Prof. P. M. Duncan on Spongiophagus Carteri. 121 
granular plasma of the same tint within enlargements of the 
axial canals was noticed; and the penetration and erosion 
were stated to be due to the organism. The cells which were 
observed within hollows on the surface of spicula, and also on 
perfect spicula in positions where erosion from without in- 
wards could readily occur, were very small,—not more than 
too inch in length, and very much less in height. Their 
imensions, however, corresponded to those of certain circular 
patches with hollowed-out bases, which are the first stages of 
the penetration through the spicule down to the axial canal. 
The penetration of the spicule down to the central canal is 
followed by the growth of the organism, which appears to 
erode the silica and enlarges the canal in a most remarkable 
manner, 
After a while the spicule suffers solution of its continuity 
by the thinning from within, and the thinnest flakes present 
a granulated appearance. 
Since writing that communication I have observed siliceous 
sponge-spicula, obtained from great depths, which are affected 
by an organism whose cells are much larger and whose pene- 
trations therefore are wider and much more visible. On the 
head of a large spinulate spicule I found many circular pits, 
each containing an organic mass without definite cell-wall, 
and yet granular and green in colour by transmitted light. 
These pits are shallow and are 5,4, inch in diameter. 
Similar pits and of the same dimensions are seen on other 
spicula; but they are deep and resemble cylindrical tubes with 
hollowed-out bottoms. Some reach the axial canal, which 
has become enlarged. The penetrations contain granular 
organic substance ; and so do the enlarged axial canals. The 
walls of the enlarged axial canals are frequently very irregu- 
larly eroded and look ‘“ worm-eaten ;” the hollows are, more- 
over, green with the very visible granular matter. 
Thus there are two dimensions of the penetrations. The 
first kind of cell found on the spicula resembles somewhat the 
simple zoospores of Achlya penetrans, nobis (Proc, Royal 
Soc. vol. xxv. pl. vi.); the second is larger; and in both 
there is a decided green tint. No ramifications of the pene- 
trating cylindrical tube occur; and it pierces perpendicularly 
to the surface of the spicule, or, it may be, slightly aslant. 
The presence of pits on the surface of sponge-spicula was 
noticed by Kolliker as a peculiar degeneration of the structure. 
Dr. Carter described and figured pits in the outer part of a 
spicule, and distinctly referred them to the action of a vege- 
table cell, in the Ann. & Mag. Nat. Hist. ser. 4, vol. xii. 
p- 407, pl. xvi. figs. 8,9, None of the pits seen by my 
Ann. & Mag. N. Hist. Ser. 5, Vol. viii. 9 
