Feeding and Breathing 



41 



gullet and the stomach ; there are neither jaws, 

 teeth, nor tongue. The absence or presence of a 

 tongue has been found to be an important item in 

 classification, and we find that some authors have 

 divided the Mollusca into two unequal groups, 

 the Tongue-bearers {Glossophora) and the Tongue- 

 less (Aglossa), the Tongue-bearers including the 

 Cuttles and the Univalves, the Bivalves constituting 

 the Tongueless. 



It is in the Cuttles that we find the highest 

 development of jaws for the purpose of biting off 

 jDortions of food. These 

 are composed of chitin, 

 formed much like the 

 beak of a parrot, and 

 working much in the 

 same fashion except that 

 in the case of the Cuttle 

 the upper jaw works 

 within the lower. The more ordinary type of jaws 

 may be observed by watching any of our common 

 snails feeding. The Land-snails (Helix) have the jaw 

 only in one portion, and this is placed behind the 

 upper lip ; but in the Pond-snails (Limncea) there 

 is an accessory plate on either side of the mouth, 

 and these act with the upper plate. If a pond- 

 snail be watched as it glides up the glass of an 

 aquarium and cleans ofif the minute vegetation, the 

 mechanism of the jaws will be understood better 

 than from a description. 



The food having been cut ofi" by the jaws is 

 not yet in a condition to be readily digested; it 

 has first to be masticated. This work is performed 



Upper (a) and lower (//) portions of the 

 beak of a Cuttle-fish (Sepia) 



