OEGAXS OF MASTICATION. 187 



as in figs. 193 and 194. The central part is called the 

 rachis, anil the lateral parts pleura. The rachidian teeth 

 sometimes form a row of plates, as in Nerita ; or they have 

 a tile-shaped disposition, with pectinated borders, as in Buc- 

 The lateral series exhibit an immense variety of 

 forms, some having fringed processes, as in Nerita (fig. 193). 

 By the aid of this singular dental organ the gasteropoda bruise, 

 ra'sp, or pierce the vegetable or animal substances on which they 

 subsist, and bore through the shells of mollusca, on which 

 they prey. The tongue of the whelk (Buccinum) is fur- 

 nished with upwards of one hundred rows of pectinated 

 teeth, but the number of the dental rows on the lingual 

 ribbon varies in different genera, and at the different periods 

 of life of the individual. The dental organ of the common 

 limpet (Patella vulgata) is more than twice the length of the 

 animal, and in a state of repose is folded back into the digestive 

 tube. The dental membrane is wide in the mouth, and con- 

 tracted in the esophagus; and after a course of nearly three 

 inches, terminates near the small transverse stomach. The new 

 teeth, like those of rays and sharks, are developed from be- 

 hind, and are brought into use when required, a new series 

 arising with the age of the individual.* T. \V.] 



337. The articulata are remarkable, as a class, for the 

 diversity and complication of the apparatus for taking and 

 dividing their food. In some marine worms, Nereis, for ex- 

 ample, the jaws consist of a pair of curved, horny instru- 

 ments, lodged in a sheath. In spiders, they are external, and 

 sometimes mounted on long, jointed stems. Insects which 

 masticate their food have, for the most part, at least two pairs 

 of horny jaws (figs. 195, 196 m), besides several additional 

 pieces serving for seizing and holding their food. Those living 

 on the fluids extracted either from plants or from the blood 

 of other animals, have the masticatory organs transformed 

 into a trunk or tube for that purpose. This trunk is some- 

 times rolled up in a spiral manner, as in the butterfly (fig. 



* Loven's Memoir on the Teeth of Mollusca is nearly all that we pos- 

 sess on this suhject. 



Figures 193 and 194 were drawn by Mr. Etheridge, of the Bristol In- 

 stitution, from specimens dissected and prepared by my friend John W. 

 n, Esq., F.R.C.S., Gloucester. The position of the dental organ of 

 the Patella (fig. 194) on the slide does not permit the left lateral teeth of 

 the specimen to be seen. 



