THE PROPULSATORY APPARATUS 401 



nearly touch that they form a transverse partition wall in the chamber. But, 

 for the last purpose, i.e. for the separation of the chambers from one another, 

 there is a very special contrivance. In the May beetle, we find, besides a valve 

 (Fig. 373, B, e}, opening into the middle of the chambers, a large, stalked cell 

 (rt)., which, in the diastole, i.e. in the expansion of the heart, hangs down free 

 on the walls of the heart ; but, in the systole or contraction, like a cork, closes 

 the middle of the valve, but does not wholly close the cavity. He has observed, 

 in the larva of Corethra, formal, interventricular valves, which also are not in 

 the middle, but are separated from one another in the interlaced ends. They 

 consist of two longitudinally membranous flaps which move against each other 

 like two valves (Fig. 373, B, ft). 



"But what is the necessity for such a complicated mechanism? All the 

 blood from behind passes into the heart, and, for its propulsion a simple mus- 

 cular tube, whose circular fibres would draw together and contract it, would be 

 thought to be sufficient. But the heart, except in some larvae, ends posteriorly 

 in a blind sac, and the blood can only pass into it by a series of pairs of lateral 

 openings. Now, as regards the reception and the propulsion of the blood 

 forwards, two modes are conceivable. The simplest way would be that the 

 tubular heart should, along its whole length, contract or expand ; that, more- 

 over, the blood should be simultaneously sucked in through all the openings, 

 and that then, also, the contraction, or systole, should take place in every part 

 of the heart at the same moment. But this would, plainly, in so long and thin- 

 walled a vessel, be highly impracticable, since, through such a manipulation, the 

 mass of blood enclosed in the heart would be crowded together rather than 

 really impelled forwards. Only the second case could be admissible, and that 

 is this, that each chamber pulsates, one after another, from behind forwards. 

 But, then, each segmental heart must be separated from the others by a valve. 

 To make the matter wholly clear, we may observe an insect heart pulsating, and 

 this is best seen in one of its middle chambers. This chamber expands (simply 

 by the relaxation of its circular muscles), the ostia, also, consequently open, 

 and a given quantity of blood is drawn in from the pericardial cavity. What 

 now would happen after the succeeding contraction if there were no valves 

 between ? The blood would not flow forwards, but seek a way out backwards. 



"But, in fact, the valve of the hinder chamber, at this time, closes itself, 

 while, by the simultaneous expansion of the anterior ones, their door opens, and 

 this section of the heart, at the same time, causes a sucking in of the contents 

 of the posterior chamber. This phenomenon is repeated, in the same way, from 

 chamber to chamber, which also acts alternately as ventricle and auricle, or by 

 a sucking and pumping action. One is involuntarily reminded of the ingenious 

 manipulation by which, by the alternate opening and shutting of the flood-gates, 

 a vessel is carried along a canal. 



"This wave-like motion of an insect's heart also has the advantage that, just 

 before a pulse-wave has reached the chambers farthest in front, the hinder ones 

 are already prepared for the production of a second, for, as a matter of fact, 

 often 60, and even 100, and, in very agile insects, 150, waves pass, in a single 

 minute, through the series of chambers, which make it very difficult to follow 

 the flowing of their waves." (Graber.) 



The propulsatory apparatus. But the heart itself is only a part of the 

 entire propulsatorial apparatus to which belongs the following contrivance, the 

 nature of which has been worked out by Graber. 



Under the dorsal vessel is stretched a sort of roof-like diaphragm, i.e. a- 

 membrane, arched like the dorsal wall of the hind-body which is attached, in a 

 peculiar way, to the sides of the body. The best idea can be gained by a cross- 



