with descriptions of new Species. 323 
On the coast of Northumberland the surface of almost every 
piece of limestone near low-water mark is riddled by Cliona: old 
shells, whether univalves or bivalves, are filled with it; it imha- 
bits nullipores ; and in southern latitudes it buries itself in corals. 
Its ravages are very extensive and appear to be rapidly effected. 
I have seen half-grown living oysters with Clona extending from 
the umbones almost to the ventral margin, and in one or two 
instances it even reaches that margin. [In these cases it is evi- 
dent that the growth of the sponge must have been more rapid 
than that of the shell; for the work of destruction could not 
commence until the oyster had attained to some size; and had its 
growth been even equal to that of the sponge, the shell ought to 
have reached its full development before the sponge had gained 
the lower margin. 
When a shell is once attacked, the operations of these crea- 
tures never cease until they have extended throughout its entire 
substance. The middle portion soon becomes almost completely 
excavated, small pieces only remaining to divide the chambers or 
branches. A thin plate is left on the outer and inner surfaces 
to protect the parasite ; and even these plates are ultimately rid- 
dled with numerous circular holes, whieh are the only indication 
‘of the work of destruction beneath, until some slight external 
influence ruptures the protecting walls, or the increasing growth 
of the tenant bursts them asunder; when the whole system of 
elaborately wrought chambers becoming exposed soon gives way, 
and Cliona, Sampson-like, perishes amidst the ruin produced by 
its own energy. 
The excavating sponges abound in the Tropics, where they will 
‘keep in check the rapid increase of calcareous matter. The coral 
reef is built up by particles, and so by particles will it be reduced 
by the antagonism of these ever-working, all-pervading beings: 
the huge massive 7ridacna falls in pieces subjected to the insi- 
dious encroachments of Cliona; and the limestone rock, almost 
bidding defiance to elemental influences, crnmbles beneath the 
touch of this the lowest of animated beings. 
It is difficult to say whether certain species of these parasites 
confine their ravages or not to certain shells or other calcareous 
bodies, though the fact of twelve species occurring in a single 
individual of Tridacna would appear to contradict such an opi- 
nion. Three or four distinct species are likewise found in the 
common oyster, and one of them occurs in limestone: Fusus an- 
_tiquus has also supplied three good species. On the other hand 
Clona radiata would appear to be confined to Triton variegatus, 
in two or three specimens of which it occurred abundantly ; 
Murex regius is frequently affected by the operations of Cliona, 
and always, as far as I have been able to ascertain, by the same 
21* 
