Crustacea. 69 



artery, and the other extending backward, the ventral ab- 

 dominal artery. 



All these arteries divide and subdivide, forming capil- 

 laries. But the capillaries do not reunite, forming veins; 

 they empty into more or less irregular spaces in the body, 

 around the muscles and other internal organs. All these 

 spaces, or sinuses, as they are called, lead into one main 

 channel, the sternal sinus, which extends along the middle 

 of the ventral region. From this sinus, passageways con- 

 duct the blood to each gill. In each gill one tube, the 

 afferent vein, conveys the blood to the gill filaments ; while 

 another tube, the efferent vein, returns the blood to another 

 set of veins, called the branchio-cardiac veins, which lead 

 to the pericardium. There are no tubes to convey the 

 blood into the heart, but the blood enters the heart directly 

 through three pairs of holes, one on each side, a pair on top, 

 and another pair below. These holes have lips on the 

 inside, which act as valves, allowing the blood to enter 

 freely, but preventing a reflow through the holes. Thus 

 the beating of the heart causes a constant flow of blood in 

 one direction : first to the tissues of the body in general, 

 where it gives up oxygen and food materials and picks up 

 carbon dioxid ; then to the gills, where it gives off carbon 

 dioxid and gains oxygen ; then back again to the heart. 

 The blood is colorless, but after exposure to the air it 

 turns bluish. 



Excretion in the Crayfish. In connection with the study 

 of the gills we have seen how the carbon dioxid is removed 

 from the body. But the nitrogenous wastes are excreted 

 by a pair of kidneys, which are called, on account of their 

 color, the green glands. They are situated in the head, 

 just below and in front of the stomach. The gland proper 

 is a button-shaped body, lying close to the ventral body 



