PHYLUM ARTHROPODA — CLASS CRUSTACEA 



273 



meut 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 Nood which circulates. It consists of the fluid plasma containing 



5 6 7 g <? 10 u 



Fig. 155. — Lateral view of a dissection of the crayfish to show many of the 

 internal organs. 1, supraesopliageal ganglion ; 2, circumesophageal connective ; S, 

 ophthalmic artery ; J/, stomach, cardiac portion ; 5, lateral teeth ; 6, median teeth : 

 7, antennary artery; 8, testis; Oj, hepatic artery; 10, ostiuin ; 11, heart; 12, dorsal 

 abdominal ganglion; 28, ventral abdominal artery; 29, nerve cord; SO, rectum; 

 gland; 17, esophagus; 18, mouth; 19, subesopliageal ganglion; SO, stomach, pyloric 

 portion; 21, opening of hepatic duct; 23, digestive gland; 23, ventral thoracic 

 artery; 2i, sternal artery; 25, opening of vas deferens; 26, thoracic ganglion; 27. 

 abdominal arterj' : IS, vas deferens; li, intestine: 15, renal opening; 16, green 

 31, anu9. (Modified from Turtox Key Card of Crayfish. Courtesy General Bio- 

 logical Supply House.) 



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, 



