THE TANGANYIKA PROBLEM. 
253 
the last two whorls of the animal’s body. There is a single 
bile duct, opening, as has already been stated, in the posterior 
chamber of the stomach. 
The heart has the normal tsenioglossate characters, and 
consists of a thin-walled auricle, a thick-walled ventricle, and 
a short aortic trunk. Between the auricle, ventricle, and 
aortic trunks there are the usual valves. The gill of 
Nassopsis is of average length, very simple in structure, and 
consists of a large number of low, broad, triangular leaves, 
the apices of which are not produced into filamentous pro- 
cesses, nor ornamented in any way. The osphradium is 
long and simple ; it lies in a groove at the base of the gill, 
and shows no tendency to become pectinated or modified in 
any way either before or behind. 
The nervous system of Nassopsis (Fig. 37) is extremely 
interesting, being one of the most archaic tsenioglossate types 
at present known. The cerebral ganglia are widely separated 
from one another, and the pleural ganglia are not only 
separated from the cerebral ganglia, but on the sides of the 
oesophagus, the cerebro-pleural connectives being conse- 
quently of considerable relative length. The supra-intestinal 
cord springs directly from the right pleural ganglion, passes 
