288 Introduction to Animal Morphology. 



stretches forward, and ends anteriorly by being at- 

 tached by a globular, toothless portion (Orbis radulae) 

 to the floor of the pharynx. The teeth are of chitin 

 or conchiolin, with calcic phosphate, and are true 

 cuticular structures ; they vary in number from ten 

 (Pontolimax) to many thousands (Trochidse) ; their 

 forms and numbers are of taxonomic value.* The 

 radula is absent in Phyllirhoe, Tethys, &c., as are the 

 pharyngeal jaws in Aplysia, Tethys, Sec. 



In Doris there is a prehensile collar of two arched lancet- 

 like conchiolin plates outside the buccal lips, covered with a 

 file-like surface of close groups of bifid pointed warts. Pon- 

 tolimax and Elysia have a prehensile, eversible sac appended 

 to the tongue ; in Pleurobranchus, Lobiger, Lophocercus, 

 (S:c., there are two such side pouches in the buccal mass, 

 attached in front and free behind, having long, outwardly- 

 curved, slipper-sole-shaped, horny plates. 



Into the mouth behind, rarely in front of the radula, open 

 the ducts of the salivary glands, which are usually two long 

 (Atlantidae, &c.), or short caeca (Carinaria, .^olis, &c.), some- 

 times branched (Lobiger, Lophocercus, &c.), sometimes 

 corkscrew-like (Voluta), or forked (Pleurophyllidia, &c.), very 

 rarely. In Terebra and Polycera only a single follicle exists. 

 In Phyllidia two lateral and one azygous. In Pleurobranchus 

 there are three long, branched caeca reaching to the liver. 



* The forms of radula are: — i. Gymnoglossa — toothless, as Acus. 

 2. Rachiglossata — with only rachis plates, as Volutidse. 3. Hamiglossata — 

 with rachis plates and one row of lateral teeth (Murex, Buccinidse, Ohvidas). 

 4. Toxoglossata — with no rachis teeth, but an intermediate and a lateral 

 row (Conidse and Pleurotomidse). 5. Tasnioglossata — with one row of 

 rachis teeth and three lateral plates on each side (Strombidae, Littorinidje, 

 Cerithiadse, &c.) 6. Ptenoglossata — with no racliis teeth, and manylateral 

 (.Scalaria, Janthina). 7. Rhipidoglossata — with one rachis row, 4—6 in- 

 termediate and many small lateral rows (Neritidoe, Trochidse, HaUotida?), 

 The paired jaws arc also variable in Pulmonates, being absent (Agnatha), 

 with a sharp tooth (Oxygnatha), striped with a crenated edge (Aulaco- 

 gnatha), or with marginal rows of teeth (Odontognatha). 



