THE HOUSE THAT THE OYSTER BUILT. 95 



observer, found, at supper-table, an oyster shell infested 

 by various creatures. On looking more intently, he disco- 

 vered, what eyes less acute would have failed to perceive, 

 some small white dots, irregularly placed on the surface of 

 the fronds of some of these parasites. Laying, therefore, the 

 oyster shell aside for further examination, and submitting 

 a fragment to the microscope on the following day, he was 

 soon rewarded with a sight of a delicate, glossy fan, and, 

 in fact, with the discovery of a new plant. Each of the 

 minute dots was now seen to consist of one or more, fre- 

 quently of a cluster, of transparent fan-shaped bodies. 

 Each dot, or cluster, crumbles under the touch, being 

 composed of carbonate of lime : not as an incrustation, 

 but as intimately incorporated with the tissues of the 

 plant, (g) 



(g] The reader will find a chemical analysis of the oyster in 

 Chapter XVIII., but I have thought it of interest to the scientific 

 reader to conclude the subject under review by quoting the substance 

 of a Paper read by Mr. Robert Irvine, T.C.S., Edinburgh, 1889, rela- 

 tive to the " Secretion of Carbonate of Lime by Animals," in which it 

 was stated that " hens supplied with any salt of lime produce normal 

 egg-shells composed of carbonate of lime. They cannot make shells 

 from magnesium or strontium carbonate. Crustacea, such as crabs, 

 cannot assimilate sulphate of lime from the sea-water to form their 

 exo-skeleton. They can form their shells from calcium chloride. In 

 the egg-shell the organic and inorganic material are both secreted by 

 cells separated from the epithelial cells. In the crab-shell, the organic 

 material (chitin) remains attached to the epithelial cells, and in this 

 the lime salts are deposited, probably by a process of dialysis, whilst, 

 in the case of bone, the cells are not epithelial in character : the matrix, 

 though separate, is closely associated with the cells, especially during 

 its formation, and the lime is deposited in the matrix apparently by a 

 process of dialysis. Phosphoric acid, combined with alkalies and 

 alkaline earths, acts as the carrier of the lime salt to the secreting 

 cells. While in the blood, the lime salt is a phosphate ; it may be 



