Polymastia harmelini Boury-Esnault &  

The specimens are cushion
shaped and cover a surface of about 100 cm2. The thickness is
about 3-5 mm. In situ the papillae only are visible.
The body is covered by sediments and particles trapped by
the hispid surface. The colour of the papillae is brown, as well
as the surface. The choanosome has a deep yellow colour in
life. The cortex is difficult to tear but it is easily detachable
from the choanosome. There are about 28 inhalant papillae
and 1 exhalant papilla bearing an oscule per specimen. A dark
ring followed by a white one surrounds the oscule.
The length of the inhalant papilla is 4-10 x 1.5-3 mm and that
of the exhalant ones is 8-12 x 4 mm.
Skeleton (Fig. 2-3): The ectosomal skeleton is about 320-
370 μm thick and composed of three layers: the upper one
is a dense palisade (150-170 μm) of tylostyles which lie on
a layer of collagen (90-120 μm) (Fig. 2A). The basal layer of
the cortex is a tangential layer (50-80 μm) made of intermediary spicules . Right below the surface is a layerof cells with granular inclusions (25 μm) which is responsible
for the brown colour of the ectosome. The basal part in contact with the substratum is constituted by the tangential layer of intermediary spicules. The palisade is absent and the sponge is fixed to the substratum bya collagen layer.
Choanosomal tracts of principal spicules can reach 340
μm in thickness at the basis. These tracts are divided into two
or three smaller ones (170 μm) below the ectosome .
They cross the ectosome and echinate the surface at distances
of approximately 400-500 μm . Ectosomal and
intermediary spicules are scattered between the choanosomal
tracts .
The skeleton of the papilla consists of ascending
multispicular tracts running through the length of the papillae
. About 25 to 35 tracts are present in a papilla and
each tract has a diameter of 50-100 μm. The central exhalant
canal is about 160 μm in diameter. It is surrounded in the
exhalant papilla by about 10 inhalant canals 80 to 150 μm
in diameter. The septa between the canals are strengthened
by intermediary spicules . The ectosomal skeleton
of the papilla is about 260-300 μm thick and composed of
two layers . Towards the periphery there is a layer
of tangentially arranged intermediary spicules (50 μm) and
followed by a palisade of ectosomal spicules (180-290 μm).
Towards the surface, the extremities of the ectosomal spicules
form a regular hispidation of about 100 μm in height. Below
the cell surface there is a layer of spherulous cells of about
50 μm thick.