REPORT ON THE TETRACTINELLTDA. 
285 
These branches — the main excurrent canals — traverse the wall of the sponge, chiefly 
obliquely outwards and downwards, sometimes extending as far as the ectosome. They 
are lined by an extension of the collenchymatous layer of the chief excurrent tube. 
Pores (PI. XXX. figs. 3, 4) generally distributed, except on the summits of the ridges, 
where they are rare or absent; leading into subdermal cavities (PI. XXX. figs. 1, 2, 5), 
which join together to form irregular sinuous canals, having a somewhat stellate arrange- 
ment, seen as dark spaces through the skin. The centre of each stellate system is situated 
within one of the depressions left between the ridges of the surface ; usually several such 
centres He within a single depression. Each centre marks the outer end of an incurrent 
canal, which descends, with a usually downward and inward direction, into the 
choanosome ; it is lined by a collenchymatous layer, similar to that which forms the walls 
of the larger excurrent canals. 
Spicules. — I. Megascleres. 1. Desma (PI. XXIX. figs. 7, 7a), characterised by 
four triangular depressions, symmetrically disposed around the centre, one in each of the 
angles formed by the four diverging epactines. Each epactine is triradiate in transverse 
section near its origin, being raised into three strong longitudinal ridges ; and it is by the 
confluence of these ridges about the centre that the triangular depressious which they 
bound are produced. At some distance from the origin the epactines become more or 
less cylindrical in section, and still more distally flatten out, so as to acquire a much 
depressed elliptical section. They may or may not bifurcate before growing out into a 
number of little elongated tubercles, which are often bifid ; if they bifurcate, the tubercles 
are produced from any or all parts of the protocladi, if they remain simple, from the ends 
of the epactines. Zygosis is terminal, and is produced by the intergrowth of the 
syzygial tubercles, which seldom form laminar processes. Sometimes the angle between 
the two protocladi of an epactine is webbed across by an extension of their adjacent inner 
margins ; in this way the epactine sometimes comes to end in a flat triangular plate, the 
distal base of wdiich grows out into syzygial tubercles. 
The epactines and cladi of the desmas are very variable in their dimensions ; most 
commonly the epactines measure, when simple, about 0T6 to 0'24 by 0‘04 mm.; when 
bifurcate, the epactine exclusive of the cladi is from about 0'05 to 0T25 mm. long. 
The axial fibre of the crepis extends from its origin into tlie epactine, as a rigidly 
straight rod, for a distance of 0*04 to 0’06 mm.; it then abruptly terminates, and its 
direction is continued by a series of granules, which branch out into twm series at the 
points of bifurcation. 
2. Phyllotrisene (PL XXIX. figs. 4, 4a -e), a short, conical rhabdome, about 0T3 to 
0T6 by 0’019 mm., rounded off at the end ; cladi of very various lengths, usually about 
0'26 to 0'32 mm. long, diverging at various angles with each other, but perpendi- 
cularly to the rhabdome ; more or less flattened in a horizontal plane ; straight, or more 
usually irregularly curved ; margin undulating ; simple or bifurcate, or with accessory 
