LESSONS IN ZOOLOGY. 271 



the tubes or blood vessels leading from the heart. In the heart 

 there are two holes above, two below and one on each side, each 

 hole guarded by a lip-like valve. From the gills the blood passes 

 into the pericardia! cavity; as the heart expands the blood flows 

 through the holes into it; as it contracts, the valves close and 

 the blood is forced into the different parts of the body, then to 

 the gills again. Under the heart are the reproductive organs, 

 the yellowish ovaries, and the oviducts leading to the genital 

 openings already mentioned, while in the male there are the 

 testes with long white coiled tubes leading to the genital opening 

 in the last thoracic leg. 



Cut away the roof of the head. The space within the head is 

 quite fully occupied by the stomach. Pass a probe through the 

 mouth into the stomach. Note carefully the gullet, stomach and 

 intestine, and near the beginning of the intestine some reddish 

 masses, the hepatic glands or liver. Observe the white muscles 

 along each side the body cavity. Cut away the roof of the 

 abdomen, and trace the muscles through it to the telson, the 

 upper thinner layer straightens the abdomen while the thicker 

 lower layer below the intestine bends it. 



Over the gullet there is a large ganglion, extending outward 

 and downward. Around the gullet are two masses of nervous mat- 

 ter connecting the ganglion above the gullet with the anterior 

 of twelve pairs of ganglia that lie along the ventral surface under- 

 neath the intestine and deeper muscles. Trace out the ganglia 

 and principal branches of this system. Try to have some cray- 

 fish for observation during their moulting season in the spring. 



Those members of the Crustacea living in the sea, as the lob- 

 sters, shrimps and crabs are the most numerous, and individuals 

 among them sometimes attain great size. And these members 

 of the class furnish large quantities of food material. 



The Barnacles belong to this class, as did the Trilobites, ani- 

 mals that were so abundant during the early geologic ages. 

 Besides the crayfish, there is a group of minute organisms called 

 Entomostraca that inhabit fresh water. They are an abundant 

 source of food for fish and other forms of aquatic life. 



