1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 341 
late spicula, with which it is probable that the membrane was amply 
furnished as secondary defences against minute enemies. 
This singular tissue is figured in the ‘ Philosophical Transactions 
of the Royal Society’ for 1862, plate 32. fig. 7, and also in my 
‘Monograph of the British Spongiade,’ vol. i. plate 21. fig. 311. 
I believe the portions presented to the eye in the picces figured to 
be the external surface, as the fragments of the dermal membrane 
which remained all seemed to cover that side of the fibres of the 
network, and the presence of the external series of the spicular 
organs is strongly indicative of the minute hispidation of the surface 
of the sponge in its natural condition. 
In the present condition of the sponge it is impossible to deter- 
mine whether this singular harrow-like dermal structure was con- 
tinuous over the whole of its surface when in the living condition ; 
but the probability is, judging from the general structure of the ex- 
pansile dermal system of every other known species of siliceo-fibrous 
sponge, that it was composed of detached sections, so as to allow of 
the usual amount of expansion and contraction that we observe to 
exist in every other such sponge. 
The reticulation of the skeleton is always angular, but the areas 
vary from square into ail imaginable varieties of the oblong figure. 
The fibre is stout and strong, with a well-defined central canal in its 
fully developed condition ; a portion of it is represented in Plate 
XXIV. fig. 1, with numerous attenuato-stellate retentive spicula ad- 
hering to the fibres. 
Occasionally in some portions of the skeleton-fibre we find two 
canals, neither of which are central: this abnormal form probably 
arises from two immature fibres, closely approximated in an early 
stage of their development, uniting longitudimally; and in one cage 
I observed as many as three irregular portions of canals in one frag- 
ment of the fibre; but this irregularity of structure is the exception 
and not therule. The spination of the skeleton-fibres is very slightly 
produced in the form of acute cones, and in some of the larger fibres 
it may be almost designated as incipient, while occasionally in some 
of the immature ones the spinules assume the forms of tubercles, 
which are sometimes more or less bifurcated. 
The interstitial tension-spicula of this sponge are very remarkable 
organs. ‘hey are simple biternate, spiculated biternate, and fur- 
cated spiculated biternate. Sometimes one termination only is spicu- 
lated, sometimes both are thus furnished. One or two of the terminal 
radii are frequently furcated; but it is of rare occurrence that the 
whole of them are produced to that extent. They occur in groups 
entangled together; in several of these groups they were numerous 
and closely packed, much in the same manner in which we find the 
spinulo-trifurcated hexradiate spicula of the interstitial membranes 
of Dactylocalyx pumicea when seen in situ. They are stout and 
comparatively of large size (Pl. XXIV. figs. 5 & 6). 
The attenuato-stellate retentive spicula are minute and very irre- 
gular in their structure and in the number of their radii. They have 
evidently been very numerous, as they are frequently found adhering 
Proc. Zoou. Soc.—1869, No. XXIII. 
