45 



fouud ; they are much more strongly developed in the walls 

 of the last two chambers than elsewhere. The endocardium 

 is an excessively thin membrane ; in certain places it can 

 be made out with the aid of an oil-immersion lens 

 (iVin.^, but only with considerable difficulty. 



The alary muscles are attached to the dorsal integu- 

 ment between the two groups of longitudinal tergal 

 muscles on either side of the body. Each muscle is 

 composed of a small bundle of striated fibres [al. ??(. in 

 fig. 41), and these latter are prolonged with a tendonous 

 strand of connective tissue [al. t.). As each tendon 

 approaches the heart it broadens out and splits into two 

 fan-shaped sheets or layers, one of which spreads out over 

 the dorsal surface of the heart and the other over its 

 ventral aspect (fig. 43). In the space between these two 

 layers is situated an ostium or lateral perforatioii of the 

 wall of the heart, and altogether there are six pairs of 

 ostia and alary muscles. 



The aorta («. in fig. 35) may be said to commence 

 immediately in front of the first ostium, and directly 

 above the junction of the oesophagus with the mid-gut. 

 At its commencement it is triangular in transverse 

 section (fig. 42j but, traced further forwards, it becomes 

 more flattened, and is closely applied to the dorsal wall of 

 the oesophagus (fig. 51). By examining a series of 

 consecutive transverse sections, it will be seen that the 

 aorta gradually comes to wrap itself round the fore- 

 gut, and eventually completely surrounds it (fig. 35). 

 Just beneath the supra-oesophageal ganglion the aorta is 

 seen to terminate in a slightly expanded funnel-shaped 

 mouth (fig. 27), which enables the blood to flow directly 

 from the heart into the hsemocoelic cavity of the head. 

 The walls of the aorta are entirely membranous, and 

 contain no muscle fibres. 



