REPORT ON THE TETRACTINELLIDA. 
233 
being 0'08 mm. thick. Large excurrent canals proceed from them, and approaching the 
poral surface their branches run radially towards it, interdigitating with the incurrent 
canals. The flagellated chambers are usually nearly spherical, the largest measuring' 
0’0276 mm. in diameter, the prosopyle from 0'008 to O'Ol mm., and the apop}de from 
0’008 to 0'012 mm. in diameter. The aphodal canals are frequently constricted into 
a series of vesicles by extension inwards of their walls, after the manner of velar 
diaphragms. 
The secondary canals formed by an extension inwards of the oscular and poral surfaces 
do not appear to bear chones in their walls, at least not in the case of those produced from 
the latter surface. The others cannot be examined without injury to the specimen. 
Fragments removed from the invaginated poral surface show first a chitinous layer 
produced by some species of Hydrozoon which infests them, and beneath this a thin 
layer representing a modified cortex ; it consists of sterrasters, two or three deep, coated 
by a single layer of the ectochrotal spherasters ; but neither in tangential fragments 
torn away, nor in transverse sections, was a vestige of a chone or any other poral 
aperture to be found. These canals cannot, therefore, be regarded as vestibular. The 
chitinous layer which loosely lines the canals contains numerous deciduous spicules, 
which have been extruded from the sponge ; the modified cortex us also hispidated, 
though no trace of hispidation can be distinguished on the outer surface, not even in 
thin slices when subject to microscopic examination. 
The cortex (PI. XXII. fig. 13) is about 0'478 mm. in thickness, almost entirely consti- 
tuted by the sterrastral layer, the ectochrote being represented merely by the thin layer 
of tissue in which the single layer of somal spherasters occurs. The innermost fibrous 
layer of the cortex is also excessively thin ; the cladomes of the orthotrisenes lie in this, 
apposed to the overlying sterrasters. They seldom extend into the sterrastral layer. 
The choanosome is sarcenchymatous, except where it forms the collenchymatous 
walls of the canals. It is infested, especially when it becomes collenchymatous, by a 
species of Oscillaria, of much smaller dimensions than that described from some of the 
Australian Stellettids. The thickness of the filaments is 0 '00395 mm., or very slightly 
less ; in length I have measured them up to 0T38 mm. ; the length of each joint would 
appear to be about 0'0022 mm. 
The megascleres are arranged partly in bundles or spicular tracts accompanied by 
fusiform cells, partly scattered singly through the choanosome ; they show very little 
constancy in direction ; some of the fibres run parallel to the walls of the larger excurrent 
canals, others, and these are more numerous, quite irregularly ; ljut on approaching the 
cortex the spicular fibres are always directed at right angles to it, and the orthotrisenes 
first appear in its immediate neighbourhood. 
The sterrasters present an appearance of coarseness, owing to the fact that, notwith- 
standing their small size, their component actines have nearly the same diameter as 
(ZOOL. CHALL. EXP. — PART LXIII. 1887.) Ell’ 30 
