SANGUTFEROUS SYSTEM. 451 



The united recurrent or venous trunks extending along the 

 posterior margin of the wings return to the great lateral abdomi- 

 nal veins, which convey the blood backwards to the posterior 

 end of the dorsal vessel. Several currents, apparently continued 

 from the abdominal veins, are also perceived moving to and 

 from the principal viscera of the trunk, and exterior to these 

 great returning abdominal currents, two smaller veins (133. 

 D. h. i.) are seen extending laterally nearer to the openings 

 of the stigmata. These smaller accessory abdominal veins 

 receive the blood from the neighbouring exterior parts, and 

 convey it by several short canals to the great returning veins. 

 Abdominal currents are also sometimes perceptible, directed 

 to the principal viscera, especially to the genital organs and 

 to the biliary tubuli. As the aortal arches of the head are 

 observed to give off small currents to the various appendices 

 of its united segments, and the great returning veins to send 

 similar currents to the locomotive appendices of the thorax, 

 so we observe the caudal (133. D. k. I.) and other appendices 

 of the abdomen to receive small streams of blood-globules 

 from the great ventral veins before they open into the poste- 

 rior segment of the heart. The prime mover of the circula- 

 tion in insects is obviously the muscular structure of the heart, 

 and the direction is given by the numerous semilunar valves 

 of its compartments. From the free aeration of the circu- 

 lating fluid by the tracheae in every part of the body, there is 

 as little difference between the contents of arteries and veins, 

 as in the structure of these canals which perform the same 

 functions in ah 1 parts of the body, and hence the high tem- 

 perature and the rapid evolution of caloric, shown by Ber- 

 thold, in the body of these most active of the invertebrata. 

 The diminished extent of the circulation in the imago state or 

 last stage of insects, when the organization is completed, 

 and the end of their career approaches, better fits these light 

 aeronauts to flit through the attenuated air, and corresponds 

 with the senile condition of this system in higher classes of 

 animals. 



The heart of the arachnida is generally suspended, like 

 that of insects, by means of transverse muscular bands, from 

 the dorsal parts of the segments, but that of the phalangium 

 appears to extend free along the middle of the back. In the 

 short abdomen of the spiders, the heart is also short, wide, 



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