420 THE MICROSCOPE. 



radiating lines, these at first appear like dark oval spots ; 

 but in a vertical section they are seen to be perforations or 

 tubes running obliquely from the inner to the outer surface 

 of the shell, and having a series of radiating lines on the 

 edge, as in fig. 198, No. 3. The outer layer has been re- 

 moved, to show a radiating structure around the perfora- 

 tions. Dr. Carpenter has accurately described the Terebra- 

 tula in the Philosophical Magazine, 1854. 



Not less curious than beautiful is the internal layer 

 of many kinds of bivalves, which present an iridescent 

 lustre, the whole surface being varied with a series of 

 grooved lines running nearly parallel to each other. One 

 of the most remarkable is the well-known Ear-shell, 

 Haliotus splendens; this has been ascertained to consist of 

 numerous plates, resembling tortoise-shell, forming a series 

 of hexagonal cells, in the centre of which the stellate pig- 

 ment is deposited (fig. 205, No. 3), alternating with thin 

 layers of pearl, or nacre; and this exhibits, when highly 

 magnified, a series of irregular undulating folds, repre- 

 sented in* the upper portion of the section. The iridescent 

 lines are often extremely pleasing ; and if a piece be sub- 

 mitted to the action of diluted hydrochloric acid, until the 

 calcareous portion of the nacreous layers are dissolved out, 

 the plates of animal matter fall apart, each one carrying 

 with it the membraneous residuum of the layer of nacre 

 that belonged to its inner surface. But the nacre and 

 membrane covering some of these horny plates remain 

 undisturbed ; and their folded or plaited surfaces, although 

 divested of calcareous matter, exhibit iridescent hues of 

 the most gorgeous description. If the membrane is 

 spread out with a needle, and the plates unfolded to a 

 considerable extent, the iridescence is no longer seen; a 

 fact which clearly demonstrates, that the beautiful colours 

 presented by the nacreous portions of shells, commonly 

 called mother-of-pearl, are produced solely by the disposi- 

 tion of single membraneous layers in folds or plaits, lying 

 more or less obliquely to the general surface. 



GASTEROPODA. Belly-creeping animals, Snails and Slugs ; 

 this family is characterised by having a fleshy disc, serving 

 as a foot upon which to creep. The back is covered with 



