A CRAYFISH OR A LOBSTER 37 
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 
very slender, 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 artery lies the intestine, which often contains 
dark-colored fecal 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. 
