CIRCULATORY SYSTEM. 147 



II. DISSECTION OF THE CRAYFISH. 



The several systems are described below in the order in 

 which it will be most convenient to dissect them : this order 

 may be varied if found desirable. 



A. The Circulatory System. 



The blood of the crayfish, which is slightly bluish, is 

 carried from the heart to the various parts of the body by 

 arteries which open into large lacunar spaces or sinuses, 

 bathing the several organs : from these it passes to the gills,- in 

 which it is aerated, and from which it is conveyed through 

 the branchiooardiac canals to the pericardial sinus. From 

 the sinus it enters the heart through the apertures in its 

 walls. 



1. The pericardial sinus and heart. 



With stout scissors carefully cut through the thoracic 

 portion of the carapace along the outer side of each branchio- 

 cardiac groove, tahing care not to inpire the structures 

 beneath. Connect the two cuts in front by a transverse cut 

 along the cervical groove, and remove the portion of carapace 

 so isolated. 



Gut through the shin underlying the carapace, and turn it 

 aside, thus laying open the pericardial simis and exposing 

 the heart. 



a. The pericardial sinus is a cavity of considerable 



size, lying in the dorsal portion of the thorax and 

 receiving the arterial blood from the gills. 



b. The heart is a polygonal thick- walled muscular 



sac lying in the pericardial sinus. If living it 

 will be seen to contract rhythmically. It receives 

 blood from the pericardial sinus by three pairs of 

 valvular apertures, the ostia, of which the dorsal 

 pair will be easily seen. The lateral and ventral 



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