REPORT ON THE TETRACTINELLIDA. 
293 
Spicules. — I. Megascleres. 1. Desma (PL XXXII. figs. 4, 5), in the young adult 
state quadriradiate, with smooth cylindrical epactines, simple or branched, with triangular 
depressions about the centre occupying each angle when three arms meet ; in the older 
adult state they are sparingly tubercled, either by simple, rounded, conical tubercles, or 
low, transverse, crescentic ridges. The zygoses are formed by the meeting of several 
cladi, the tubercles intertwining to form an inextricably tangled knot. So thorough is 
the union that no distinction can be drawn between adapted tubercles and those to 
which they are adapted, every tubercle is both one and the other. At the base of the 
sponge the tubercles run out into elongated twig-like processes of most varied form, 
and these adapt themselves accurately to the asperities of the surface of attachment. The 
simple epactines of the desma are on average 0'25 mm. long. The axial rod extends 
from the centre for a distance of from 0’032 to 0’064 mm. into each epactine ; in a 
few cases the axial rod was observed to have lost its tetrad character, the number of 
its rays being reduced to three (PI. XXXII. fig. 6) or two ; in others it was seen to have 
retained it in spite of the loss of tetrad form in the desma ; in one case a desma near the 
base of the sponge was found reduced to a single cylindrical shaft, branched at the 
ends, but with the tetrad axis preserved in its midst. 
2. Phyllotrisene (PI. XXXII. figs. 7-9) ; (a) of the oscular surface ; rhabdome short, 
conical, rounded at the inner end, distally expanding into thin plate-like cladi flattened 
in a horizontal plane; the three protocladi usually bifurcate, sometimes trifurcate, giving 
rise to deuterocladi, which subdivide into terminal branches with rounded margins. 
{b) Of the poral surface ; similar, but in addition discotriaenes having cladomes with 
entire margins more or less circular in outline or broadly lobate. The phyllotrisenes are 
arranged in several layers, and when discotriaenes occur they overlie the space enclosed 
by the curved or semicircular angles formed by the cladi of the underlying phyllotriaenes. 
The rhabdome of the discotriaene descends perpendicularly through this space. 
In some of the branched phyllotriaenes a ridge runs from the rhabdome to each 
protocladus, producing three triangular depressions similar to those which characterise 
the young choanosomal desma. The cladi of the deeper-lying phyllotriaenes are thicker 
than those of the more superficial. 
The rhabdome of the phyllotriaene is about 0T6 mm. long ; the cladi of the branched 
phyllotriaenes measured from origin to end are very various in length, the largest 
attaining a length of from 0‘32 to 0'45 mm. The discotriaenes measure from 0T8 
to 0*2 mm. in diameter. The axial rod extends throughout the length of the rhabdome, 
but not more than from O’OllS to 0’028 mm. into the cladome. The three protocladi of 
the discotriaene commence like the epactines of the desma in granular or reticular silica, 
and they retain this granular appearance for some time, up to a length of 0'08 mm. 
3. Rhahdus. — This was not observed, but it does not follow that it is not 
present. 
