

CIRCULATORY SYSTEM. 155 



E. Demonstration of the Respiratory Current of Water 

 through 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 issuing in front from the mouths of the cervical canals. 



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 forming the 

 body-cavity or haemoccel. From this it passes to the gills, in 

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

 the branchiocardiac canals to the pericardial sinus. From 

 the sinus it enters the heart through the apertures in its 

 walls. 

 1. The pericardial sinus and heart. 

 Insert one blade of a stout pair of scissors into the slit 

 behind the carapace. Cut forwards, holding the blade horizon- 

 tally, right up to the rostrum, and then across and downwards 

 to the outer side of the eyes. Remove the carapace completely. 

 Next cut backwards along the outer edge of the abdominal terga 

 on each side, avoiding injury to the muscles, and carefully 

 remove the tergal sclerites. Clean away the pigmented epidermis 

 with forceps. 



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. 



