432 NATURAL HISTORY AND ANATOMY OF 



surrounds it and its muscles by dividing and again closing. 

 It then proceeds to the commencement of the oesophagus, and 

 joins a vessel to he described immediately. The ventral 

 vessel, after giving off this great trunk, proceeds to the oral 

 extremity of the pharynx, round winch it forms a vascular 

 circle. The latter sends branches back upon the pharynx ; 

 and a branch forwards, which forms a second circle or vascu- 

 lar zone round the lip, on the surface of the nervous ring, and 

 a large trunk which, running to the middle of the pharynx, 

 dilates into a sacculated sinus, which probably owes its peculiar 

 appearance to the transverse contractions into which this por- 

 tion of the tube is generally thrown. This sinus runs along 

 the second portion of the pharynx, and at the commencement 

 of the oesophagus receives the trunk formerly described as 

 proceeding from the ventral vessel. It then terminates by 

 ramifying on the oesophagus, and supplying this portion of the 

 tube with aerated blood. The use of the large trunk which 

 comes off from the ventral vessel, is evidently to supply the 

 mouth, trunk, and anterior part of the digestive tube, with 

 arterial blood, when the animal has projected the anterior part 

 of the body, and when the constriction of the snout, and the 

 pressure of the sand in which it is boring, would prevent the 

 free circulation of the blood in the two vascular circles, at 

 times when the supply is absolutely necessary from increased 

 muscular action. 



The nervous system is very simple, being merely a ring 

 surrounding the anterior part of the pharynx when it becomes 

 continuous with the shin. From this ring a nervous cord 

 runs along the under side of the animal to the extremity of 

 the body, terminating abruptly by sending off a few branches. 

 Along its course the cord gives off numerous lateral twigs, 

 which are unsymmetrical, and continue free for a short dis- 

 tance from their origin, and then disappear in the muscular 

 parietes of the body. When the animal is contracted, tie 



