II.] THE CRAYFISH AND LOBSTER. 223 



A chamber {the pericardial sinus) is thus laid bare 

 in which lies the heart (D. 1. b). 



a. The alary muscles; delicate strands passing from 

 the angles of the heart to the adjacent pericardial 

 wall. 



b. The two dorsal cardiac apertures; oblique openings 

 in the cardiac roof, guarded by valves. 



c. Carefully dissect off the roof of the heart, and 

 note 



a. Its wall ; thick and spongy, the muscular fibres 

 being arranged in irregular bundles. 



/?. Its cavity; central and single ; i.e. there are no 

 auricles. 



y. The ventral apertures and valves; identical in 

 their characters with b ; they are situated at the 

 anterior end, transversely to the long axis. 



<S. The lateral apertures ; similar perforations of the 

 middle region of the side walls. Dissect to the 

 level of one and examine its valves ; they are 

 pocket-shaped, and so arranged as only to admit 

 of a passage of the blood from the pericardial 

 sinus into the heart. 



d. Compare the heart of a similar specimen, after 

 removal from the body. 



3. The great arteries. Obtain a fresh uninjured specimen 

 and pin down under water on its side; lay bare the 

 pericardial sinus with extreme care, and inject from 

 the lateral cardiac aperture. Work leisurely and 

 apply a steady gentle pressure. 



Cut along the post-mandibular portion of the ex- 

 oskeleton to the level of the middle line (with the 



