Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/668

Rh 640 MOLLUSCA [CLASSES AND ORDERS. which 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. h ., e i IT Fio. 0. Oclontophore of Glossophorous Mollusea. A. Diagram showing mouth, oesophagus, and lingual apparatus of a Gastro pod in section, aw, upper lip ; al, lower lip ; b, calcareo-corneous jaw of left side ; c, outer surface of the snout ; d, cesophagus ; 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 ; g, l)ase of the radular sac or recess of the pharynx ; h, 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 ; k, posterior muscles attached to the carti lage ; /, muscle acting as a retractor of the buccal mass ; m, muscle attached to the lower lip ; n, posterior extremity of the radular sac ; o, the bed of the radula or layer of cells by which its lower surface is formed ; p, the horny 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 &quot;cuticular secretion &quot; the rows of teeth of the upper surface of the radula. B. Radula or lingual ribbon of Paludina vivipara, stripped from its bed, a horny, cuticular product. C. A single row of teeth from the radula of Trochus cinerarius. Rhipido glossate ; formula, x.5.1.. r ).x. D. A single row of teeth from the radula of Faultimafmgilis. Ptcnoglossate ; formula, x.O.x. E. A single row of teeth from the radula of Chiton cinereus. Too elaborate for formulation. F. A single row of teeth from the radula of Patella vulyata. Formula, 3.1.4.1.3. G. A single row of teetli from the radula of Cyprasa helvola. Tamioglossate ; formula, 3.1.3. II. A single row of teeth from the radula of Nassa- annidata. 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 Mollusea may be conveniently described at once. Essentially it is a tube-like outgrowth the radular sac (fig. 9, A, &amp;lt;/, ) in the median line of the ventral floor of the stomodaeum, upon the inner surface of which is formed a chitinous band (the radula) beset with minute teeth like a rasp (p). Anteriorly the ventral wall of the diverticulum is converted into cartilage (/i), to which protractor and re tractor muscles are attached (, 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 Avitnessed 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 (n), 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 floor 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 off, 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 Rachiglossa (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 Ttenioglossa (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 Ehipi doglossa (formula, x.6, 1.6.x, where x stands for an inde finite number of lateral pieces). The Toxoglossa have