76 ON T11E ANATOMY OP THE PLY. 



The internal or elastic coat is constricted at intervals, 

 forming five chambers, with a valvular fold between each. 

 The three posterior chambers are very narrow, slightly enlarged 

 at their anterior extremities ; but the two anterior form a 

 considerable dilatation. Each has a pair of small lateral openings 

 protected internally by a valvular fold. The elasticity of this 

 coat causes the vessel to curl up when detached from the 

 abdominal wall ; it probably keeps the chambers open* during 

 life, whilst they are emptied successively by the contraction of 

 the muscles which surround them. So that the action of the 

 vessel is analogous to that of Dr. Richardson's sether spray 

 apparatus. The contraction of the whole length of the mus- 

 cular coat is not simultaneous, but takes place by a series of 

 undulations commencing at its posterior extremity. 



The anterior abdominal chamber is prolonged anteriorly as 

 a tubular vessel, which lies above the chyle stomach and 

 beneath the great dorsal thoracic muscles. This vessel enters 

 the head above the oesophagus, and ends by branching into six 

 or eight minute tubes, which terminate in open mouths in tho 

 cephalic cavity. The muscular wall of the dorsal vessel in the 

 abdomen is continued in the thoracic portion of the vessel and 

 its terminal branches ; but the elastic coat is either absent or 

 very little developed. 



The development of the dorsal vessel has not hitherto been 

 worked out. The difficulties of the necessary observations are 

 very great, from its great transparency and elasticity. It is 



* This is the only theory I can suggest to account for the dilatation of 

 the dorsal vessel, since the influx of fluid can have no effect in dilating 

 the chambers, as in animals having a closed circulation. I think 

 it probable that the wings of the dorsal vessel of the cockchafer enclose 

 an elastic tube exactly as the muscular tubs encloses the elastic one in the 

 fly, except that their direction is transverse instead of longitudinal. 



