REPORT ON THE TETRACTINELLIDA. 
289 
the collenchymatous lining of the cloaca similar fibrous strands are longitudinally and 
concentrically arranged ; fusiform cells also concentrically surround the fenestrse by 
which the excurrent canals open into the cloaca. Isolated fusiform cells are always 
present near the margins of the fenestrse, arranged radiately, and with one end in con- 
nection with the outer epithelium. 
Flagellated Chambers (PI. XXX. figs. 7, 8).- — These are almost spherical, measuring 
0’024 by 0’026 mm. They are aphodal, the apopyle frequently measuring 0’012 mm. 
in diameter. The choanocytes, which are about 0'008 mm. in length, are of the 
usual nature, a deeply stained basal part containing the nucleus contrasts with a 
transparent coUum which does not stain, and is indicated by two sharply marked 
marginal lines. So clearly marked are these lines as to suggest sometimes the appear- 
ance of a conical tube rather than a collum ; they terminate in the usual fenestrated 
membrane (PI. XXX. fig. 8). 
Symbiosis or Parasitism. — The sponge is infested throughout with a form of 
Cyanophyceous protophyte, which is so abundant as to constitute a really important part 
of the tissues. It usually presents itself as narrow jointed threads, 0'004 mm. in 
diameter, and of considerable length ; the joints of the threads are sometimes square 
in lateral optical section, like those of an Oscillaria, sometimes round, like those of a 
Nostoc; they measure about 0‘004 mm. in length. Besides the threads isolated spherical 
protophytes are very abundant in places, particularly in the ectosome ; they measure 
about 0'007 mm. in diameter, and present a thick double-contoured wall and finely 
granular contents. These are represented in PI. XXX. figs. 12, 14, the thread-like forms 
are shown in the neighbourhood of the flagellated chambers in fig. 7, and amidst the 
fusiform cells of a fibrous thread in fig.' 14. In the latter it will be noticed that they 
run parallel with the fusiform cells, and this they do invariably. 
Spicules. — The strongyles, which are usually collected into spicular fibres, are best 
studied where they traverse the collenchyma of the cloaca, since the matrix of this tissue 
in this place is perfectly colourless and transparent, and not affected by staining reagents. 
Many of them are here met with, surrounded by a deeply-stained, faintly granular spicular 
sheath, in which in a few instances I thought I detected an oval nucleus with its nucleolus ; 
if so, this sheath may be regarded as the scleroblast ; most of the strongyles are without 
the sheath, and this would suggest that when the spicule is fully formed the scleroblast 
disappears ; in other words, the scleroblast remains active for only a limited period, during 
which the spicule increases in size ; when its secretive power is exhausted, the spicule 
has attained its limit of growth, and the scleroblast is absorbed. 
Spongin. — In numerous instances a homogeneous non-granular substance, taking a 
deep stain with hsematoxylin, was noticed in association with the strongyles ; it is most 
obvious at the points where two strongyles cross each other, frequently forming at 
the junction a comparatively large mass ; it has all the appearance of spongin. 
(ZOOL. CHALL. EXP. — PART LXIII. — 1887.) Err 37 
