Some Causes of the Disintegration of Shells. 139 



action of any acid upon carbonate of lime is well known. 

 Perforating seaweeds perform the same office as acid, though 

 in a less rapid manner, upon the shells which they penetrate. 

 Bornet was the first to point out that they must therefore be 

 most powerful factors in the work of disintegration. 



All these algae attack the shell in the same manner. First 

 they form a horizontal layer on the outside of the shell, from 

 this branches develop, and enter into the substance of the 

 shell until it may be quite honeycombed by them. The 

 calcareous matter is thus by their agency returned to the 

 water, where it may again enter into the composition of 

 marine plants or animals. 



One of the commonest species is Goinontia polyrhiza, fre- 

 quently found, especially in razor-shells. It is readily distin- 

 guished by the naked eye, owing to its patches of green, mark- 

 ing the shell, sometimes on one side, sometimes on both, 

 the depth depending on the age of the plant. At certain 

 times we notice dark green specks scattered over the sur- 

 face of the shell ; these are the reproductive organs or 

 sporangia. 



Under the microscope Gomoiitia is recognised by its long 

 branched green septate filaments, and by the sporangia, 

 which take definite forms when j'oung. • As they reach 

 maturity their walls become thick and stratified, root-like 

 filaments are developed, and the sporangia being detached, 

 look like independent plants. They were at one time de- 

 scribed as such under the name of Codiohi7ii polyrhizimi. The 

 filaments of Gomontia vary greatly in size ; so much so, that 

 one is extremely liable to mistake the different preparations 

 for distinct and separate species. 



With Gonio7itia we often find Mastigocoleits and Hyella, both 

 of which stain blue-green or grey. The former can be dis- 

 tinguished microscopically from the latter by the presence of 

 heterocysts, and by its tortuous filament, which is of equal 

 thickness throughout. Looked at with the naked eye, Hyella 

 shows more numerous patches, the filaments of which are 

 closer together than Mastigocoleus. On microscopic ex- 

 amination Hyella usually shows two kinds of filaments, 

 some long and branched, others composed of a number of 

 cells. 



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