292 CHORDATA. 
groove, and has paired dranchials leading down the pharyn- 
geal walls into the ventral vessel which lies immediately 
below the endostyle. ‘lhe dorsal vessel runs back to the 
stomach and intestine, over which it breaks up into sinuses. 
The 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. ‘Ihe heart has a single chamber 
and is clearly ventral in 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 Ba/anoglossus. 
ll vessel Se 
System 
Pharynx a 
soe vessel<— Hearf 
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 swéneural 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 
papilla 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. 
Nervous. 
Excretory. 
