TEGUMENTARY ORGANS 4OI 



simply of a multitude of thin layers successively thrown off, super- 

 imposed and coherent, all the peculiarities of their structure arising from 

 subsequent modifications, which are altogether independent of cells. 

 This view is in perfect agreement with all that is known of the nature 

 of the shells of larval Gasteropods and Acephala, which are invariably 

 either of an absolutely structureless, thin, transparent, membranous 

 character, or at most present a delicate striation. It may be added that 

 not the slightest trace of a cellular structure is to be met with in the 

 pellucid shells of the Heteropoda and Pteropoda. So much for the 

 two primary forms of shell structure, the membranous and the 

 prismatic. A most interesting variety of the former is the nacreous 

 (mother-of-pearl) lining which is presented by many shells, both of 

 Acephala and Cephalophora. The pearly iridescence proceeds, as 

 Dr. Carpenter has well shown, from the folding of the membranous 

 layer into close plaits, and not, as has been supposed, from the 

 alternate cropping out of calcareous and membranous layers. Dr. 

 Carpenter proved this by decalcifying with acid a layer of nacre from 

 Haliotis splendens. The iridescence remained ; but if the plaits of the 

 layer were pulled out by stretching it with needles, the iridescence 

 disappeared. 



Another variety of structure usually, but not alone found in the 

 membranous shell substance, is the tubular. " All the different 

 forms of membranous shell structure are occasionally traversed by 

 tubes which seem to commence from the inner surface of the shell, 

 and to be distributed to its several layers. These tubes vary in size 

 from about the -^-^^^^ to the 2-oVo of ^"^ inch, but their general 

 diameter in the shells in which they most abound is about ^^Vf of an 

 inch. The direction and distribution of these tubes are extremely 

 various in different shells ; in general, when they exist in considerable 

 numbers, they form a network which spreads itself out in each layer 

 nearly parallel to its surface, so that a large part of it comes into 

 focus at the same time in a section which passes in the plane of the 

 lamina. From this network some branches proceed towards the 

 nearer side of the section as if to join the network of another layer, 

 whilst others dip downwards, as if for a similar purpose " (Carpenter, 

 /. c. p. 14.). In other instances the tubes run obliquely through all the 

 layers. The former structure was found by Dr. Carpenter in the 

 outer yellow layer of Anomia ephippium ; the outer layer of Lima 

 scabra and in Chania, the latter in Area Pectunculus, and Trigonia. 

 In the latter case, the tubules are not continuous, but are seen under 

 a high power to be formed by rows of isolated vacuities one for each 

 lamina ; corresponding, I imagine, with the appearance, " as if they 



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