28 PHYLUM ARTHROPODA 



under-skin. Study the organs as they He, without disturbing them. 

 Notice in the cephalothorax, first, the large sadike stomach just 

 back of the rostrum and connected by muscles with the anterior 

 body wall. On each side of the stomach will be seen the cut ends 

 of a mass of muscle fibers. These are the mandibular muscles. 

 Demonstrate their connection with the mandibles. Just back of 

 the stomach is the white, shield-shaped heart, from the ante- 

 rior 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 stomach 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 is 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 development 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 

 sperm duct on each side. 



Study the musculature and the other organs of the abdomen. 

 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 dehcate, 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 abdo- 

 men are the flexor muscles, which are very complex, and 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. 



