=The Circulation.=--The blood is a liquid containing _white corpuscles_. It lacks red corpuscles and is colorless. The heart is in the upper part of the thorax. It is surrounded by a large, thin bag, and thus it is in a chamber (called the _pericardial sinus_). The blood from the pulmonary veins enters this sinus before it enters the heart. The origin of this pericardial sinus by the fusing of veins is shown in Fig. 130. Does one artery, or do several arteries, leave the heart? There is a larger dorsal artery lying on the intestine and passing back to the telson; there are three arteries passing forward close to the dorsal surface (Figs. 89, 91). One large artery (the sternal) passes directly downward (Figs. 88, 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 several arteries. The arteries take a supply of fresh blood to the eyes, stomach, muscles, liver, and the various organs. After it has given oxygen to the several organs and taken up carbon dioxid, it returns by veins to pass through the gills on each side, where it gives out the useless gas and takes up oxygen from the water. It is then led upward by veins into the pericardial sinus again. A _double nerve chain_ of ganglia supplies nerve force to the various nerves (Fig. 92). This main nerve chain lies along the ventral surface below the food tube (Fig. 90), except one pair of ganglia which lie above the esophagus or gullet (Fig. 88), and are called the supra-esophageal ganglia, or brain. [Illustration: FIG. 91.--Showing heart and main blood vessels.] [Illustration: FIG. 92.] =Crustacea.=--Because of the limy crust which covers the crawfish and its kindred, they are placed in the class called _Crustacea_. [Illustration: FIG. 93.--CRAB FROM BELOW.] [Illustration: FIG. 94.--HERMIT CRAB, using shell of sea snail for a house.] =Decapods.=--All crustacea which have ten feet belong in the order called decap′oda (ten-footed). This order includes the crabs, lobsters, shrimp, etc. The crabs and lobsters are of considerable importance because of use as food. Small boys sometimes catch crawfish, and in some instances are known to cook and eat them for amusement, the only part cooked being the muscular tail. The crab’s tail is small and flat and held under the body (Fig. 93). [Illustration: FIG. 95.--DEVELOPMENT OF A CRAB. _a_, nauplius just after hatching; _b_, _c_, _d_, zoëa; _e_, megalops; _f_, adult. =Question:= Which stage is most like a crayfish? Compare with metamorphoses of insects.]