104 



MOLLUSCA 



1.0.1, the central tooth being absent and the lateral teeth 

 peculiarly long and connected with muscles. The term 

 Ptenoglossa (fig. 9, D) 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 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, 4). 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 



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 the 



B C v ' '--" A 



Fio. 10. Three views of Chiton. A. Dorsal view of Chiton Wosnessenksii, 

 MM'l., showing the eight shells. (After Middendorf.) 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) ; kr, 

 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. 6, buccal mass ; m, retractor muscles of the buccal mass ; m, 

 ovary ; od, oviduct ; i, coils of intestines ; ao, aorta ; c 1 , left auricle ; c, 

 ventricle. 



number of sixteen or more ; an osphradium or area of 

 " olfactory epithelium " (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 

 Chitonidx, with the genera: Chiton, Lin. (figs. 10, 15, &c.); 

 Cryptochiton, Midd., 1847 ; and Cryptoplax ( = Chitonellm), 

 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 



A BCD 



Fio. 11. Neomenia mrinata, Tullberg (after Tullberg). A. Lateral view. B. 

 Ventral view. C. Dorsal view. D. Ventral view of a more extended speci- 

 men, o, 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 Neomeniae contains the two genera Neomenia, 

 Tullberg (Solenopus, Sars) (fig. 11); and Proneomenia, 

 Hubrecht. 



Order 3. Chsetoderma. 



Characters. Gastropoda Isopleura devoid of a shell, 

 which is replaced by numerous minute calcareous spines 



Fio. 12. Chsetoderma nitidulum, Loven (after Graff). The cephalic enlarge- 

 ment is to the left, the anal chamber (reduced pallial chamber, containing 

 the concealed pair of ctenidia) to the right. 



standing up like hairs on the surface of the body ; body 



