45 



the upper side. Between these the odontophorc is placed, 

 and has, in this part alone, a wide transparent membranaceous 

 expansion. From the under side and back of both jaws two 

 muscles proceed and meet on the under side of the ribbon, 

 and continue to its point, which is tongue-shaped, and with 

 a gradually diminishing number of teeth. These muscles I 

 call the retractors. There are also two extensor muscles 

 proceeding from the point of the jaws and meeting under the 

 odontophore, about half way down the membranaceous expan- 

 sion, and continuing to its inner end. The membranaceous 

 expansion itself appeared to have free movement and not to 

 be attached in any way. Now,on turning a limpet on its back 

 in the water and watching its mouth with a lens, the ribbon 

 is seen to be in constant movement. First there is the outer 

 lip of the mouth with cirrhi and notched below. Secondly, a 

 mouth opening vertically. Within this the odontophore is 

 seen to be constantly moving, being drawn back over the 

 curved cartilaginous jaws like a strap, and when drawn back 

 the mouth would close. It seemed also as if it was pushed in 

 between and not over the jaws in returning to its place. The 

 whole operation was much like the action of a strap drawn 

 over a drum wheel. 



The action of the odontophore, however, does not rest here. 

 It is not only a rasp for tearing away from sea weeds, etc., the 

 necessary portions, but it lines the whole of the ceso- 

 phagous, and is continued along a considerable portion of the 

 intestinal tube. Its length is consequently very much more 

 than the length of the shell. In Patella Umbata the length 

 is enormous, being ten to eleven inches ; while the longest 

 diameter of the shell is seldom over two. Some naturalists 

 have imagined that the length depends upon the age. I have 

 not found it so. The proportionate length is generally the 

 same in young as in old animals. The distal end of the ribbon 

 is soft, and with rudimentary transparent teeth. This may 

 have led to the idea that the odontophore was constantly being 

 added to. It seems to me, however, that the rudimentary teeth 

 were of a different characterand structure from those of the rest 

 of the ribbon, and serve some other purpose. All the teeth 

 are hooked backwards,so that by the movement of the odonto- 

 phore the fragments of sea-weed are torn off, and continually 

 by the same action forced down the cesophagous, and literally 

 shredded as they pass over the innumerable fine points. Pro- 

 perly speaking this process combines the office of mastication 

 and digestion, teeth, gizzard, and stomach all in one, and as 

 far as my examinations have gone there is really no stomach, 

 that is to say, a cavity where food is specially reserved for 

 assimilation. There is a dilatation of the intestinal tube 



