CIRCULATORY SYSTEM IN ARACHNID A 171 



lead to the thoracic sinuses, whereas in the scorpion the cephahc 

 arteries arise from the aortic arch. This means simply a further 

 extension of the arch in the spider, so that the aortic arch of the 

 scorpion corresponds with the thoracic and connecting arteries 

 of the spider. The homology is unmistakable, and it may be 

 wiser to speak in the spider also of an aortic arch instead of a 

 thoracic and a connecting artery. 



The thoracic arch, then, of the spider opens into the thoracic 

 sinus at the base of the second pedal artery as in the scorpion. 

 As in the scorpion, the pedipalpal and the first pedal arteries 

 are branches of the first arterial stem given off by the thoracic 

 sinus. The aorta recurrens of the spider, showTi in my drawings, 

 is the supraneural artery. But for the comparison of the supra- 

 neural and subneural arteries of the scorpion and spider we have 

 to consult the description of these arteries given by Causard, 

 and his figures on plate IV. Instead of quoting passages in the 

 original, I translate them with such omissions as have no rela- 

 tion to our subject. 



''We will now consider the arteries which issue from the aortic 

 arches. For a long tim.e two roots were described issuing from 

 the posterior end of each goose-foot [my thoracic sinus, — A. P.], 

 forming by their junction a sort of supraganghonic anastomosis 

 which gives rise to a longitudinal artery directed backward and 

 "running along the dorsal surface of the ganghonic mass. Schnei- 

 der gave this artery the name supraneural. He has also shown 

 that in front of this anastomosis there are five others. There 

 are therefore altogether six anastomoses which this author de- 

 scribes as thin and delicate. This is true of the five anterior ones, 

 but cannot be accepted as characteristic of the last one, which 

 has a considerable diameter. Moreover, the supraneural artery 

 is rather stout ; how could it be fed by two such fine roots? These, 

 as he shows, are often incomplete, the supraneural artery aris- 

 ing from a single root which may be either the right or the left 

 one. When the root is complete this anastomosis has the shape 

 of a V. 



''The anterior anastomosis has the shape of a V open poste- 

 riorly [i.e., of an angle with a vertex directed forward — A. P.]. 



