194 



PHYSIOLOGY OF THE INVERTEBRATA. 



of the blood. The blood enters the heart from the cavity of 

 the body by a series of valvular openings, when it is gradually 

 driven forwards by the successive contraction of the divisions 

 of the heart, until it escapes in the neighbourhood of the 

 head. After this it is no longer confined within vessels, as 

 neither arteries nor veins have been observed in the Insecta; 



Fig. 40.— Circulation in the Insecta. 

 a = dorsal vessel or heart, i = principal lateral currents. 



but the blood or nourishing fluid is spread about in the 

 lacunae and interstices of the organs. Even in these lacunae 

 the blood is still animated by the action of the heart, and is 

 ultimately forced back until it again reaches that organ. 

 These lacunse all communicate with a sinus or vessel on the 

 ventral side of the body. Thence the blood passes to the 

 respiratory Organs, and then to the so-called pericardium or 

 venous sinus surrounding the heart. 



It may be mentioned that in the L&pidoptera, Orthoptcra, 

 &c., a ventral vessel has been observed. Dr. V. Graber* 

 discovered a ventral vessel in Stetheopliyma grossum, (grass- 

 hopper) and various species of Lihdlula, and states that it 

 should be regarded in the light of an artery to a dorsal vein. 

 * D%e Imecten, 1877, vol. i, pp. ,328-345. 



