1875. DR. J. S. BOWERBANK ON THE SPONGIADE. 283 
This sponge is parasitical on the shell of a large specimen of 
Spondylus spathuliferus (?), which measures from the hinge to the 
front of the shell nearly 3 inches. It completely covers every part 
of the shell and all its spines, many of which exceed 1 inch in length ; 
and in no part does it appear to exceed in thickness a stout sheet of 
paper. The surface is abundantly but minutely hispid by the pro- 
jection of the skeleton-spicula through the dermal membrane for 
about half their entire length, thus performing the offices of external 
defensive spicula as well as those of the skeleton. ‘The oscula are 
simple orifices, so minute as not to be readily detected even with the 
aid of a lens of 2 inches focus. In the specimen under description 
there are appearances very liable to deceive an observer in search of 
these organs, as they assume a form very likely to deceive the eye, 
that of little conical elevations terminating in small circular orifices. 
These fallacious appearances are thus produced. Before the shell 
was covered by the sponge a considerable number of small Balani 
had built upon its upper valve and also upon some of the spines; 
and these shell-parasites have become so completely covered by the 
sponge as to leave only the terminal orifice of each visible, simula- 
ting in a remarkable manner what might readily be mistaken for the 
oscula of the sponge. The dermal membrane is pellucid; it is 
situated about midway between the bases of the skeleton-spicula 
and their apices; and a few spicula of the same size and form as 
those of the skeleton are distributed on its surface. The skeleton- 
spicula all spring from the basal membrane of the sponge, and are 
projected through the dermal one for about half their lengths: they 
vary to some extent in length; a fully developed one measured =; inch 
in length ; and as they pass for about half their length through the 
substance of the sponge, its entire thickness will be about ~}, inch. 
The skeleton-spicula are all more or less fusiform, and in many of 
them the spinulate base is very slightly produced; their greatest 
diameter is 5,),~ inch. 
The internal defensive spicula all spring from the basal membrane : 
their average length is ;+; inch; and their diameter is rather less 
than that of the skeleton-spicula; some of the largest of them reach 
very nearly to the inner surface of the dermal membrane. 
This species is closely allied to the British Hymeraphia simplez ; 
but it differs from it in having the skeleton-spicula much more 
slender and delicate, and in their fusiform shape, and also by the total 
absence of attenuato-spinulate incipiently spinous defensive spicula. 
RAPHIODESMA PARISHII, Bowerbank. 
Sponge sessile, coating thinly, even, minutely hispid; spicula acuate, 
same as those of the skeleton, visibly reticulated. Oscula simple, 
dispersed, small, rather fewin number. Poresinconspicuous. Der- 
mal membrane abundantly spiculous, reticulated ; rete multispicu- 
lous, strongly developed ; areas rather irregular in form, but nearly 
equal in size ; spicula of rete acuate, same size and form as those of 
the skeleton; tension-spicula biclavate cylindrical, very slender, 
dispersed or loosely fasciculated, and tricurvate acerate, small and 
