392 bowerbank's researches : 



other body, although I have found, at the junctions of the 

 adductor muscle with the shells, both in Pinna and Ostrea, 

 a layer of most elaborate and comjjlex minute vascular tissue, 

 and amid these other vessels, few in number and much 

 greater in size. These vessels were evidently not those 

 which appertained only to the mussel itself, as they were 

 found in no other part of it, but concentrated in a single 

 complex stratum at this point." 



You observe, then, that Mr. Bowerbank concludes the 

 shell to be highly organized and vascular, and that it re- 

 tains, during life, a vascular communication wdth the animal 

 it protects. It has a structure analogous to bone in some 

 respects, and is formed much in the same manner by the 

 deposition of carbonate of lime within the cells of the mem- 

 branes of which shell is composed, or by the aggregation and 

 coalescence of the calcigerous cells when the membrane is 

 very sparingly prodviced. Let us go back to the Mollusk 

 when yet within its egg, but far enough advanced to be 

 endued with the rudiment of its future shell. We may sup- 

 pose this rudiment to have been the result of the excretion 

 of some mucus or lymph, and it is in fact nothing more than 

 a very thin transparent membrane, with a determinate figure 

 dependent on the figure of its species. In this membrane 

 organizing cytoblasts and cells are produced and multiplied 

 in rapid sviccession until, by their increase and apposition, 

 a cellular structure is formed in it. On their first appear- 

 ance the cells are transparent and globular, but, pushed on 

 by the law of growth, which regulates their development, 

 they very soon begin to secrete, from their inner surfaces, 

 carbonate of lime. The cells being filled with it, a solicl 

 structure is the result of their close packing and aggregation, 

 and this structure must, of course, exhibit the cellular struc- 

 ture whence it has been derived, the pattern being modified 

 only by the form and degree of condensation of the calci- 

 gerous cells in which it has been secreted. As this deposi- 

 tion of calcareous matter proceeds, canals are also formed, 

 which penetrate the layer, and another system of vessels 

 which are destined to maintain its life by leading through it 

 a circulation of fluids. A layer or stratum of shell being 

 thus formed, another is produced from its inner surface, by 

 the same production of a basement membrane and the same 

 development and aggregation of calcigerous cells ; and then 

 others until the normal number set for the species is com- 

 pleted, the whole being, however, kept together as one by 

 the living tissues and vessels. Mr. Bowerbank thinks that 

 the truth of this is proved not only by the structures he 



