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outer and inner surfaces of the shell. Upon subjecting fragments of 

 the shell to the action of dilute hydrochloric acid, until the whole of 

 the carbonate of lime was removed, I obtained an abundant residuum 

 of animal matter, which consisted of a scries of fleshy membranes 

 adhering to each other, and having their planes parallel to the sur- 

 faces of the shell. If the dissolution of the shell in the acid and 

 water be stopped when there is but an exceedingly thin plate of it 

 remaining, and the membranous matter in which it is enveloped be 

 carefully removed from the surface by gently rubbing it between the 

 finger and thumb, and it be examined, immersed in water, by trans- 

 mitted light, with a linear power of 700, it presents the appearance 

 represented at PL xvi. fig. 1. The calcigerous cells, filled with car- 

 bonate of lime, are observed closely packed together, and in the cen- 

 tre of the area of each we observe an indistinct grouping of points, 

 as if these were the remains of cytoblasts ; and this I am the more 

 inclined to believe to be the case, as we observe, although more indis- 

 tinctly, the same appearances in the animal remains of the cells 

 which are preserved in the fleshy membranes after complete solution 

 of the earthy matter contained in them had been effected by the con- 

 tinued action of the acid. It is not easily to be determined whether 

 this curious appearance within the areas of the cells be due to the 

 presence of cytoblasts, or to other causes, from the extreme minute- 

 ness of the cellular tissue, which does not upon an average exceed 

 ^ s Vb of an inch in diameter. Numerous canals are observed traver- 

 sing amid the cells in every possible direction. Their courses are 

 tortuous, and seldom in a right line for any considerable distance. 

 Sometimes they are seen pursuing their way for a short space parallel 

 to the natural planes of the shell, but they are usually disposed in 

 lines more or less oblique to these surfaces ; and, under such circum- 

 stances, a section of them at nearly right angles to their axes may be 

 observed in all parts of the fragment of shell under examination, as 

 represented at PI. xvi. fig. 1, a a. The best view of these organs is 

 to be obtained by an examination of the membranous matter of the 

 shell when totally deprived of the carbonate of lime. In this case 

 we observe them permeating the tissues in every direction, and pre- 

 senting an appearance exceedingly like that of the same description 

 of organs as they exist in the oyster, and which are represented in 

 PI. xvi. fig. 5. At the first view these canals may readily be mistaken 

 for vessels, as their parietes are lined with a thick coat of gelatinous 

 matter, but which is not always of uniform thickness like the parietes 

 of a true vessel. Upon a careful examination of the undisturbed 



