168 ALEXANDEE PETRUNKEYITCH 



but the length of the posterior valve is many times smaller than 

 that of the anterior valve. Indeed, the posterior aorta which 

 begins at this place is a thin vessel gradually becoming smaller 

 as it traverses all the segments of the postabdomen. It may 

 be traced through the poison gland into the sting, where it ends 

 apparently without any ramifications. 



I have stated that the valves are muscular in structure. It may 

 be objected that I have adduced no evidence in support of this 

 assertion and that one may just as well claim that the valves are 

 internal projections of the same connective tissue which as its 

 adventitia surrounds the heart. Indeed, I have no sections 

 through either the anterior or posterior aortic valves to prove 

 or disprove either of the contentions. But I have already men- 

 tioned the fact that spiders possess the same types of valves. 

 A comparison of figure 6 \\ith figure 1 will show that the position 

 and appearance of the anterior valve in both scorpions and spiders 

 is the same. Similarlj'', a comparison of figures 4 and 7 will 

 disclose the identity in structure of the anterior aortic valve in 

 these two orders of Arachnida. Now I happen to have a great 

 many sections through young spiders, and these show the intima 

 lining both surfaces of the valve and continuing directly as 

 intima of the heart itself. In many cases there is a slight loosen- 

 ing of the intima from the muscularis, with the consequence 

 that it appears as an uninterrupted Hne. The intima of the 

 heart being the sarcolemma of the muscle fibers, it is not con- 

 ceivable that it could line any but muscular tissue. 



Let us now turn our attention to the anterior aorta and the 

 arterial blood vessels of the cephalothorax (fig. 1). The anterior 

 aortic valve, having the exact direction of and lying immediately 

 in front of the diaphragm represents the exact demarkation line 

 between the heart and the aorta. Otherwise, the transition from 

 the heart to the aorta would be scarcely perceptible. Shortly 

 beyond the valve the aorta gives rise to a pair of small arteries 

 supplying with blood the pair of dorsoventral muscles which 

 separate the first cephalothoracic diverticula of the midgut from 

 the second. 



