? 

 Make a drawing of the heart, showing on one side 

 the dorsal surface, on the other the internal aspect. 



Eunning from the posterior end of the heart 

 find 



c. The superior abdominal artery, — How far does 



it extend ? Notice the branches. At what points 

 are they given off? To what parts do they run? 



