400 TEGUMENTARY ORGANS 



trically or radially-striated structure, resembling sections of urinary 

 calculi on a small scale, and still more the corresponding bodies in the 

 integument of the shrimp {supra.). For these reasons I think it must 

 be granted that the appearances in question, however cell-like, are, in 

 reality, not the expression of the development of a cellular structure 

 at all, but merely that of the mode in which the deposit of calcareous 

 matter takes place in the membranous basis of the shell. In fact, I 

 ■believe that the calcareous matter appears first in small and distinct 

 globules (the " cytoblasts "), and that more or less concentric deposits 

 take place round these, the result of which is, that the membranous 

 basis is more and more displaced, and that the deposited masses 

 eventually come almost into contact. The regularity of the ultimate 

 prismatic structure results from that of the distances of the granules 

 primarily deposited, and the even rate of addition to each subse- 

 quently. 



There appears to me to be but one interpretation to be placed 

 •upon these facts ; viz. that cells as such do not enter into the 

 formation of the shell of the Naiades at all, but that it is con- 

 stituted by the successive excretion of membranous laminae from the 

 surface of the epidermis of the mantle.^ The outer laminae retain 

 their membranous nature, only becoming so far altered as to assume 

 the horny aspect of the so called " epidermis '' ; in the next laminae, 

 which are added to the inner surface of the young shell, calcareous 

 matter is deposited in granules, additions to which are made in such 

 a manner as to constitute the cella;form concretions, and ultimately, 

 the process going on in the same way in successive layers, the 

 prisms ; in the innermost lamina;, finally, the calcareous deposit 

 results in an even, homogeneous, folded or striated layer. By 

 scraping with a sharp knife the inner surface of the shell of Anodon, 

 freshly detached from the mantle, I have obtained a distinct tough 

 membranous layer, scattered through which were a vast number of close- 

 set irregular granules of calcareous matter. A similar structureless layer 

 without the granules constitutes the outermost surface of the ecderon 

 of the mantle {fig. 313. C, b') and may occasionally be detached as 

 such. Such a layer consisting of the thickened outer portion of the 

 periplast of the ecderon of the mantle is by no means an anomalous 

 structure, as we have a formation of exactly the same kind in the 

 "cuticle" of plants, and in the chitinous lining of the intestine in 

 Insects ; and I believe that the shells of molluscs in general consist 



' This is, after all, only a return to the opinion of Poll, whose observations on shell 

 structure are remarkably accurate, and should never be overlooked. See his Testacea 

 utriusque SiciliiE. Pars prima " in qua de Testarum natura atque affectionibus disputatur." 



