R. W. H. ROW—REPORT ON THE SPONGES : NON-CALCAREA. 363 
Skeleton arrangement. 
The skeleton consists of a reticulation of spongin-fibre covered with foreign 
bodies, and there is also a thin and rather scanty dermal coating of small sand- 
grains. The structure of the spongin-fibres and their arrangement differ 
considerably in the regular branches of the sponge and in the secondarily 
formed tissue around the barnacles. 
The fibres in branches themselves can be distinguished roughly into 
primary and secondary fibres, according as they possess a core of foreign 
bodies or not. The average diameter of the main fibres is 0-1 mm., and the 
core within them usually measures about 0°07 mm. 
The foreign bodies which form the fibre-core seem to have undergone some 
peculiar change ; they do not seem to possess any definite shape, but all lie 
fused up together into a continuous and irregular cylinder. Ina very few 
places it is possible to see a few sand-grains clearly marked out from the 
surrounding confused mass, but, as a rule, it is quite impossible to ascertain 
what the various foreign bodies are. 
The fibres are not always cored ; sometimes the core stops quite suddenly, 
and in these cases the distinction between primary and secondary fibres breaks 
down, or else a primary fibre becomes a secondary. 
The primary fibres branch and run fairly regularly. The reticulation 
formed by them and the secondary fibres is rather irregular in the centre of 
the sponge, but at the surface they form an extremely regular meshwork 
whose meshes are rectangular ; the primary fibres are not quite so thick here 
as in the middle of the branch. ‘The secondary fibres have no foreign bodies 
within them, and are very mach slenderer than the primary fibres, measuring 
only 0-015 mm. to 0:02 mm. in diameter. 
The fibres in the secondarily formed tissue are midway between the 
primary and secondary fibres of the true branches in point of size, measuring 
from 0°03 mm. to 0°05 mm. in diameter, but the most striking difference lies 
in the fact that in these latter fibres the foreign bodies, chiefly sand-grains, 
of the core are quite distinct, and not fused up together at all. The reticu- 
lation that they form is fairly regular, but not definitely oriented to the 
sponge surface. 
Canal-system and Chambers. 
The pores lead direct into large horizontal subdermal canals, from which 
large inhalant canals lead into the interior of the sponge. These canals 
ramify throughout the sponge, and cause the interior of the sponge to appear 
very cavernous. They measure from 0-3 mm. to 0°4 mm. in diameter. 
The chambers are eurypylous, and measure 0:06mm. to 0°07 mm. in diameter. 
They open direct into the exhalant canals, which are similar to the inhalant. 
