144 THE INTERNAL STRUCTURE OF THE FULL-GROWN LARVA 



direction. The swollen posterior region (fig. 63), which is called 

 the heart, lies in the last three or four segments, its anterior limit 

 being hard to define. It consists of three distinguishable chambers, 



which, however, are not divided by septa. 

 Three pairs of ostia (os.), each provided 

 with a pair of internal valves (v.), are 

 situated laterally, and place the cardiac 

 cavity in communication with the peri- 

 cardium, in which this portion of the 

 dorsal vessel lies. There are three pairs 

 of alar muscles controlling the action of 

 this posterior cardiac region of the dorsal 

 vessel. Lowne describes other openings 

 in the wall of the " heart " of the blow-fly 

 larva, but I have been unable to find 

 others than those already described in 

 this larva; it has three pairs only. 



The dorsal aorta is the anterior con- 

 tinuation of the dorsal vessel, which 

 gradually diminishes in diameter. When 

 it reaches the fifth segment and lies above 

 the ganglion, it terminates in a peculiar 

 cellular structure (fig. 53, c.r.), which in 

 the blow-fly has a circular shape and was 

 called by Weismann the " ring." In the 

 larva of M. domestica it has not so pro- 

 nounced a ring-like appearance, but is 

 more elliptically compressed and rather 

 A-shaped. The cells of Avhich it is com- 

 posed have a very characteristic appear- 

 ance, and are rather similar to a small 

 gi-oup of cells lying on the neck of the 

 pruventriculus and at the anterior end of 

 the dorsal vessel of the fly. From the 

 lower sides of this cellular structure (c.r.) the outer sheaths of the 

 major cephalic imaginal discs depend, and extend anteriorly to the 

 pharyngeal mass, enclosing between them the anterior portion of 

 the great ventral blood sinus. 



A, 



Fig. 63. Horizontal section 

 of posterior or "cardiac" 

 region of the dorsal vessel. 

 (From camera lueida draw- 

 ings.) 



OS. Ostium. t\ Valvular 

 flaps guarding the same. 



