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structure and arrangement of the tissues may be obtained. If we 

 submit the shell of the common edible muscle to the action of dilute 

 hydrochloric acid, we obtain the animal matter in the form of a series 

 of thin membranes loosely adhering together ; and in these, with a 

 power of 300 or 400 linear, in many parts of the tissue, we may trace 

 the developmentof the tessellated cells from the cytoblast upwards, un- 

 til they are seen closely packed together, and only to be distinguished 

 from the membrane, with which they have now become incorporated, 

 by their junctions with each other resembling small patches of reti- 

 culated structure.* 



In Pinna we find another form of cellular tissue, which is perhaps 

 one of the most beautiful and interesting of all the modes of dispo- 

 sition of these organs that can be found within the entire range of 

 conchological science. 



If we fracture any tolerably thick species of this genus at right 

 angles to the natural planes of the shell, either parallel to the lines 

 of growth or in an opposite direction, and view the surface thus ob- 

 tained as an opaque object by the aid of the Lieberkuhn, with a linear 

 power of 150, the structure appears to be entirely composed of elon- 

 gated prismatic cells, having their axes at right angles to the natural 

 planes of the shell, and looking exceedingly like regular prismatic 

 crystals of carbonate of lime. If we separate these prismatic bodies 

 from each other, — which may be readily done, by very gently striking 

 a fractured edge of Pinna affinis from the Bognor rock of the London 

 clay formation, — and view the prisms immersed in water or Canada 

 balsam, by transmitted light, with a power of 500 linear, they assume 

 an appearance which very strongly resembles that of the fasciculi of 

 muscular fibres, where the banded appearance of the sarcolemma is 

 not very strongly marked ; as a series of fine striae is observed, 

 crossing each prism at right angles to its axis, at somewhat unequal 

 distances. These striae are so fine that it is necessary to inflect the 

 light at right angles to their lines of direction, before they can be 



* Mr. Bowman, in his admirable paper on mucous membrane, in the 'Cyclopaedia 

 of Anatomy and Physiology,' describes the basement membrane, as displayed in the 

 kidney, as " an extremely thin, transparent and homogeneous lamina, simple and en- 

 tire, without any aperture or appearance of structure : " he says also, " it readily 

 wrinkles, and such is its tenuity, that it is sometimes only by the folds thus occasioned 

 that it becomes visible at all." This description is perfectly applicable to the base- 

 ment membranes of the shell tissues, whether they be situated in the periostracum, 

 lining the internal surface of the shell, or distributed in repeated laminae throughout 

 its substance. 



