PRACTICAL MICROSCOPY. 



required. The dissection should be carried on under 

 water, in the usual way, though the mollusc needs but little 

 pinning down. 



Let the student start with the common periwinkle, 

 Littorina littoralis, or, perhaps better still, with Patella 

 vulgata, or common limpet. In the former, the lingual 

 ribbon will be found coiled up like a watch-spring by open- 

 ing the back of the animal, and this place will be found 

 generally the best of all to examine first. 



In Patella vulgata it is only necessary to remove the 



foot, or broad flat 

 disc forming the lower 

 surface of the body, 

 when the lingual rib- 

 bon is exposed to 

 view, and appears as 

 shown in Fig. 165. 



The Trochus zizi- 

 pkinus, or pearly-top, 

 may also form a sub- 

 ject, as it is found in 

 tolerable profusion on 

 our shores ; and the 

 whelk, Buccinum un~ 

 datum, will also fur- 

 nish new specimens. 



In T.ziziphinus,\he 

 floor of the mouth 



must be exposed from above, when the lingual ribbon will 

 be found lying upon it. In the whelk, the trunk contains 

 the whole of the ribbon, and may be seen by opening the 

 back just behind the tentacles. 



In Fig. 1 66 the periwinkle is well displayed : / is the 

 foot, m the muscle for withdrawing the animal into his 



FIG. 



