346 i)K. R. HORST. 



they can retract into the shell. At the dorsal side another 

 muscle is situated, passing transversely from the right valve 

 to the left, and serves to shut the shell (sp). The prse-oral cili- 

 ated ring is composed of a double row of cilia, which surround 

 a vaulted area, in the middle of which is found the thicken- 

 ing already mentioned before, now consisting of several 

 layers ofepiblastic cells (tp); this thickening gives rise to 

 the supra-oesophageal ganglion. On its upper side it is 

 traversed by a groove, which seems to divide it slightly into 

 two halves. 



Meanwhile the alimentary tract has augmented consider- 

 ably in length and in width, the stomach is differentiated 

 into a superior and an inferior division, and the intestine 

 begins at the junction of these two divisions. The superior 

 portion of the stomach gives rise at both sides to a large lobe 

 / (d), the origin of the liver. The digestive tube is covered 

 with cilia on the whole of its inner surface, without, perhaps, 

 including the hepatic sacs. 



At the ventral side of the larva, almost in the region of 

 the foot, a bur-shaped thickening of the epiblast is present ; 

 this is probably the rudiment of the pedal ganglia, although 

 I have not found the otocysts, which Lacaze-Duthiers says 

 he has observed ; also I have not succeeded in discovering an 

 excretory organ, though I did my utmost to find it, because 

 the oyster-larva offers so much resemblance with the Tro- 

 chophora of Teredo, described by Hatschek. Perhaps later 

 investigations will show that provisional renal organs are 

 not wanting here. 



