16Sf Dr Grant on the Structure 



ends of the clavate spicula hang over and converge around the 

 entrances of the pores, and seem to have a relation to that func- 

 tion. As these tubular spicula have no external opening, they 

 cannot be the cells of polypi, or contribute in any way to pro- 

 duce the currents of this sponge. 



The Spongia nivea, Gr. is a small sessile flat spreading 

 species, of a pure white colour, which I have only found on the 

 under surface of sheltered rocks at Prestonpans Bay, during the 

 ebb of stream-tides : it is not very uncommon there ; it appears 

 like patches of mineral agaric, or rock -milk, on the roofs of 

 small caves, is about two lines in thickness, spreads to the ex- 

 tent of one or two inches in diameter, and is smooth on the sur- 

 face. Its pores are just visible to the naked eye, and its fecal 

 orifices are regularly and beautifully constructed ; there is a 

 gentle rise of the surface to the margins of the fecal orifices, the 

 margins are quite circular, and have thin transparent termina- 

 tions ; the orifices are never produced so far as to form distinct 

 papillae, and their currents are directed perpendicularly down- 

 wards, in the natural position of the animal. When the nivea 

 is checked in its growth, and prevented from spreading by the 

 crowding of other animals around it, its surface becomes waved, 

 and in many places presents elevated sharp ridges, which allow a 

 greater space for the distribution of the pores. (See specimens 

 in the Museum of the University, ^S*. nivea, Gr.) The axis of 

 this sponge is composed almost entirely of large tri-radiate spi- 

 cula, some of which are more than half a line in length, and 

 thick in proportion ; their forms are seen by the naked eye. 

 These triradiate spicula occur of different sizes, to the minute- 

 ness of the fiftieth of a line in length, their rays taper regu- 

 larly from their place of junction to their sharp-pointed extre- 

 mities, their internal cavities are very distinctly seen in the large 

 spicula. The second form of spiculum in the *S'. nivea is the 

 most remarkable, though the rarest ; it consists of a straight 

 line, with two opposite lateral projections in its middle, which 

 are generally a little curved. When these lateral processes are 

 large and straight, it becomes a regular quadriradiate spiculum, 

 but they are generally much shorter than the other two rays ; 

 and when they are placed near one extremity of the spiculum, 

 it appears under the microscope like a small dagger with a 



