TONGUES OF MOLLUSCA. 79 



Theiij 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 forceps 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 difierent 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 with- 

 drawn, these movements being almost continually repeated. 

 The action has the appearance of licking ; but when the 

 light falls suitably on the protruded structure, it is seen to 

 be armed vdth a number of bright points, which are the 

 lingual teeth, so arranged as to give the organ the character 

 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 are 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. In the 

 Plate of anatomical details you will see the parts repre- 

 sented. 



