CIRCULATORY SYSTEM IN ARACHNID A 173 



is the eleventh neuromere proper, while the posterior portion 

 represents the remnant of the abdominal neuromeres, whether 

 contracted and fused or lost altogether makes no difference as 

 regarding our proposition. We thus have at the most eleven 

 interganglionic surfaces, if we count the partition of the eleventh 

 neuromere as complete. Therefore, there cannot be more than 

 eleven vertical arteries, since arteries passing through instead 

 of between ganglia are not known. 



The heart of the spider has four pairs of ostia in the Theraph- 

 osidae and only three pairs in the true spiders, as against seven 

 pairs in the scorpions. From the position of the aortic valve, 

 it is safe, however, to accept that the reduction in the number of 

 ostia took place in a progressive direction from the rear end of 

 the heart forward. What has happened to the rear portion of the 

 heart, which has lost the ostia? I think it must have shrunk 

 in size, become considerably thinner and changed into what 

 became the proximal end of the posterior aorta. We have 

 seen that the posterior aortic valve has a structure distinctly 

 different from that of the anterior valve. It w^ould be scarcely 

 necessary to assume a progression of the posterior aortic valve, 

 a shifting of its position with the loss of ostia. Is it not more 

 likely that the posterior aortic valve is a modified remnant of 

 the last pair of ostia valves which have become functionless as 

 such, when the ostia themselves closed? With other words, 

 that the posterior aortic valve of a Theraphosid is the remnant 

 of the fifth pair of ostia valves, while in true spiders it is the rem- 

 nant of the fourth pair? 



From the above comparison of the circulatory system of the 

 scorpion with that of the spider we may now draw the following 

 important conclusions : the scorpion represents the more genera- 

 lized and therefore more primitive circulatory system among 

 A*rachnida, the spider the more modified and therefore the more 

 advanced. The most permanent structure in the circulatory 

 system of Arachnida is the anterior aortic valve which is at- 

 tached at the anterior edge of the first abdominal tergite and 

 therefore marks the hmit between cephalothorax and abdomen. 

 The reduction in the number of ostia stands in direct relation- 



