VIIl] THE CIRCULATORY SYSTEM 159 



posterior end. Hence the eighth chamber lies in the second 

 abdominal segment. 



In most insects, each separate chamber can not only propel 

 the blood forward, but can also receive blood from the haemocoele 

 by openings, or ostia, in its walls. The heart of the Dragonfly 

 larva is, however, differentiated into two very distinct portions, 

 which we shall call the Receptive or Hind Heart, and the Conductive 

 or Fore Heart respectively. 



The Receptive Heart consists of the first two chambers only, 

 viz. those in the ninth and eighth abdominal segments (fig. 71, 

 i and ii). Each chamber not only performs the ventricular 

 function of pumping the blood forward (into the chamber in front 

 of it), but it also carries out the auricular function of receiving 

 the blood from the haemocoele by means of ostia. Each chamber 

 has a pair of these openings (ost) situated dorso-laterally near the 

 middle of its wall. The wall is more muscular, and its powers of 

 contraction and expansion greater, than in the case of the chambers 

 of the Fore Heart. 



The first chamber (i) is firmly suspended above the posterior 

 end of the rectum. Its walls are held in position by two strong 

 alary muscles (a.m.), attached to the side walls of the ninth segment, 

 and also by two posterior ligaments (p.lig.) passing backwards, and 

 attached to the wall of the tenth segment above the anus. Pos- 

 teriorly the chamber is rounded off blindly; the anterior half is 

 almost cylindrical. The ostia (ost) open, one on either side, above 

 the alary muscles. Their openings are guarded by valves directed 

 slantingly forwards. Each valve is continued forwards along the 

 side wall of the heart to end in a larger flap (v) which forms the 

 valve separating this chamber from the second. 



The second chamber (ii) resembles the first, except that it is 

 cylindrical throughout. It is supported by two strong alary 

 muscles (a.m.) attached to the side walls of the eighth segment, and 

 in front by two anterior ligaments (a.lig.) attached to the partition 

 between the seventh and eighth segments. Its ostia and valves 

 resemble those of i. 



Close to each ostium of the Receptive Heart lies a small 

 phagocyte organ (not shewn in the figure). This is simply a mass 

 of corpuscles enclosed in a skeletal network of fibres. These 



