790 
DE. J. S. BOWEEBANK ON THE ANATOMY 
As the ternate spicula thus united for the support of the dermal membrane 'would 
afford it little or no protection against the voracity of its smaller enemies, -we find the 
necessary defence in innumerable short, stout, entirely spined cylindrical spicula not 
exceeding -sinrof^^ of i’^oh in length ; thus minute, there is no conceiving a predaceous 
creature with a mouth so small that they would not enter and become a subject of 
annoyance so great as to interfere seriously with its attacks on the membrane; and 
they are so numerous, and. so closely packed together, that no portion of it equal in size 
to the length of a spiculum could be removed without one or two of them accom- 
panying it. 
A still further advance in this system of dermal support and defence is exhibited in 
the beautiful harrow tissue of Dr. A. Faree’s siliceo-fibrous sponge, to which his speci- 
men of Eu])lectella cucumer, Owen, is attached. In this case we have a perfect and 
regular quadrilateral network of smooth siliceous fibre, from the angles of which a 
double set of short conical spicular shafts are projected, each about of i^oh in 
length and entirely spined. Each set are at right angles to the plane of the network, 
one series pointing inward, and serving the purposes of attachment to the mass of the 
sponge beneath, while the other set are directed outward, serving as defensive weapons ; 
so that a small piece of this tissue beneath the microscope closely resembles an agricul- 
tural harrow, with the difierence that it has two sets of teeth in opposite directions 
instead of one. The dermal membrane has been nearly all destroyed ; but entangled 
with the fibres of the skeleton there are some attenuato-stellate spicula, with which it 
is probable the dermal membrane was amply furnished as secondary defences against its 
minute enemies. 
I believe the surface presented to the eye in the portion represented in Plate XXXII. 
fig. 7 to be the external surface, as the fragments of the dermal membrane which 
remain all seem to cover that side of the fibres. Generally speaking there is some 
difficulty in detecting the double series of spicular organs at the angles of the network, 
but a reversal of the object beneath the microscope immediately removes all doubt on 
that subject. 
In Grantia compressa and ciliata the intermarginal cavities appear to attain their 
highest degree of development, and are multiplied and expanded to such a degree as to 
almost supersede every other organ. The whole sponge in these species is formed of 
a great accumulation of elongated cells or cavities, closely adjoining each other and 
angular by compression. Their conical distal terminations, abounding in pores, repre- 
sent the external surface of the sponge, while their valvular proximal ends form the 
inner surface, in conjunction with the shallow cavities, into the distal ends of which 
each cell discharges its contents. These shallow depressions, intervening between the 
intermarginal cavities and the cloaca, are all that remains to represent the incurrent 
portion of the interstitial systems so largely developed in the Halichondroid sponges, the 
great cloacal cavity entirely superseding the excurrent spaces and canals (Plate XXXIII. 
figs. 1 & 2). 
