1875. | SILICEO-FIBROUS SPONGES. 277 
skeleton-structure appears to consist of two and sometimes three 
layers of this beautiful network. The outer and the inner layers are 
powerfully armed by a single series of stout harrow-like teeth pro- 
jected from the angles of the network at right angles to the surface ; 
and each of these teeth is furnished with numerous imbricated scales, 
like the similar organs in Farrea occa, described in the ‘ Proceed- 
ings of the Zoological Society of London,’ for May 13, 1869, p- 339. 
These organs in the species under consideration differ from those in 
F. ocea in being longer and more slender in their proportions. If 
the specimen be examined by direct light with a power of 50 linear, 
these structures are exhibited in a very beautiful manner, projecting 
outwardly from the angles of the dermal layer of the quadrangular 
network at right angles to its surface, and from the internal skeleton- 
layer in like manner into the interior cavity of the sponge, while the 
intermediate layer or layers of the skeleton are entirely destitute of 
these organs; but in place of them they are abundantly furnished 
with attenuato-acuate entirely spined internal defensive spicula, 
which are based on the sides of the skeleton-fibres, and are projected 
at various angles into the areas of the rete: These internal defen- 
sive spicula are very numerous where the fibre is well coated with 
the dark amber-coloured sarcode ; but when the sarcode is absent 
not a single spiculum is to be seen on the fibres. The skeleton- 
fibres of the interior surface do not appear to be furnished with in- 
ternal defensive spicula; but those of the outer and intermediate 
layers are abundantly furnished with them. These spicula vary to 
a considerable extent in size, the longer being frequently twice the 
length of the shorter ones. The basal half of both is abundantly 
spinous, while the distal half is usually entirely spineless. The spines 
are short and acutely conical. I could not detect any interstitial 
membranes ; but a few large acerate spicula were observed in the in- 
terstitial spaces ; but whether they belonged to the sponge or were 
adventitious I could not determine, as several other forms of spicula 
were observed on both the outer and the inner surfaces of the sponge. 
On examining the interior of the sponge with a lens of two inches 
focus through one of the large terminal orifices I observed a small 
patch of flocculent-looking sponge-tissue ; and on removing a portion 
of this and mounting it in Canada balsam, it proved to be a very 
young Hymedesmia johnsonia in an excellent state of preservation, 
with numerous specimens of that remarkable form of spiculum, the 
trenchant contort bihamate, dispersed among the acerate skeleton- 
spicula of that sponge, and closely resembling the portion of the type 
specimen represented in vol. i. plate xviii. fig. 293, ‘ Monograph of 
British Spongiade.’ The occurrence of this well characterized 
sponge parasitical within the Farrea is a remarkable circumstance, 
and it seems to indicate that the open tubular structure of that sponge 
is probably its normal condition. Beside Hymedesmia johnsonia, 
naturally parasitical on its inner surface, there were two other small 
groups of sponge-spicula that were evidently adventitious. One 
consisted of numerous short entirely spined cylindrical spicula dis- 
tributed irregularly over the fibres of three or four areas of the skele- 
