80 DR. T. ALCOCK ON THE 



But the peculiarity of the toothed membrane which makes 

 its name of " ribbon " so appropriate is, that there is always 

 a considerable length of it behind the mouth, perfectly 

 formed, and ready to come forward and supply the place 

 of that at the front, which is continually wearing away 

 by use. 



In the Limpet this reserve ribbon is of great length, 

 being nearly twice as long as the body, and the whole of it 

 is exposed to view on simply removing the foot of the 

 animal : nothing, then, can be easier than to extract the 

 tongue of the common Limpet. But, unfortunately, what 

 you find in one kind of MoUusk is not at all what you 

 find in another. In the Acmseas for instance, which 

 are very closely related to the Limpets, and have shells 

 which cannot be distinguished, the reserve portion of the 

 ribbon has to be dug out from the substance of the Kver, 

 in which it is imbedded, that organ being, as it were, 

 stitched completely through by a long loop of it, as shown 

 in the drawing (PL VII.) of one species of this animal. 



It might be thought a comfortable reflection, that, at all 

 events, one end of the ribbon can always be found in the 

 mouth j but in many cases this is about the worst place to 

 look for it. Perhaps it may appear strange, but in some 

 of the smaller species, with a retractile trunk, a beginner 

 may very likely fail altogether in his attempt to find the 

 mouth ; if, however, the skin of the back is removed, com- 

 mencing just behind the tentacles, there will be very little 

 difficulty in making out the trunk, which either contains 

 the whole of the ribbon, as in the Whelk, or the front 

 part of it, as in Purpura and Murex, where a free coil 

 is also seen to hang from its hinder extremity. Ex- 

 amples of these two forms are represented in the drawings 

 (PL VII.). 



In the Periwinkles the same plan of proceeding, by at 

 once opening the back of the animal, is best ; and, on doing 



