Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/669

Rh ISOPLEUROUS GASTROPODS.] 1.0.1, the central tooth being absent and the lateral teetl peculiarly long and connected with rmiscles. The term Ptenoglossa (fig. 9, D) is applied to those Glossophor? in which the radula presents no median tooth, but ar indefinite and large number of admedian teeth, giving the formula x.O.x. When the admedian teeth are inde finite (forty to fifty), and a median tooth is present, the term Myriaglossa is applied (formula, x.l.x). It must be understood that the pieces or teeth thus formulated may themselves vary much in form, being either flat plates, or denticulated, hooked, or spine-like bodies. We shall revert to the terms thus explained in the systematic descriptions of the groups of Glossophora. The muscular development in connexion with the whole buccal mass, and with each part of the radular apparatus, is exceedingly complicated, as many as twenty distinct muscles having been enumerated in connexion with this organ. In addition to the radula, and correlated with its development, we find almost universally present in the Glossophora a pair of horny jaws (usually calcified) de veloped as cuticular productions upon the epidermis of the lips (fig. 9, A, b). The radula and the shelly jaws of the Glossophora enable their possessors not only to voraciously attack vegetable food, but the radula is used in some in stances for boring the shells of other Mollusca, and the jaws for crushing the shells of Crustacea, and for wound ing even Vertebrata. PHYLUM MOLLUSCA. BRANCH A. GLOSSOPHORA. Characters. Mollusca with head-region more or less prominently developed ; always provided with a peculiar rasping-tongue the odontophore rising from the floor of the buccal cavity. The Glossophora comprise three classes, chiefly distin guished from one another by the modifications of the foot. Class I. GASTROPODA. Characters. Glossophora in which (with special excep tion of swimming forms) the FOOT is simple, median in position, and flattened so as to form a broad sole-like sur face, by the contractions of which the animal crawls, often divided into three successive regions the pro-, meso-, and meta-podium by lateral constrictions. The Gastropoda exhibit two divergent lines of descent indicated by the term sub-class (see p. 649). Sub-class 1. GASTROPODA ISOPLEURA. Characters. Gastropoda in which not only the head and foot but also the visceral dome with its contents and the mantle retain the primitive BILATERAL SYMMETRY of the archi-Mollusc. The anus retains its position in the median line at the posterior end of the body. The whole visceral mass together with the foot is elongated, so that the axis joining mouth and anus is relatively long, whilst the dorso-pedal axis at right angles to it is short. The CTENIDIA, the NEPHRIDIA, GENITAL DUCTS, and CIRCULA TORY ORGANS are paired and bilaterally symmetrical. The pedal and visceral NERVE-CORDS are straight, parallel with one another, and all extend the whole length of the body ; the ganglionic enlargements are feebly or not at all deve loped. The Isopleura comprise three orders. Order 1. Polyplacophora (the Chitons). Characters. Gastropoda Isopleura with a metameric re petition of the shell to the number of eight. The shells of the primitive type are partially or wholly concealed in shell- sacs comparable to the single embryonic shell-sac of other Mollusca. On the surface of the mantle-flap numerous 641 calcified spines and knobs are frequently developed. The ctenidia are of the typical form, small in size and meta- merically repeated along the sides of the body to tho B FIG. 10. Three views of Chiton. A. Dorsal view of Chiton Wosnesscnksii, Midd., showing the eight shells. (After Jliddemlorf.) B. View from the pedal surface of a species of Chiton from the Indian Ocean, p, foot ; o, mouth (at the other end of the foot is seen the anus raised on a papilla) ; AT, oral fringe ; br, the numerous ctenidia (branchial plumes) ; spreading beyond these, and all round the animal, is the mantle-skirt. (After Cuvier.) C. The same species of Chiton, with the shells removed and the dorsal integument reflected, b, buccal mass ; m, retractor muscles of the buccal mass ; ov, ovary ; od, oviduct ; i, coils of intestines ; ao, aorta ; c*, left auricle ; c, ventricle. number of sixteen or more ; an osphradium or area of &quot; olfactory epithelium &quot; (Spengel) is found at the base of each ctenidium. The other organs are not subject to metameric repetition. The odontophore is highly devel oped ; the teeth of the lingual ribbon are varied in form,- several in each transverse row (fig. 9, E). Paired genital ducts distinct from the paired nephridia are present. The order Polyplacophora contains but one family, the Chitonidas, with the genera: Chiton, Lin. (figs. 10, 15, &amp;lt;fec.); Cryptockiton, Midd., 1847; and Cryptoplax ( = Chitonellus Blainv., 1818. Order 2. Neomeniae. Characters. Gastropoda Isopleura devoid of a shell, which is replaced by innumerable microscopic calcified plates or spicules set in the dorsal epidermis ; mantle-flap not lateral, but reduced to a small collar surrounding the

b A BCD Fio. H.Neomenia carinata, Tullberg (after Tullberg). A. Lateral view. B. Ventral view. C. Dorsal view. D. Ventral view of a more extended speci men, a, anterior; 6, posterior extremity; c, furrow, in which the narrow foot is concealed. anus ; ctenidia represented by a symmetrical group of bran chial filaments on either side of the anus; foot very narrow, sunk in a groove; odontophore feebly developed, but the radula many-toothed ; gonads placed in the pericardium discharging by the nephridia ; no special generative ducts. The order Neomeniaj contains the two genera Neomenia, Tullberg (Solenojnts, Sars) (fig. 11); and Proneomrnia, Hubrecht. Order 3. Chaetoderma. Characters. Gastropoda Isopleura devoid of a shell, which is replaced by numerous minute calcareous spines Fio. 12. Chrtoderma nitidulum, Loven (after Graff). The cephalic enlarge ment is to the left, the anal chamber (reduced pallia! chamber, containing the concealed pair of ctenidia) to the right. tanding up like hairs on the surface of the body ; body