THE CRAYFISH 23 



pentagonal sac, the heart, contained within a delicate 

 membrane, the pericardium. Remove the pericardium 

 and note a pair of dorsal openings into the heart, called 

 ostia. (There are also two lateral pairs and a ventral 

 pair of ostia.) Note passing anteriorly from the heart 

 along the median line to the eyes a blood-vessel, the 

 ophthalmic artery. Arising from the anterior portion of 

 the heart are the antennary arteries, running to the 

 antennae. Yet another pair running anteriorly from the 

 heart to the stomach and digestive glands are called the 

 hepatic arteries. From the posterior end of the heart 

 arises the dorsal abdominal artery, running back to the 

 telson. Below this arises the sternal artery, which will 

 be seen later. 



In the region below the heart are located the reproduc- 

 tive organs. They are whitish glandular masses from 

 each of which runs a tube which opens at the base of the 

 last pair of walking-legs in the male, and at the base of 

 the third pair of walking-legs in the female. 



TECHNICAL NOTE. Cut longitudinally through the dorsal wall 

 of the abdomen on either side of the median line and remove the 

 piece of shell. 



Note the powerful muscles within which flex and extend 

 the abdomen. By a rapid contraction of these muscles 

 the tail is brought beneath the body, propelling the animal 

 strongly backwards. When the crayfish crawls it gen- 

 erally goes forward, but in swimming it reverses this 

 direction. 



Make a drawing showing, in their natural position, the 

 internal organs which have been studied. 



Examine the alimentary canal for its whole length. 

 Note that the large bladder-shaped stomach is attached 

 to the mouth-opening by a short tube. What part of the 

 canal is this ? From the posterior end of the stomach is 

 a short thick-walled part, the small intestine, followed by 



