REPORT OR THE TETRACTINELLIDA. 
The false incurrent canals formed by invaginations of the cortical layer are some of them 
lined by a chitinous layer produced by a species of Hydroid ; one contains an Ophiurid, its 
disc lying about 15 mm. from the exterior, with one arm extending from it along the 
false canal, protruded at the exterior. 
The false canals are apparently of the same nature as the cavities described by von 
Lendenfeld as vestibules, and they may therefore be appropriately named vestibular canals. 
The large excurrent canals run chiefly longitudinally, and are traceable from the base 
to the oscular chones. 
The cortex (PI. XXIII. fig. 15) of the oscular surface is about 0'63T mm. thick, and 
composed as follows : — An outer epithelium covering the ectochrote, which is crowded 
with minute spherasters (5) ; this, which is very thin (O’Ol to 0’04 mm.), is succeeded 
by the sterrastral layer, 0‘143 mm. thick, including the ectochrote; then follows a dense 
felt of deeply staining fibres 0'334 mm. thick, and finally a fibrous collenchymatous 
layer 0'16 mm. thick. Immediately about the oscules the total thickness of the cortex is 
increased to about 0’828 mm. On the poriferous surface (PI. XXIII. fig. 16) the cortex 
is thicker, about 1’33 mm. on an average; it is made up as follows : — First, the ectochrote 
0’0318 mm. thick, then the sterrastral layer 0191 mm. thick, and, finally, the fibrous 
layer 1114 mm. thick. 
About the margins of the oscules and the roofs of the poral chones the small oxeas 
(No. 2) appear within the outer half of the cortex; and, projecting slightly from the surface, 
give it a slightly pilose appearance. 
The spicules of the body do not run radiately to the cortex ; only those immediately 
next the cortex are directed at right angles to it ; the majority run in spicular fibres, 
(accompanied by numerous fusiform cells and collenchyma) in various and apparently 
irregular directions through the choanosome. Near the oscular surface many of these 
fibres run transversely to the excurrent canals, and subdivide the choanosome, containing 
flagellated chambers, into curiously restricted areas. The sterrastral spicules are the 
smallest I have yet seen, possibly the smallest known. By this character the sponge may 
be at once distinguished from Synops pyriformis, Vosmaer. The spines of the actines 
are few and small, but they still serve for the attachment of the connecting fibres. On 
the oscular surface the sterrasters form a layer only two or three thick. The ortho- 
trisenes lie with their cladomes immediately below the fibrous layer of the cortex, 
occasionally extending into it. 
The roofs of the poriferous chones lie in depressions below the general surface of the 
poriferous area, and the sterrastral layer curves downwards towards the chones as it 
approaches them. The chone traverses the greater part of the fibrous layer before it 
becomes constricted by the sphincter. 
The flagellated chambers vary from about 0'0237 by 0’0276 to 0 ’02 76 by 0'0315 mm. 
in diameter. 
