180 COMPARATIVE ANATOMY chap. 



B. The Pharynx and Jaws, the Tongue and Salivary Glands. 



The mouth or buccal cavity is followed in all Molluscs except the 

 Lamellibranchia by the pharynx or oesophageal bulb (buccal mass). 

 The pharyngeal cavity opens anteriorly into the buccal cavity, and 

 posteriorly into the oesophagus. The pharynx is characterised by the 

 possession of (1) jaws, which lie anteriorly at the boundary between 

 the buccal and pharyngeal cavities ; (2) a lingual apparatus at its 

 base, and (3) salivary glands, which usually open laterally near its 

 posterior boundary. 



1. Jaws are almost universal, and are sometimes, especially in 

 carnivorous animals, very highly developed ; less frequently they are 

 rudimentary or wanting. They are hard cuticular formations of the 

 epithelium of the anterior pharyngeal region, and no doubt composed 

 of conchyolin or some related substance, in a few cases hardened by 

 calcareous deposits (e.g. JVautilns). 



The jaws serve for seizing prey or pai'ticles of food. The great variations in 

 number, form, and arrangement of the jaws can best be understood by assuming 

 that they originally extended completely round the entrance to the pharynx ; and 

 that of this ring sometimes only upper and lower or sometimes only lateral portions 

 have been retained. 



Such a complete circle of jaws is found at the entrance to the pharynx in some 

 forms, such as Umbrella and Tylodina {Opisthobranchia). 



The fresh-water Pidmonatcs have an upper and two lateral jaws. 



Most Prosobranchia and Opisthobi'anchia have two lateral jaws. These may 

 approach so near one another as almost to touch {Haliotis, Fissurella). Terrestrial 

 Pulmonata have an uj)per jaw and occasionally a weak lower jaw as well. 



The jaws are particularly strongly developed in the Cephalopoda, which have an 

 upper and a lower jaw, the two together resembling in shape the beak of a parrot. 



In the OpisthohrancMate family Aj)hjsiida', Notarchus, Accra, Dolabella, and 

 Aplysiella have, besides the lateral jaws, numerous hooks or small teeth on the roof of 

 the pharyngeal cavity. The hook sacs (Fig. 17, p. 11) of the Pteropoda gymnoso- 

 mata, which are wanting only in Halopsiiche, are perhaps to be derived from these 

 pharyngeal teeth. 



The hook sacs are paired dorsal outgrowths of the pharyngeal cavity, which vary 

 in length and lie in front of the radula. The walls of the sacs carry hooks project- 

 ing inward. When the proboscis of these carnivorous animals is protruded, the sacs 

 are completely evaginated, so that the hooks come to lie outside (Fig. 17, p. 11). 



Jaws are wanting or rudimentary in the Amphineura and the Scaphopioda ; 

 among the Prosobranchia, in the Toxoglossa, Pyramidellidw, Eulimidcc, many 

 Trochidce, the Hctcropoda, and in many Nudibranchia {Tcthys, Mclibe, Doriclopsis, 

 Phyllidia) ; in the Ascoglossa, and in certain Tectibranchia [Adccon, Doridium, 

 Philinc, Utricidus, Scaphander, Lohigcr). Among the Pulmonata they disappear 

 in a series of Testacellida;, being present in Daudebardia rufa, rudimentary in 

 D. Saulcyi, and wanting in Testacclla. 



2. The lingual apparatus (Figs. 153, 154) is highly characteristic 

 of all Molluscs except the LameUibrandiia, i.e. of all Glossophm-a. It 

 may be said that every animal with a radula is a Mollusc. 



