MOLLUSCA 



103 



wMch. merely points to the possession of a head is objec- 

 tionable, since this is common to them and the hypotheti- 

 cal archi-Mollusca from which they descend. The term 

 Odontophora, -which has been applied to them, is also un- 

 suitable, since the organ which characterizes them is not a 

 tooth, but a tongue. 



Fia. 9. — Odontophore of Glossophorous Mollusca. 



A. Diagram showing mouth, oesophagus, aud lingual apparatus of a Gastro- 



pod in section, au, upper lip ; al, lower lip ; 6, calcareo-comeous jaw of 

 left side ; c, outer surface of the snout ; d, oesophagus ; e, fold in the 

 ■wall of the oesophagus behind the radular sac (n) ; f, anterior termina- 

 tion of the radula and its bed, the point at which it wears away ; fir, 

 base of the radular sac or recess of the pharynx ; 7t, cartilaginous piece 

 developed in the floor of the pharynx beneath the radula, and serving 

 for the attachment of numerous muscles, and for the support of the 

 radula ; i, anterior muscles ; fc, posterior muscles attached to the carti- 

 lage ; I, muscle acting as a retractor of the buccal mass ; m, muscle 

 attached to the lower lip ; 7t, posterior extremity of the radular sac ; 

 0, the bed of the radula or layer of cells by which its lower surface is 

 formed ; p, the homy radula or lingual ribbon ; q, opening of the radular 

 sac into the pharynx or buccal cavity ; r, cells at the extreme end of 

 the inner surface of the radular sac which produce as a "cuticular 

 secretion " the rows of teeth of the upper surface of the radula. 



B. Badula or lingual ribbon of Palvdina vivvpara^ stripped from its bed, — a 



homy, cuticular product. 



C. A single row of teeth from the radula of Trochus ciTiercmus. Rhipido- 



glossate ; formula, x.5.1.5.x. 



D. A single row ofteeth from the radula of yaWKmayrusiiis. Ptenoglossate; 



formula, x.O.x. 



E. A single row of teeth from the radula of Chiton cinereus. Too elaborate 



for formulation. 



F. AsinglerowofteethfromtheradulaofPafe^^VMZgFato. Formula, 3.1.4.1.3. 



G. A single row of teeth ftom the radula of Cyprsea helvola. Tasnioglossate ; 



formula, 3.1.3. 

 H. A single row of teeth from the radula of Nassa, amivZala. Rachiglossate ; 

 formula, 1.1.1. The Common Whelk is similar to this. 



The general structure of the odontophore ( = tooth- 

 bearer, in allusion to the rasp-like ribbon) of the glosso- 

 phorous Mollusca may be conveniently described at once. 

 Essentially it is a tube-like outgrowth — the radular sac (fig. 

 9, A, ff, n) — in the median line of the ventral floor of the 

 stomodseum, upon the inner surface of which is formed a 

 chitinous band (the radula) beset with minute teeth like a 



rasp (j»). Anteriorly the ventral wall of the diverticulum 

 is converted into cartilage (h), to which protractor and re- 

 tractor muscles are attached (k, i), so that by the action of 

 the former the cartilage, with the anterior end of the ribbon 

 resting firmly upon it, may be brought forward into the 

 space between the lips of the oral aperture (au, al), and 

 made to exert there a backward and forward rasping action 

 by the alternate contraction of retractor and protractor 

 muscles attached to the cartilage. But in many Glosso- 

 phora {e.g., the Whelk) the apparatus is complicated by the 

 fact that the diverticulum itself, with its contained radula, 

 rests but loosely on the cartilage, and has special muscles 

 attached to each end of it, arising from the body wall ; 

 these muscles pull the whole diverticulum or radular sac 

 alternately backwards and forwards over the surface of the 

 cartilage. This action, which is quite distinct from the 

 movement of the cartilage itself, may be witnessed in a 

 Whelk if the pharynx be opened whilst it is alive. It has 

 also been seen in living transparent Gastropods. The chi- 

 tinous ribbon is continuously growing forward from the 

 tube-like diverticulum as a finger-nail does on its bed, and 

 thus the wearing away of the part which rests on the car- 

 tilage and is brought into active use, is made up for by 

 the advance of the ribbon in the same way as the wearing 

 down of the finger-nail is counterbalanced by its own for- 

 ward growth. And, just as the new substance of the 

 finger-nail is formed in the concealed part, sunk posteriorly 

 below a fold of skin, and yet is continually carried forward 

 with the forward movement of the bed on which it rests, 

 and which forms its undermost layers, so is the new sub- 

 stance of the radula formed in the compressed extremity 

 of the radular sac (w), and carried forward by the forward 

 movement of the bed (o) on which it rests, and by which 

 is formed its undermost layer. This forward-moving bed 

 is not merely the ventral wall of the radular diverticulum, 

 but includes also that portion of the floor of the oral cavity 

 to which the radula adheres (as far forward as the point / 

 in fig. 9, A). At the spot where the radula ceases, the for- 

 ward growth-movement of the fioor also ceases, just as in 

 the case of the finger-nail the similar growth-movement 

 ceases at the line where the nail becomes free. 



The radula or cuticular product of the slowly-moving 

 bed can be stripped ojBf, and is then found to consist of a 

 ribbon-like area, upon which are set numerous tooth-like 

 processes of various form in transverse rows, which follow 

 one another closely, and exactly resemble one another in the 

 form of their teeth (fig. 9, B). The tooth-like processes in a 

 single transverse row are of very different shape and num- 

 ber in different members of the Glossophora, and it is pos- 

 sible to use a formula for their description. Thus, when 

 in each row there is a single median tooth with three teeth 

 on each side of it more or less closely resembling one 

 another, as in fig. 9, G, we write the formula 3.1.3. When 

 there are additional lateral pieces of a different shape to 

 those immediately adjoining the central tooth, we indi- 

 cate them by the figure 0, repeated to represent their 

 number, thus 0000.1.1.1.0000 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 ad- 

 median tooth on each side of it (fig. 9, H) ; these are 

 termed Eachiglossa (formula, — .1. — or 1.1.1). In a large 

 number of Glossophora we have three admedian on each 

 side and one median, no lateral pieces (fig. 9, G) ; these 

 are termed Tsenioglossa (formula, 3. 1.3). Those with nume- 

 rous lateral pieces, four to six or more admedian pieces, 

 and a median piece or tooth (fig. 9, C) are termed Khipi 

 doglossa (formula, x.6. 1.6.x, where x stands for an inde- 

 finite number of lateral pieces). The Toxoglossa have 



