124 MOLLUSCA. 



which it creeps will be seen. At the upper part of 

 this is the head, with its two horns (ten'tacles) . 

 Below the roots or bases of the tentacles, and be- 

 tween them and the upper part of the foot, is 

 the little round mouth. On slitting this up with 

 scissors, a cavity will be opened, and in it will be 

 seen a reddish tube (the proboscis), about as large as 

 a goose- quill, with an aperture at the end. This 

 must be carefully slit up, when the tongue, which is 

 of about the size of a crow-quill, will come into view. 

 The tongue is moveable in the proboscis, and can be 

 protruded or withdrawn by the animal at will. If 

 the surface of the tongue be viewed under a hand- 

 lens, the rows of teeth will be seen at once. It is 

 better not to pull the tongue out with forceps, as 

 the teeth are easily displaced and injured. The best 

 plan is to dissect away the muscular structures with 

 forceps and a pair of fine-pointed scissors, then to 

 cut off the tongue at its root, and to soak it in water 

 for some hours, when the skin or epidermis containing 

 the teeth can be separated with the mounted needles 

 under a simple lens or microscope. After any loose 

 particles have been washed away with a hair pencil, 

 the object may be spread flat on a slide, and dried 

 between two slides. The upper slide should then be 

 removed, the tongue soaked in oil of turpentine, and 

 mounted in balsam with the least possible heat. 



As thus prepared, the horny teeth (PL IX. fig. 21) 

 are seen to be arranged in rows, united by a colourless 

 membrane, so as to form a long ribbon. The teeth 

 form three longitudinal parallel rows, a central and 

 two lateral. Each tooth, considering the separate 

 pieces as constituting distinct teeth, has little teeth 

 or denticles at its lower edge. These are curved 

 inwards, four in number, and connected by a basal 

 plate in the side teeth ; while the middle teeth have 

 six or seven straight denticles. These teeth serve 

 to enable the animal to scrape or rasp the algae, 



