

127 



their parts, and composed of a single series of cells, each exhibiting 

 a distinct nucleus or cytoblast, as represented in PI. xiv. fig. 4. 

 These bodies are ^vn of an incn in diameter, and vary from y^ to 

 .^tj. in their length. Upon endeavouring to trace whence these curi- 

 ous bodies came, I found them embedded in bands, corresponding in 

 their direction with the lines of growth of the new lip of the shell, 

 immediately beneath the newly-formed stratum of calcigerous cells, 

 and the direction of the cylindrical bodies, when not disturbed, being 

 in lines, which were diagonals to the lines of growth of the shell, and 

 crossing each other at about right angles, — a mode of disposition 

 which we shall hereafter find to be in accordance with the arrange- 

 ment of the cellular structure in the adult shells. Amid the layers of 

 well-developed cylindrical bodies, and immersed in the membrane 

 beneath, I observed a number of faint pellucid lines, disposed pre- 

 cisely in the same manner as the distinct and well-developed cylin- 

 ders, and having every appearance of being the incipient state of 

 those tissues. Upon many of the well-defined cylindrical bodies I 

 frequently observed cytoblasts or cells in progressive stages of deve- 

 lopment. These additional cells sometimes appear to be irregularly 

 dispersed over the whole surface, but are usually developed upon two 

 sides only, and those opposite to each other. As the tissues which I 

 have described approach maturity, the periostracum, progressively 

 advancing from the old lip, covers the new tissues, and the whole be- 

 comes firm and consolidated like the older portions of the shell. 



From a careful examination of many new lips in various stages of 

 progressive development, I do not think that the whole of the coagu- 

 lable lymph, of which the new membrane is formed, is exuded from 

 the surface of the animal at one operation, but rather by a series of 

 exudations, each of which emerges from beneath the terminal margin 

 of the last one j for if the membrane be carefully examined from the 

 line of advance of the newly-formed cells towards its extreme edge, 

 it will be seen that there are several lines of thickened tissue, which 

 correspond in distance and appearance with the lines of growth on 

 the old shell ; and this is the more probable, as I have observed in 

 young shells which I have taken from their places of hybernation, in 

 November, that the extension of the lip is still going forward, or ra- 

 ther perhaps has been so up to the very period of their retirement for 

 the season. 



On proceeding to investigate the structure of adult shells, I selected 

 for examination thirteen specimens, belonging to the eight following 

 genera, — Cyprcea, Conus, Pyrula, Cassis, Ampullaria, Oliva, Voluta 



