402 ESSENTIALS OF ZOOLOGY 



over the gills to facilitate the exchange of oxygen and carbon di- 

 oxide between the blood in the capillaries of the gills and the sur- 

 rounding water. The aerated blood is then carried to all of the 

 tissues of the body. . 



The digestive system is in the form of a modified canal and is 

 composed of mouth, esophagus, stomach, and intestine. The mouth 

 opens between the mandibles on the ventral side of the third seg- 

 ment. From this the short, tubular esophagus leads dorsally and 

 joins the ventral side of the stomach almost directly above the 

 mouth. This larger anterior portion of the stomach is the cardiac 

 chamber. Within its wall are a number of hard chitinous bars, 

 known as ossicles, which bear teeth capable of mastication of food 

 when moved over each other by the muscular activity of the wall. 

 This grinding apparatus is known as the gastric mill. Between the 

 cardiac chamber and the posterior or pyloric chamber is an arrange- 

 ment of bristles which serve as a strainer that allows only properly 

 masticated food to pass through. The pyloric chamber is consider- 

 ably smaller and curves downward to continue posteriorly as the 

 tubular intestine which extends almost directly posteriorly through 

 the center of the abdomen to the anus in the last segment. Large 

 digestive glands (hepato-pancreas) lead into the pyloric chamber 

 through hepatic ducts. The secretion of these glands contains diges- 

 tive enzymes. 



The vascular system consists of a heart, the pumping organ; the 

 arteries, definite vessels; the sinuses, a series of blood spaces; and 

 the Mood which circulates. It consists of the fluid plasma containing 

 white corpuscles but without red ones. The hemocyanin which ab- 

 sorbs oxygen is dissolved in the plasma. Fresh blood is almost clear 

 and colorless, but it takes a blue color after standing in the air for a 

 short time. The heart is somewhat flattened and angular in outline, 

 and has a muscular wall which is perforated with three pairs of slitlike 

 ostia. When the muscular wall of the heart is relaxed, the slits open, 

 and blood is drawn in from the surrounding pericardial sinus in 

 which the aerated blood accumulates. When the heart contracts, 

 blood is forced into the anterior region of the body through the single 

 anterior median artery, paired antennary, and paired hepatic arteries 

 all of wlivch arise from the anterior end of the heart. The large dorsal 

 abdominal artery extends from the posterior tip of the heart pos- 

 teriorly through the abdomen just dorsal to the intestine. It sup- 

 plies the intestine and muscles of the body wall. The sternal artery 



