86 THE COCKROACH [CH. Ill 



openings in each segment, and forces it forward towards 

 the head, all backward flow within the heart being pre- 

 vented by the funnel-like communication of chamber with 

 chamber. The rate of heart-beat varies, but is on an 

 average from 70 80 beats per minute. Around the 

 heart is a space partitioned off from the rest of the body 

 spaces (blood cavities), excepting at certain holes through 

 which the blood streams when the pericardial space 

 is enlarged by the pull of the paired, fanshaped alary 

 muscles in each segment: the handle of each "fan" is 

 attached to the side of the dorsal covering of the body 

 while the expanded part is spread out below the heart 

 in the partition membrane. When the heart is relaxed 

 blood flows into it from the pericardial space through the 

 valved openings. Thus by the alternate contractions of the 

 alary muscles and of the wall of the heart the circulation 

 of the blood is maintained. Another space with pulsating 

 walls of very similar structure covers the ventral nerve 

 cord and urges the blood from the head towards the tail 

 by waves of contraction, which run from before backwards. 

 Nervous System and Special Senses. The cerebral 

 ganglia (brain) are the centres of all voluntary action, and 

 exercise, as in the crayfish, a general controlling influence 

 over the activities of the ganglia of the ventral chain, which 

 are, to a very great extent, reflex centres. Reference has 

 already been made to their action in connexion with the 

 respiratory movements; they are also responsible for the 

 protrusion of the sting in decapitated wasps and their 

 allies, and again for the phenomenon of " shamming dead " 

 that may be witnessed in many insects. 



