176 ALEXANDER PETRUNKEVITCH 



these tendinous bands, which is certainly not the case with the 

 heart hgaments of the subsequent metameres. 



The aorta is exceedingly short and forms almost at once two 

 vessels which are rightly regarded as the aortic arch. These 

 vessels are large and long, run at first forward, then curve down- 

 ward, pass the oesophagus on each side and open into the Vascu- 

 lar' ring a little to the inside of and above the base of the first 

 pedal artery. The entire ventral circulatory system of Limulus 

 is perineural; i.e., it sheaths completely the nervous system. Not 

 only the postoral neuromeres of the suboesophageal ganglionic 

 mass, but the supraoesophageal forebrain as well is enclosed in 

 this perineural circulatory system. The haemal sheath extends 

 through the entire length of the ventral nervous cord in the 

 abdomen. Accordingly, neither supraneural, nor subneural, nor 

 interneural or vertical arteries are present. ' The cheliceral ar- 

 teries issue from the ventral surface (actual, not morphological) 

 of the vascular ring. In all this Limulus is very different from 

 the scorpion and other Arachnida. Yet the similarity is never- 

 theless quite striking. If the forebrain portion of the vascular 

 ring were removed, the rest of it would present an identical 

 appearance with the two thoracic sinuses of the Arachnida. The 

 similarity is increased by the existence of five nervous bridges 

 connecting the right and left ganglia of the five pedal neuromeres. 

 These nervous commissures are naturally ensheathed by the 

 corresponding perineural vessels which, therefore, represent the 

 five arteries in the scorpion connecting the thoracic sinuses with 

 the supraneural artery. But in what way could we explain 

 the origin of the scorpion tj^e of neural circulatory system from 

 the Limulus type or vice versa? Has the perineural system 

 broken up into two sinuses and neural blood vessels, or have the 

 latter altogether a separate origin? 



The relatively great size of the oesophagus and the position 

 of the forebrain in front of and not above the suboesophageal 

 ganglionic mass in Limulus may have something to do with the 

 differences between this animal and Arachnida. But this posi- 

 tion itself is by no means original. Notice the position of the 

 mouth in the middle of the ventral surface of the cephalothorax 



