592 S. B. MITEA. 



itself (Anodon). A structure of sncli a comparatively great 

 length canuot be regarded as a vestigial structure unless 

 there are cogent reasons for taking it in that light; but there 

 are none. 



(2) The caecum (a diverticulum of the alimentary canal 

 starting from the pyloric end of the stomach)^ in which in 

 some Lamellibranchs the style is lodged^ and which the 

 hypothesis under consideration would take to represent the 

 sac proper of the radula of a glossophorous Mollusc, is lined 

 with epithelial cells bearing active cilia which are much 

 longer and better developed than the cilia of the epithelial 

 cells that line the alimentaiy canal itself. Here we might 

 say parenthetically that the alimentary canal of the Lamelli- 

 branchia is lined throughout with ciliated epithelium. Now, 

 as cilia always perform an important function in the economy 

 of the organism that possesses them, we should, if the csecum 

 were really a vestigial structure, expect to see either that it 

 was not lined with ciliated epithelium at all, or that if it 

 was lined with ciliated epithelium at all, the cilia were shorter 

 and less well developed than the cilia of the cells that line 

 the alimentary canal itself. The fact that the epithelial 

 cells lining the caecum bear such highly active and longer 

 cilia, suggests forcibly the idea that the csecum performs 

 some important function, and so cannot be regarded as a 

 vestigial structure. In fact, there are reasons (as we shall 

 see later on) for regarding it as a higher stage in the evolu- 

 tion of the receptacle for the lodgment of the crystalline 

 style, the more primitive stage being still found in some 

 species of the Laraellibranchia. 



(3) The cEecum in which the crystalline style is lodged in 

 some Lamellibranchs starts from the pyloric end of the 

 stomach, whereas the radular sac is formed by a diverticulum 

 of the wall of the oesophagus. If the former represented the 

 latter, we should expect to see the c^cum start from some 

 part of the oesophagus. We admit that, taken alone, this 

 reason is not very forcible, as there may be considerable 

 change in the position of the same fundamental structure in 



