292 CHORD AT A. 



groove, and has paired branchials leading down the pharyn- 

 geal walls into the ventral vessel which lies immediately 

 below the endostyle. 'i he dorsal vessel runs back to the 

 stomach and intestine, over which it breaks up into sinuses. 

 1 he ventral vessel also runs back to these sinuses ; but in 

 its course, just after leaving the pharynx, it is modified into 

 a simple contractile heart. '\ he heart has a single chamber 

 and is clearly ventral va position. It contracts rhythmically, 

 driving the blood forwards to the pharynx for a certain 

 number of beats, and then, reversing its action^ drives it 

 backwards to the viscera; hence it is alternately systemic 

 and respiratory. For this reason it is impossible to speak 

 of arteries or veins. 



In the accompanying diagram the heart is shown in its 

 respiratory phase, during which the dorsal vessel may be 

 directly compared with the dorsal aorta of Vertebrata^ the 

 ventral vessel with the ventral aorta and the part of the 

 ventral vessel between the heart and the system with the 

 main subintestinal vein. A reversible heart such as this 

 also occurs in some allies of Balanoglossus. 



Ocsal ^/cssel 



Pha 



rynx 



V^\ 



^cnWsl vessel <— Heart 



The main nerve-ganglion or brain lies dorsally between 

 the mouth and atriopore, just under the ectoderm, and gives 

 off fine branches to the muscles. A main nerve- 

 trunk runs back dorsally to the stomach. Under 

 the brain lies a subneural gland, which communicates by a 

 duct with the front part of the pharynx just inside the ring 

 of tentacles. It may possibly be an excretory organ. 



No special sense-organs are recognisable, though the 

 papillae around the mouth apparently function for testing 

 the quality of the incurrent water. 



Excretory products are said to accumulate in solid 

 „ . masses in parts of the body, and to be extruded 



only on the death of the individual. No 

 definite excretory organs have been described. 



