A CRAYFISH OR A LOBSTER 31 



is the white, shield-shaped heart, from the anterior end of which 

 five delicate arteries proceed, a median artery, and two pairs of 

 lateral ones. Find these arteries and trace them forward as far 

 as possible without breaking them. On both sides of the stom- 

 ach and the heart and partly beneath them are the liver and the 

 reproductive organs. The former is a pair of large, soft, and 

 usually light-green organs which may fill a large portion of the 

 cephalothorax and may extend back into the abdomen. The 

 latter, if the animal be a female, are a pair of brownish or 

 yellowish organs, the ovaries, in which the ova can often be 

 seen ; they are situated beneath the heart and in front of and 

 behind it, and vary in size and also in color with the develop- 

 ment of the ova. When these are approaching maturity the 

 ovaries are the most prominent organs in the body-cavity, and 

 often extend far back into the abdomen. In the male animal 

 the reproductive glands, the testes, are white in color and 

 occupy the same position as the ovaries in the female. Note 

 the coiled vas deferens on each side. 



Study the musculature and the other organs of the abdo- 

 men. There are two systems of muscles here. On the dorsal 

 side are longitudinal muscles, the extensors, which extend or 

 straighten the abdomen. Separate these muscles carefully 

 along the median line and observe beneath them the delicate, 

 colorless abdominal artery which carries the blood from the heart 

 throughout the abdomen. Trace it forward to the heart. 

 Notice the lateral branch-arteries. How many pairs are there? 

 Just beneath this arteiy lies the intestine, which often contains 

 dark-colored faecal matter. Beneath it and filling most of the 

 space within the abdomen are the flexor muscles, which are very 

 complex, whose function it is to bend or flex the abdomen. It 

 is by the use of these two sets of muscles that the animal swims. 



Exercise 19. Draw an outline of the animal's body, showing the 

 segmentation and the above-mentioned organs in situ, and 

 label them all. 



