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structures as thoroughly as possible, in order to decide which tubular 

 structures of the hard tissues of animals are typical and which are 

 not ; and for the botanist a new field of investigation is opened, which 

 not only draws attention by the somewhat strange forms offered for 

 investigation, but is also of great interest in a physiological point of 

 view. It seems to me probable that the parasites dissolve the car- 

 bonate of lime of the hard structures into which they penetrate, by 

 means of exudation of carbonic acid, which secretion would seem to 

 take place only at the growing ends of the fungial tubes, as they 

 never lie in larger cavities, but are always closely surrounded by the 

 calcareous mass. In some cases, as in the horny fibres of sponges, 

 it seems probable that the parasites simply bore their canals by 

 mechanical force, as is the case when vegetable parasites make their 

 way through the cell-membranes of Conferva or other plants. 

 Besides this, it deserves also to be remembered that nearly all the 

 parasites here spoken of occur in marine animals. 



In concluding this notice, I may further mention that these 

 parasites afford an excellent means for demonstrating the double- 

 refracting power of the shells of the several genera mentioned in 

 this communication. I was first struck with this fact in examining 

 a horizontal section of Lima scabra obtained from Dr. Carpenter, 

 and finding that many tubuli appeared double. In following this 

 matter, it was easy to show that all the tubuli running in a certain 

 direction, and in an oblique way through the section, appeared 

 simple at the upper surface of it, and became double in the inferior 

 layers, so that the distance of the two images increased with the 

 shortening of the focus. When the preparation was inverted, the 

 reverse was the case. The same phenomena as in Lima were also 

 observed in Anomia, Ostrea, Murex truncatus, Turbo rugosus, 

 Tritonium cretaceum, and Balanus, the shells of which animals have 

 therefore all such a structure, that they refract the light in the same 

 way as the well-known double-refracting crystals*. 



* According to Brewster (Bibl. Univ. de Geneve, 1836. ii. 182), who seems the 

 only person who has hitherto observed the double-refracting power of a shell, 

 viz. of the mother-of-pearl, that shell (Meleagrina) shows the same phenomena 

 as the double-axed double-refracting Arragonite, on which question I am not as 

 yet able to give an opinion. 



