122 Dr Grant’s Observations on the Structure 
pins had been thrust into its surface, parallel and near each 
other, I struck, with a red hot wire, the exposed part of the 
surface between the pins ; but, what I little expected at that 
period of my inquiry, the parallelism of the pins was not dis- 
turbed, nor did they seem to approach each other in the slight- 
est degree. Lest the pins might have approached each other, in 
a small degree, without disturbing their parallelism, I placed 
them on a part of the animal newly raised from the water, and 
measured their distances with a pair of compasses ; but, after 
receiving some smart strokes with a red hot iron, on the surface, 
between the pins, half an inch distant, the points of the com- 
passes still coincided with the heads of the pins ; there seemed to 
be no more effect produced on the living animal than would be 
produced on a piece of common moistened sponge. The sur- 
face of the urens , when young, is somewhat transparent, and of 
a yellowish-grey colour ; but, as it advances, it acquires a bright- 
er yellow colour, and more opacity ; when looked closely into, it 
appears covered with a net-work of the finest gauze, the pores 
being visible to the naked eye. I had hopes of inducing mo- 
tion in these pores ; but, on observing them through a glass, 
while I irritated them with a needle, I could perceive no change 
in their dimensions. When this sponge spreads on the sides of 
rocks, its fecal orifices are observed to be more raised from the 
surface than in portions of it surrounding fuci, corallines, or 
other moveable bodies ; and they are likewise more thin and 
transparent on their margins ; so that, when the urens , taken 
from such situations, is kept for a time out of water, the first 
parts which begin to collapse or contract, by drying, are gene- 
rally these transparent lips of the orifices. This takes place equally 
in dead and living specimens, and might be mistaken for an ef- 
fect of irritability ; it is the only kind of motion I have ever been 
able to produce in these parts. 
In Prestonpans Bay, the tide has excavated, in many places, 
the beds of soft slate-clay from beneath the outgoings of the sand- 
stone strata, and has thus formed innumerable small caves which 
are sheltered from the direct force of the waves, by lofty ridges 
of trap-rocks extending to a great distance from the shore. In 
these sheltered recesses, far from the main current of the Frith, 
numerous species of Alcyonium , Lobularia , Sertularit , Coralli- 
nes > Tubularity Flustrty branched sponges, and other zoophytes. 
