CIKCULATORY SYSTEM 137 



water, and observe the sculling movements of the scapho- 

 gnathite, driving the water forwards out of the cervical canal. 



II. DISSECTION OF THE CEAYFISH. 



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 almost colourless, is 

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

 arteries, which open into large lacunar spaces bathing the 

 several organs : from these it passes to the gills, in which it is 

 aerated, and from which it is conveyed through the branchio- 

 cardiac canals to the pericardial sinus. From the sinus it 

 enters the heart through 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, taking care not to injure 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. 



Cut through the skin underlying the carapace, and turn it 

 aside, thus laying open the pericardial sinus 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 readily seen. The lateral and ventral 

 pairs will be better seen at a later stage of the 

 dissection. 



