210 THE PROBOSCIS, PHARYNX, AND JAWS chap. 



derived the long proboscis which is so prominent a feature of 

 many genera (compare Figs. 1, B, and 99), and in some (e.g. 

 Mitra^ Doliimi) attains a length exceeding that of the whole 

 body. As a rule, Mollusca provided with a proboscis are carniv- 

 orous, while those whose mouth is on the surface of the head 

 are Vegetable feeders, but this rule is by no means invariable. 

 The mouth is thickened round the aperture into ' lips,' which are 

 often extensile, and appear capable of closing upon and grasping 

 the food. In the Pelecypoda the mouth is furnished, on each 

 side, with a pair of special external lobes, the 'labial palps,' 

 which appear to be of a highly sensitive nature, and whose 

 object it is to collect, and possibly to taste, the food before it 

 passes into the mouth. 



2. The Pharynx^ Jaivs^ anl Radula. — Immediately behind 

 the lips the mouth opens into the muscular throat, pharynx, or 

 buccal mass. Tiie pharynx of the Glossophora, i.e. of the Gas- 

 teropoda, Scaphopoda, and Cephalopoda, is distinguished from 

 that of the Pelecypoda,^ by the possession of two very charac- 

 teristic organs for the rasping or trituration of food before it 

 reaches the oesophagus and stomach. These are (a) the jaw or 

 jaws., and (5) the radula^'^ odonto^liore., or lingual rihhon. The 

 jaws bite the food, the radula tears it up small before it passes 

 into the stomach to undergo digestion. The jaws are not set 

 with teeth like our own ; roughly speaking, the best idea of the 

 relations of the molluscan jaw and radula may be obtained by 

 imagining our own teeth removed from our jaws and set in 

 parallel rows along a greatly prolonged tongue.^ 



In nearly all land Pulmonata the jaw is single, and is placed 

 behind the upper lip. If a common Helix aspersa be observed 

 crawling up the inside of a glass jar, or feeding on some succu- 

 lent leaf, the position and action of the jaw can be readily dis- 

 cerned. It shows very black when the creature opens its mouth, 

 and under its operation the edge of a lettuce leaf shows a regular 

 series of little curved indentations, in shape not unlike the semi- 



1 There is practically no pharynx in the Pelecypoda, the mouth opening 

 directly into the oesophagus. 



2 Badere, to scrape ; 65oi;j, tooth ; (p^pcLP, to carry. 



3 The mechanism of the radula has been dealt with by Geddes, Trans. Zool. 

 Soc. X. p. 485. Riicker has observed (Brr. Oherhess. Gesell. JVat. Heilk. xxii. 

 p. 207) that the radula in Helix pomatia is the product of five rows of cells ; the 

 use of the first row is uncertain, the second forms the membrane of the radula, 

 while rows three to five originate the teeth. 



