710 
DE. WYV1LLE THOMSON ON HOLTENIA. 
that of the outer wall; it is apparently the same as that of the oscular wall. It is sup- 
ported and its spaces are defined by a very irregular elastic meshing of spicules, which 
resemble closely the smaller five-rayed spicules of the outer network, only they are some- 
what more delicate in their proportions and their rays are longer (Plate LXX. figs. 1,2). 
The sarcode contains irregularly scattered through it, but not in very great numbers, 
the feathered five-rayed spicules, and in this situation they exhibit every variety in form. 
They are most usually extremely delicate, with curved rays (Plate LXVIII. figs. 12, 13, 14); 
but some of the stoutest and most rigid spicules (Plate LXVIII. fig. 11) were found asso- 
ciated with these. These feathered spicules are arranged in the substance of the sponge in 
the same way as in the membranes, the crosses imbedded in the sarcode, and the feathered 
styles projecting into the sinuses. Amphidisci are much less abundant in the sponge- 
substance than in either of the walls, but in some places the minute fusiform spicules 
(Plate LXVIII. figs. 8, 8 a) are numerous. Here and there throughout the sponge there 
is one of the bihamate spicules (Plate LXVIII. fig. 5), and over the lower third of the 
sponge, and over the upper third, wherever a column or rod of sponge rises to the outer 
network, a fascicle of siliceous fibres and spicules passes along its axis, becoming free 
beyond the outer wall of the sponge. In the lower third of the sponge the bundles already 
described of 20 to 100 long cylindrical fibres originate in the perpendicular columns 
of sponge-substance. The spicules commence in a fine point, gradually acquiring their 
full diameter as they approach the surface of the sponge (Plate LXX. fig. 2, a) ; they then 
pass out into the oozy mud, in which they spread. The layer of sarcode covering these 
fibres throughout their entire length is equal to about the semidiameter of the fibre. 
The sarcode is soft and nearly colourless. It contains a few scattered granules and com- 
pound granular masses, and apparently no spicules. It is so abundant in the silky 
beard, that when the fresh sponge is held up it falls from it in flakes and glutinous 
drops loaded with Globigerince and chalk-mud. The sarcode of this sponge appears to 
be perfectly uniform and continuous. There is no trace of its differentiation into cell- 
like bodies surrounding endoplasts. 
Reproduction. 
In the specimen of Holtenia which was sacrificed in order to investigate the details 
of internal structure, any indications of the function of reproduction were unfortu- 
nately very obscure. Near the base of the sponge, among the granular sarcode bounding 
the sinuses round the lower part of the exhalent cavity, there were numerous soft, 
granular, pyriform gemmules. At the particular stage at which they were observed 
these gemmules were of a bright orange-colour, destitute of spicules, and perfectly 
uniform in their dense granular structure. Although gemmules and specimens of all 
ages, of other sponges such as Tisiphonia and Hyalonema , were detected abundantly in 
the mud, and even among the fibres of the beard of Holtenia, it is singular that neither 
gemmules nor young specimens of this species were met with. 
