1915. | N. ANNANDALE: Parasitic Sponges. 475 
growth. If the material into which they have penetrated is at all 
soft or crumbling this causes it to split or even to fall in pieces, 
and the final result of the parasitism of most of the invading 
sponges must be to produce a state of affairs in which it is neces- 
sary for them, unless they are to perish altogether, to assume again 
an independent form of existence. Sooner or later they destroy 
the walls of their retreat and so are once more exposed. 
The species of Stoeba and Coppatias do not depend solely on 
expansion as a means of penetration, for they are able to break 
off fragments of calcareous matter. These are more or less 
rounded in form and are stored up in the interior of the sponge. 
How the fragments are broken off we do not know, but it is evi- 
dent that the sharp points of the spicules play an important part 
in the operation. Even in the case of the Clionidae the precise 
method by which the burrows are excavated is not yet by any 
means clear. It has been shown! that the action of acid is absent, 
and it seems most probable from the disposition of the spicules in 
the growing points of the sponge that little pieces of shell or coral 
are broken off, not merely by impact of the spicules, but also by a 
rotary action. The points of a number of macroscleres are pro- 
bably directed in a circle covering a small area of the surface on 
which they are to work. The heads of these spicules may be then 
rotated by what would be called in an animal more highly organ- 
ized than a sponge, muscular action. The fragments observed in 
the interior of Coppatias and Stoeba are as a rule larger and of 
less regular shape than those produced by the activities of Cliona 
or Thoosa, and it seems probable that the operation by which they 
are produced is of a less specialized nature than in the case of 
the Clionidae. Moreover, the manner in which the spicules are 
arranged appears to be much more haphazard, and we can only 
suppose that their action is less concerted. 
The fragments of calcareous matter removed by Rhabderemia 
prolifera are certainly separated by an entirely different process. 
The species of Coppatias and Stoeba that invade Clionid burrows 
grow forwards as bodies that are practically solid, whereas the 
Rhabderemia merely coats the walls of the excavations it invades 
as an extremely thin film. This film grows round projecting frag- 
ments of coral and separates them from the walls by constricting 
itself round their bases. There is no evidence that the contained 
particles of calcareous matter are of any utility to the other spe- 
cies, but to this sponge they are probably directly useful. The 
film that surrounds each fragment contracts away from the main 
body of the sponge and forms a bud that separates itself from its 
parent and doubtless aids in the distribution of the species by so 
doing. The fact that it has a solid core of relatively heavy 
material must aid it considerably by causing it to fall away more 
readily. 
! For a full discussion see Topsent, Arch. Zool. expévim, (2) V 2, pp. 59-71 
(1887). 
