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TEXTBOOK OF ZOOLOGY 



branchiae, which arise from the coxopodites of the thoracic append- 

 ages. Several of the segments have lost the pleurobranchiae. The 

 scaphognathite moves in such a way over the external surface of 

 the gills as to move the water in an anterior direction. The water 

 is brought under the free edge of the branchiostegite or branchial 

 area of the carapace and moved forward to be discharged by an 

 anterior aperture. An almost constant stream of water is pumped 

 over the gills to facilitate the exchange of oxygen and carbon 

 dioxide between the blood in the capillaries of the gills and the 

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

 tissues of the body. 



Carapace 

 removed 



Hasc/e ^ 



Ventral thoracic 

 artery 



Wntral sinus 



Pericardial sinus 

 Heart 

 Ostium 

 hasck 



Gonad 



Intestine 

 Digestive $land 

 Efferent vessel 



Gill 



Nerve cord 

 Carapace 



Fig. 154. — Diagram of cross section throug-h the posterior thoracic region of a 

 crayfish. Arrows indicate flow of blood. 



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- 





