416 Prof. M. Schultze on Polytrema miniaceum. 



hesiou after the solution of the calcareous shell, and then its 

 diffusion, in certain cases, even into the central parts of the 

 shell. Polythalamia which exhibit a yellowish-brown coloration 

 of the body in the central chambers always, according to my 

 observations, present the same or nearly the same colour through 

 all the chambers, with the exception only of the few last-formed 

 ones. This was also the case in several Polytremata which con- 

 tained no siliceous spicules, and as to the true Polythalamian 

 nature of which there could be no doubt. Now, may the origi- 

 nally brown substance become colourless again during the ap- 

 pearance of these spicules ? Our previous experience furnishes 

 no reason for such a supposition ; and therefore we must argue 

 against it, so long as another course is open to us. 



Of such, three present themselves : — Either the siliceous spi- 

 cules have penetrated accidentally, or they have been taken in as 

 food, or, lastly, they belong to a parasitic Sponge. I have 

 already mentioned that many specimens of Polytrema exhibit 

 erosions of their surface, especially at the apices of the comb- 

 like or tooth-like elevations — apertures by which a glimpse is 

 afforded of the inner system of cavities. The siliceous spicules 

 are always to be found in abundance at such spots, in the cham- 

 bers nearest to the orifice. Frequently, as was observed by 

 Krohn at Nice, the spicules project freely from the apertures, so 

 as to be detected at once by the microscope. The siliceous spi- 

 cules, if not produced in the Polythalamian body, have certainly 

 penetrated from the apertures. It might be in vain to attempt 

 to prove that they have not penetrated accidentally, or been 

 taken in as nourishment. But no probable grounds can be ad- 

 duced for either of these views. How could sponge-spicules, 

 however numerous they might be in the water surrounding the 

 Polytremata, find their way into the innermost chambers of the 

 labyrinthic system of cavities, still at least partly filled with 

 organic substance ? Must not the filling of the peripheral 

 chambers with such spicules, crossed in all directions, prevent 

 the penetration of the latter into the deeper layers ? And if 

 Sponges were the favourite food of the Polytremata, how could 

 adherent Polythalamia get at adherent Sponges in order to de- 

 vour theni ? 



There is consequently no reason why we should not embrace 

 the last possibility, and assume that Polytrema is infested by a 

 parasitic Sponge. That Sponges bore into many calcareous 

 structures, and live like parasites, is well known. The genus 

 Cliona, upon which Lieberkiihn * has recently published some 

 exact observations, is one of these boring Sponges. Its extra- 

 ordinary diffusion appears from the fact that on many coasts 

 * Miiller's Archiv, 1859, p. 515. 



