01" THE SPONGIAJD^. 107 



tear it. The dermal membrane of the Turkey sponge of 

 commerce, Spongia qfficinalis, is abundantly supplied with 

 primitive fibrous tissue. It curves round the margins of 

 the porous areas, thickening and strengthening the whole 

 of the dermis to a very considerable extent, but it exists to 

 a very shght extent in the pellucid membranes of the areas 

 in which the pores are opened. When alive, it is replete 

 with powers of life and action of a very remarkable descrip- 

 tion. Without the slightest appearance of nerves or 

 muscles, it has the power of opening pores on any part of 

 its surface, and of closing them again at pleasure, without 

 leaving a trace of their existence to indicate the spot they 

 occupied ; and there is no amount of laceration or destruc- 

 tion that it does not seem capable of repairing or replacing 

 in a very short period, reproducing itself over extensively 

 denuded surfaces in a very few hours. It also shares, in 

 common with the interstitial membranes, the power of 

 strongly and quickly adhering to other sponges of the same 

 species with which it may be brought in contact, but never 

 with those of a different species, however long the two may 

 remain pressed against each other. In some sponges the 

 distal extremities of the skeleton pass through and project 

 beyond the surface of the dermal membrane, while in other 

 cases the whole of the skeleton is confined within it. 



I will not describe at length these remarkable powers of 

 the dermal membrane, but refer the reader to a series of 

 observations on the " Vital Powers of the Spongiadie," 

 published in the ' Reports of the British Association ' for 

 1856, p. 438, and for 1857, p. 121, in which I have de- 

 scribed in detail a series of observations and experiments 

 on living sponges^ which demonstrate in a satisfactory 

 manner the extent of the vital powers and capabilities of 

 this highly sensitive membrane. 



In some species of sponges the outer surface of -the 

 skeleton is especially modified to strengthen and support 

 the dermal membrane. Thus, in some of the keratose 

 sponges of commerce, in parts of the sponge which have 

 been in contact with other sponges, or with rocks or stones, 

 we find a fine network of stout fibres immediately beneath 



