CIECULATOEY SYSTEM 139 



E. Demonstration of the Respiratory Current of water 

 througli the gill-chamber. 



Place a living crayfish in a shallow dish of water ; and, 

 when the animal is at rest, run into the dish close to the bases 

 of the hinder legs, by means of a pipette, a few drops of water 

 coloured with suspended carmine or other pigment. Watch the 

 currents entering under the edges of the branchiostegites behind, 

 and issu/ing in front from the mouths of the cervical canals. 



Open the cervical canal of one side by making two cuts, 

 one immediately behind the cervical groove, the other parallel 

 to it and a quarter of an inch further back ; and removing the 

 si/rip between the two cuts. Lay the animal on its side in the 

 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 crayiish, 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 giUs, 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 the apertures ia 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 



