THE LARVA OF TIPULA MAXIMA. 133 



Taking any one point of the heart, the pulsation may be said to include four 

 phases : — 



(a) The resting position, during 'which the heart is expanded to its normal diameter 

 and the blood is streaming rapidly forward. As the pressure from behind increases, or 

 perhaps as the resistance in front increases, the second phase is reached. 



(b) A slight expansion occurs, due to the stretching of the elastic walls by the rapid 

 increase in internal pressure. This is followed at once by 



(c) A forceful and sudden contraction, which drives the contained blood forward, a 

 back flow being prevented by the closing of the intersegmental valves. After a slight 

 pause, 



(d) finally a recovery by expansion to the original position of rest, in which phase 

 (i. e. rest) the heart remains for a period equal to about half the total period. 



Movements of Ostial Valves. 



During the pulsation of the vessel the ostial valves pass through a regular succession 

 of movements. 



(«) During the resting phase {a, above) the valves are open, i. e. the pouch is bulged 

 inwards, and blood is entering in a steady stream and joins the forward flow in the 

 heart. They remain in this position until the heart expands (b, above), when they 

 suddenly bulge outwards and close. This movement does not appear to be due to 

 muscular movement of the valve, but to be the result of the increase in internal 

 pressure. 



During contraction (c, above) they are closed, and the blood is flowing forward 

 through the intersegmental valve to the chamber in front. 



The moment the heart relaxes and begins to expand (d, above) the relief of pressure 

 within causes the ostial pouch once more to suddenly invert, and the valve opens and 

 blood enters and joins the forward flow. 



Blood returns posteriorly along the ventral sinus and also along each side of the 

 heart in the pericardial sinus, some entering the heart at the ostia. No signs of any 

 special flow into the pericardial sinus through the diaphragm could be detected. 



In the posterior segment of the body the periodic rush and stoppage of the blood 

 is well seen. The movement here appears largely due to the suction exerted by 

 the expansion of the posterior chamber of the heart and the opening of the posterior 

 ostium. 



The blood-flow throughout the body is intermittent, and synchronous with the 

 pulsations of the dorsal vessel. 



On the average, a pulsation occupies 2-5 seconds, or at the rate of about 24 per minute, 

 the resting phase occupying about half the whole period. 



During the pulsations of the heart no regular and corresponding elevation and 

 depression of the diaphragm could be observed, and, at any rate in this larva, I am 

 inclined to believe that its chief function is that of support, and that it plays no part, 

 or very little, in the directive action on the blood-flow. 



SECOND SERIES. — ZOOLOGY, VOL. XI. 20 



