2 10 THE PROBOSCIS, riiARYNX, 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, Doliuvi) attains a length exceeding that of the whole 

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

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

 are vegetable feeders, but tliis rule is l)y 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 ibod. 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, Jaws, and Radula. — Immediately behind the 

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

 buccal mass. The pharynx of the Glossophora, i.e. of the 

 Gasterojioda, Scaphopoda, and Cephalopoda, is distinguished from 

 that of tlie Pelecypoda,^ l)y the possession of two very characteristic 

 organs for the rasping or trituration of food before it reaches the 

 oesophagus and stomach. These are («) the jaiv or jaivs, and 

 (&) the radula^ odontoioliore, 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 j'aws and set in parallel rows along 

 a greatly prolonged tongue.^ 



In nearly all land I'ulmonata the jaw is single, and is placed 

 behind the upper lip. If a common Helix aspcrsa 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- 



^ There is practieall}' no pharynx in the Pelecypoda, the mouth opening directly 

 into the oesopliagus. 



- lladcrc, to scrape ; 65ovs, tooth ; (pepnv, to carry. 



•* The meclianisni of the radula has been dealt with by Geddes, Trans. Zool. Soc. 

 X. p. 485. Riicker has observed {Be7\ Oberhcss. Gcscll. Nat. Ilcilk. 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 teetli. 



