224 PROCEEDINGS OF THE ACADEMY OF [1883. 



As has already been described by Semper/ the tongue-papilla 

 consists of two layers. The internal layer is formed of loose 

 connective tissue, the fibres of which run in every direction, and 

 in which can be distinctly seen the large fusiform nuclei; most of 

 these nuclei are bipolar, although here and there a cripolar one 

 can be seen. 



The external layer of the tongue-papilla is made up of cells 

 which possess a large nucleus, and the cell-wall, if seen at all, 

 is very faintly evident ; this la3'er seems more to be a homo- 

 geneous mass of protoplasm, in which are embedded large numbers 

 of nuclei ; here and there fine lines may be seen, which may be 

 regarded as the cell-walls (PI. X, fig. 5 m). This layer comes in 

 close contact with the radula and its teeth. The axes-of these oval 

 nuclei seem to have a definite direction. In the posterior part 

 they are all directed to the point where the radula begins, while 

 those further forward become perpendicular to the radula itself. 



When the object is well stained the difference between these 

 two parts of the tongue-papilla is distinctly seen ; the loose 

 internal part being of a light color, while the external part, rich 

 in nuclei, takes a very dark shade. 



In Ancylus the demarkation between these two parts is not so 

 pronounced as in Helix. The peripheral part of the tongue-papilla, 

 rich in nuclei, passes gradually into the loose, pale, internal part 

 (PL X, fig. 5 a). 



The epithelium of the radula, s (PI. X, fig. 5), is composed of a 

 single layer of long cylindrical epithelial cells, with well-defined 

 nuclei and distinct cell-walls. These cells are much longer at the 

 posterior part of this la3'^er, i. e., at the point where they lie in 

 contact with the matrix of tlie radula, than those nearer the mouth. 

 The larger cells rest obliquely on the tunica and parallel to the 

 large cells of the matrix ; as they become shorter they become 

 more and more perpendicular, as is seen in the figure (PL X,fig. 5 s). 

 The nuclei are small, although with a high power they can bedis- 

 tinctlj^ seen. When thus examined they have the same general 

 appearance of nuclei, and are placed in that part of the cell 

 nearest to the tunica. 



Between these long cylindrical cells of the epithelium of the 

 radula and the posterior part of the odontophore are seen five 



1 Zuni feineru Bau der MoUusken-Zunge. Zeitschr. f. wiss. ZooL, Bd. 

 ix, 1858. 



