112 PREPARATION AND MOUNTING 



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mals very soft ; and as it does not harden their mucus at all, they 

 are very slippery and difficult to work upon when so preserved. 



" Then as to the apparatus required for dissection. In the first 

 place, all the work is to be done under water, and a common 

 saucer is generally the most convenient vessel to use. No kind of 

 fastening down or pinning out of the animal is needed ; and, in 

 fact, it is much better to have it quite free, that you may turn it 

 about any way you wish. The necessary instruments are a needle- 

 point, a pair of fine-pointed scissors, and small forceps ; the for- 

 ceps should have their points slightly turned in towards each 

 other. 



" A word or two on the lingual apparatus generally, and on its 

 special characters in a few different animals, will conclude what I 

 have to say. 



" The mode of using the tongue can be easily seen in any of 

 the common water-snails, when they are crawling on the glass 

 sides of an aquarium ; it may then be observed that from between 

 the fleshy lips a thick mass is protruded, with a motion forwards 

 and upwards, and afterwards withdrawn, these movements being 

 almost continually repeated. The action has the appearance of 

 licking ; but when the light falls suitably on the protruded struc- 

 ture, it is seen to be armed with a number of bright points, which 

 are the lingual teeth, so arranged as to give the organ the charac- 

 ter and action of a rasp. 



"If you proceed to dissection, and open the head of one of 

 these mollusca (say, for instance, a common limpet), you will find 

 the cavity of the mouth almost filled with the thick fleshy mass, 

 the front of which is protruded in the act of feeding ; and on its 

 upper surface, extending along the middle line from back to front, 

 is seen the strong membranous band upon which the teeth arc 

 set. The mass itself consists of a cartilaginous frame, surrounded 

 by strong muscles ; and these structures constitute the whole of 

 the active part of the lingual apparatus 



" But the peculiarity of the toothed membrane, which makes its 

 name of 'ribbon' so appropriate, is, that there is always a con- 

 siderable length of it behind the mouth, perfectly formed, and 

 ready to come forward and supply the place of that at the front, 

 which is continually wearing away by use. 



