. Beginners' Zoology . y (the sternal) passes directly downward (Figs. ^Z, 91), and sends a branch forward and another backward near the ventral surface. The openings into the heart from the sinus have valvular lips which prevent a backward flow of blood into the sinus. Hence, when the heart contracts, the blood is sent out into the sev-eral arteries. The arteries take a supply of fresh bloodto the eyes, stomach, muscles, liver, and the variousorgans. After it has given oxygen to the several organsand taken up carbon dioxide, it returns by veins to passthrough the gills on each side, where it
. Beginners' Zoology . y (the sternal) passes directly downward (Figs. ^Z, 91), and sends a branch forward and another backward near the ventral surface. The openings into the heart from the sinus have valvular lips which prevent a backward flow of blood into the sinus. Hence, when the heart contracts, the blood is sent out into the sev-eral arteries. The arteries take a supply of fresh bloodto the eyes, stomach, muscles, liver, and the variousorgans. After it has given oxygen to the several organsand taken up carbon dioxide, it returns by veins to passthrough the gills on each side, where it gives out the use-less gas and takes up oxygen from the water. It is thenled upward by veins into the pericardial sinus central nervous system consists of a doublechain of ganglia (Fig. 92). This main nerve chainlies along the ventral surface below the food tube(Fig. 90), except one pair of ganglia which lieabove the oesophagus or gullet (Fig. 88), and areFig 02 called the supra-oesophageal ganglia, or Fig. 91. — Showing heartand main blood-vessels. Crustacea.—The crayfish and its kindred are placed inthe class called Crustacea. 6o BEGINNERS ZOOLOGY Decapods. — All Crustacea which have ten feet belongin the order called decapoda (ten-footed). This orderincludes the crabs, lobsters, shrimp,etc. The crabs and the lobsters are ofconsiderable importance because ofuse as food. Small boys sometimescatch crayfish, and in some instancesare known to cook and eat them foramusement,
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