536 THE MICROSCOPE. 



the contraction of the circular muscular fibres of which 

 the walls of the tube are composed, is propelled from the 

 base to the extremity, thereby unrolling, as he believes, 

 the spiral coils. One side of each arm is fringed with a 

 vast number of long filaments : these are ciliated. The 

 shell is opened by a peculiar process, which has given to 

 the Terebratula the name of Coach-spring Shell. In the 

 shell there are minute openings surrounded by a series of 

 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. 240, No. 3. The outer layer has been re- 

 moved, to show a radiating structure around the perfora- 

 tions. Dr. Carpenter fully describes Terebratula 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. The 

 well-known gorgeously coloured univalve, the Ear-shell, 

 Haliotus splendens, has been ascertained to consist of 

 numerous plates, resembling tortoise-shell, forming a series 

 of hexagonal cells, in the centre of which the stellate 

 pigment is deposited (fig. 246, No. 3), alternating with 

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

 highly magnified, a series of irregular undulating folds, re- 

 presented in the upper portion of the section. The iride- 

 scent lines are often extremely pleasing ; and if a piece be 

 submitted 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 carry- 

 ing 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 be 

 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 



