186 KOLLIKER, ON VEGETABLE PARASITES. 



they appear to occur only in cases where an external uncal- 

 cified layer is not present, as in Balanus and Serpula ; whilst 

 in the opposite case they are absent. In corals and forami- 

 nif'era again parasitic growths are very general, whilst in the 

 Spongiadas they are often wanting. 



3. The penetration of the parasites appears to take place 

 in two ways — a mechanical and a chemical. The latter 

 is doubtless the case in all calcareous skeletons, in which 

 scarcely any other supposition can be entertained except 

 that the parasitic growth, as it advances, dissolves the car- 

 bonate of lime contained in the tissue, by the secretion of an 

 acid. Whether this be carbonic acid, or one of an organic 

 nature, must be determined by future inquiry. All that can 

 now be remarked is, that the supposition of the agent being 

 carbonic acid is hardly supported by the fact stated by 

 Bischoff ('Lehrb. der Chemischen Geologie/ ii, p. 1136), that 

 oyster shells are far more difficultly soluble in water contain- 

 ing carbonic acid than are chalk or powdered calcspar; and- 

 which is in accordance with the preservation of shells and 

 the other hard tissues in question in sea- water Avhich con- 

 tains carbonic acid. Were it the case that the hard struc- 

 tures concerned contained more organic material than 

 at any rate the bivalve shells and stony-corals actually pos- 

 sess, it might also be supposed that the fungi first attacked the 

 organic substance (which, it is true, is very doubtful), and then 

 removed the carbonate of lime by the secretion of carbonic 

 acid. However this may be, it appears that in any case the 

 solution of the calcareous matter takes place only at the ter- 

 minal, growing end, since the fungus-filaments are never 

 lodged in wide vacuities, but, on the contrary, throughout their 

 course are closely surrounded by the hard tissue. It is also to 

 be remembered, that it is difficult to perceive what becomes of 

 the dissolved carbonate of lime. It cannot well remain in 

 the fungus-filaments ; whilst, on the other hand, when their 

 often very considerable length is considered, it is difficult to 

 suppose that it is conveyed through them and deposited on 

 the exterior j and yet any other supposition seems scarcely 

 possible, and the more especially as the probable existence 

 of a continuous reciprocal action between the water and 

 the filaments at the surface of the shell should not be lost 

 sight of. 



In the case of the sponges, a mechanical penetration of the 

 fungi may be supposed to take place, since it is impossible 

 to conceive in what way they can be enabled to dissolve 

 such a resistant substanee as the horny skeleton of these 

 creatures. An analogous mechanical penetration is wit- 

 1 in the passage of parasites through cellulose-mem- 



