Phylum Arthropoda 501 



pericardial sinus. Blood enters the heart through three pairs of ostia 

 from the pericardial sinus, and is then pumped by the heart to all parts 

 of the body. The blood leaves the heart through a series of six arteries. 

 The exits of all these arteries are guarded by valves which prevent the 

 backflow of the blood. Five of these arteries pass anteriorly, but one 

 passes posteriorly ; thus this heart like that of the molluscs pumps blood 

 in both directions. 



Anteriorly, the arteries are as follows : the ophthalmic which sup- 

 plies the head region, the esophagus, and the cardiac portion of the 

 stomach ; the paired antennaries which arise on either side of the ophthal- 

 mic and supply the antennae, muscles of the head, the excretory organ, 

 and part of the cardiac stomach; and the paired hepatics which supply 

 the midgut and digestive gland and which arise ventral to the anten- 

 naries. Posteriorly, a single vessel, the dorsal abdominal, carries the 

 blood to the dorsal portion of the abdomen. Shortly after leaving the 

 heart, this latter vessel gives rise to the sternal artery which passes 

 ventrally to the region of the nerve cord. In the regions of the fourth 

 and fifth thoracic ganglia, the sternal artery bifurcates into two vessels, 

 the ventral thoracic and the ventral abdominal. Both of these pass below 

 the nerve cord. These arteries branch and rebranch, finally forming 

 capillaries which gradually become spaces between tissues. 



After the blood has given up oxygen to the tissues, it is collected 

 in the large sternal sinus. From the sternal sinus, afferent channels 

 carry the blood to the gills where it is oxygenated. Finally efferent 

 channels bring the blood into the branchiocardiac sinuses which in turn 

 enter the pericardial sinus. Blood then flows into the heart through 

 the three pairs of ostia, one dorsal, one lateral, and one ventral. 



The blood itself is very nearly colorless, but does contain amoe- 

 bocytes. The respiratory pigment is hemocyanin. The blood clots 

 readily if the animal is injured ; this is of importance inasmuch as the 

 crayfish is able to break off appendages and later regenerate them. 



The respiratory system is a series of plumelike gills located in the 

 branchial chamber between the branchiostegites and the body wall. The 

 exopodite of the second maxilla is modified into a flat plate, the scapho- 

 gnathite, whose movements draw water through the channels of the 

 branchial chamber. Water enters posteriorly and ventrally and emerges 

 anteriorly. Each gill consists of two parts, a podobranch and a arthro- 

 branch, and they are arranged in two rows with the podobranch of each 

 gill arising from the coxopodite of appendages 7 to 12 and the arthro- 

 branchs arising from the membranes that join these appendages to the 



