AN ATOM V AND PHYSIOLOGY Of A SNAtL. 2$ 



repeated to represent their number. Thus oooo.i.i.i.oooo is the 

 formula for the lingual teeth of Chiton Stelleri. A single median 

 tooth, an admedian series, and a lateral series may be thus dis- 

 tinguished. In some Glossophora * only median teeth are present, 

 or large median teeth, with a single small admedian tooth on each 

 side of it ; these ar * termed Rachiglossa (formula, i. or i.i.i.). 

 In a large number of Glossophora we have three admedian on 

 each side and one median, no lateral pieces ; these are termed 

 T?enioglossa (formula 3.1.3.)' Those with numerous lateral 

 pieces, four to six or more admedian pieces, and a median piece 

 or tooth, are termed Rhipidoglossa (formula x.6.i.6.x., where x. 

 stands for an independent number of lateral pieces). The Toxo- 

 glossa have i.o.i., the central tooth being absent, and the lateral 

 teeth peculiarly long and connected with muscles. The term 

 Ptenoglossa is applied to those Glossophora in which the radula 

 presents no median tooth, but an indefinite and large number of 

 admedian teeth, giving the formula x.o.x. When the admedian 

 teeth are indefinite (forty to fifty) and a median tooth is present, 

 the term Myriaglossa is applied (formula x. i.x.). It must be 

 understood that the pieces or teeth thus formulated may 

 themselves vary much in form, being either flat plates, or denti- 

 culated, hooked, or spine-like bodies" (Professor Ray. Lankester, 

 Art. "Mollusca." Ency. Brit., vol. ix., pp. 640, 641). In some of 

 the Helices the total number of teeth present on one radula may 

 amount to more than thirty-nine thousand. The radula is sup- 

 ported by a subradular membrane, under which there are a pair 

 of odontophoral cartilages to which intrinsic muscles are attached. 

 The buccal mass is moved by numerous muscles. The refractor 

 muscle is a large sheet which passes back from its floor to be 

 inserted into the columella ; \b% protractors are a number of deli- 

 cate bundles of muscle tissue, which pass from its side walls to be 

 inserted into the integument of the head ; the levators are delicate 

 like the last, and arise just above them to be inserted near the 

 ventral tentacles ; the depressors pass obliquely backwards, and 

 are situated just below the protractors. MacMunn has found a 

 colouring matter, termed myohaematin, in these muscles, and 

 Prof. Lankester has found that haemoglobin is present in the 

 * Molluscs with an odontophore and radula. 



