THE CIRCULATORY SYSTEM. 39 



with very many perfect Insecta and their larvae. The vascular 

 walls, supposed to have been seen at certain points, are, un- 

 doubtedly, the result of some error of observation or interpre- 

 tation. This is also true of the pulsatile organs supposed to 

 have been observed in the legs of many water-bugs, and which 

 were thought to affect the circulation." 



Blanchard and Agassiz believe in a "peritracheal circula- 

 tion," and other observers agree that the course of the circula- 

 tion is along the tracheae, i. e. that the blood circulates in the 

 space between the loose peritoneal envelope and the trachea 

 itself. Professor H. J. Clark objects to this view that the blood 

 disks are too large to pass thi'ough such an exceedingly minute 

 space as the distance between the trachea and its peritoneal 

 wall. McLeod has proved that such a circulation does not exist. 



Newport thinks that there are actual blood vessels distrib- 

 uted from the heart and "passing transversely across the 

 dorsal surface of each segment in the pupa of Sphinx. If 

 they be not vessels distributed fro7n the heart, it is a some- 

 what curious circumstance that the whole of the blood should 

 be first sent to the head of the insect, and the viscera of the 

 abdominal region be nourished only by the returning blood, 

 which has in part passed the round of the circulation." 



Newport also describes in Sphinx the supir a- spinal, or gi-eat 

 ventral vessel which lies in the abdomen just over the nervous 

 cord, and which is also found in the Scorpion and Centipede. 

 He believes "this vessel to be the chief means of returning 

 the blood from the middle and inferior portion of the body to 

 the posterior extremity of the dorsal vessel or heart." He 

 strongly suspects that anteriorly this great ventral vessel is 

 connected with the aorta. The circulation of Insects, there- 

 fore, is probably as much a closed one as in the Myriapods, for 

 he states that the "blood certainly flows in distinct vessels, at 

 least in some parts of the body in perfect insects, and that 

 vessels exist even in the larva." Observations on the vascular 

 system are exceedingly difficult from the delicate structure of 

 the vessels, and the subject needs renewed observations to 

 settle these disputed points. 



The blood is forced through the vessel into the body by regu- 

 lar pulsations. Herold counted thirty to forty in a minute in a 



