CIRCULATORY SYSTEM IN ARACHNID A 161 



This description of the gross anatomy of the circulatory system 

 is, as we shall see, not quite correct, but served to show the simi- 

 larity between the scorpion and Limulus. On the other hand, 

 it created differences between scorpions and spiders especially, 

 since the circulatory system of the latter had only recently been 

 described correctly. Moreover, the exact position of the heart 

 in both groups remained unnoticed, and yet therein hes its value 

 from the morphological point of view. 



The best material for the study of the circulatory system 

 in the scorpion is furnished by that stage of embryos represented 

 in our text figure A, in which all organs are already fully de- 

 veloped, but the nervous system has not yet reached its final 

 state of contraction. The postabdomen is still short, the length 

 of each segment being smaller than the diameter. The last 

 neuromere has already begun to fuse with the preceding one and 

 is almost entirely withdrawn into the fourth postabdominal 

 segment. The first and second abdominal neuromeres have 

 moved forward, passed the diaphragm, are completely within the 

 cephalothorax, and are fused with the thoracic ganglia into 

 one mass in which the separate neuromeres remain however 

 clearly defined as they do, even in adult scorpions. The third 

 abdominal neuromere is just on the verge of passing the dia- 

 phragm, while the fourth, which in the adult scorpion forms the 

 last portion of the thoracic ganglionic mass, is still in the abdomen 

 just behind the genital opening and on a level with the basal 

 plate of the comb. In this connection we may state that Buxton 

 had recently shown that the comb receives its nerves from the 

 eighth postoral ganghon of the suboesophageal mass, i.e., from 

 the third abdominal neuromere — an observation which I am 

 able fully to confirm. 



In median sagittal sections the diaphragm appears as a 

 thin line. Its ventral portion runs from the ventral body wall 

 just in front of the genital opening to the endosternite which 

 lies above the nervous system. Its dorsal portion arises from a 

 vertical transverse crest of the endosternite and proceeds up- 

 ward to the dorsal body wall, where it is attached between the 

 carapace and the first abdominal tergite. 



