TYPE MOLLUSC A. 



323 



ure whose concaye surface is furnished with ciliated cells and 

 also towards the margin with unicellular glands. These 

 structures have been supposed to 

 represent the ctenidia of the other 

 Mollusca, but this view cannot, in the 

 present condition of our information 

 concerning their structure and devel- 

 opment, be accepted without reser- 

 vation. The mouth opens into a 

 short oesophagus provided with a 

 single chitinous jaw-tooth apparently 

 formed by a fusion of two chitinous 

 masses, and behind this there is a 

 pharynx provided with a radula and 

 opening posteriorly into the some- 

 what U-shaped more or less convolu- 

 ted intestine {i) which terminates in 

 the anus {a) lying in the mid- ventral 

 line behind the foot. Into the intes- 

 tine at the turn of the U there open 

 the ducts of the digestive gland (Z), 

 and into the posterior portion of the Fig. 145. — Stktjctttkb op 

 intestine, the rectum, there open in Dentalium (after leuckaet). 

 Dentalium several ducts from a rectal 

 gland which surrounds this portion 

 of the digestive tract and whose sig- 

 nificance is quite obscure. 



The nervous system presents the 

 majority of the ganglia characteristic 

 of the Gasteropoda, and the pleuro- 

 visceral connectives do not cross one 

 another. The cerebral ganglia (ce) 

 lie at the base of the proboscis an- 

 terior to the oesophagus and have 

 closely associated with them the 

 cerebro-pedal and pleuro-pedal connectives fusing with one 

 another to pass downwards and forwards to the pedal ganglion 

 (pe) situated in the foot. Posteriorly in the vicinity of the 

 rectum lie the two visceral ganglia (vi) from which long 



a = anus. 



ce = cerebral ganglion. 



/ = foot. 



i = intestine. 



I = liver. 

 m = mouth. 

 mc = mantle-cavity. 

 pe = pedal ganglion. 



r = reproductive organ. 

 7'n = right nephridium. 

 s7i = shell. 



t = tentacle. 

 i)i = visceral ganglion. 



pleural ganglia, the 



