TONGUE. 123 



Mollusca may be illustrated by reference to that of 

 the oyster. Two kinds of shell-substance are at once 

 distinguishable in an oyster-shell, an outer brown, and 

 an inner pearly or nacreous. The brown portion ex- 

 hibits under the microscope the appearance of a cell- 

 structure (PL IX. fig. 28), the angular forms from 

 mutual pressure being very distinct. The component 

 bodies of this portion are seen to be more or less 

 elongated and flattened in the side view, forming 

 prisms (fig. 29). The structure of the pearly part of 

 the shell is more difficult of examination, and can only 

 be seen distinctly in ground and polished sections. 

 In these, under a high power, it exhibits numerous 

 fine, somewhat parallel wavy lines (fig. 19), which are 

 the indications of thin layers, or laminae, of which it 

 is composed. 



Shell consists of a basis of animal matter in which 

 carbonate of lime (chalk) is deposited, the whole 

 being poured out or secreted by the skin or mantle 

 of the mollusk. 



Pearls, which possess the same structure as the 

 nacreous part of shell, consist of the nacre formed 

 around some foreign body, as a grain of sand, &c., 

 by which the mantle has been wounded. 



Tongue. The structure of the tongue of the Mol- 

 lusca is very interesting, on account of the curious 

 teeth which are found upon it. It may be illustrated 

 by the common Whelk (Budcinum unddtum), which 

 is sold at the street- stalls. As, to one unacquainted 

 with the anatomy of the Mollusca, there is some diffi- 

 culty in finding the tongue, it may be well to point out 

 how it is to be found. If the shell containing the 

 animal be placed so that its orifice is directed up- 

 wards, the point or apex of the spire being towards 

 the reader, the lid (oper'culum) which closes the shell 

 will be at once evident. On drawing the animal from 

 the shell by means of the lid, the foot or portion 

 which is applied by the animal to the surface upon 



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