4 8 pone/. 
SPONGI/E. 
Anatomy. 
Schulze (34) describes the structure of the Hexactinellida. Beneath 
wide di ff erences in detail, they present a fundamental similarity of struc- 
ture, so that it is easy to construct for all known forms a common 
scheme. Disregarding the hard parts, they consist of a simple sac, the 
outer surface of which is formed of a thin poriferous skin — the dermal 
membrane ; beneath this is a cavity traversed by fine trabeculae — the 
subdermal trabecular space ; this is succeeded by the flagellated cham- 
ber-layer. The chambers, one layer thick, lie close together, with their 
convex-closed ends turned towards the exterior and their open mouths 
towards the interior ; their wall is a reticulate membrane perforated by 
the chamber pores. As a direct process of the chamber wall, the “ con- 
necting membrane ” extends from the mouths of the chambers, closing 
the spaces which remain between them. The chambers open into a 
cavity broken up by trabeculae — the subgastral trabecular space — which. is 
separated from the large gastral cavity by porous or reticulate membrane 
— the gastral membrane. 
Of those five layors every Ilexactinollid is composed ; but modifica- 
tions of form may occur, so that tho gastral and dermal faces are not 
to bo distinguished by their position with rospect to the centre of tho 
Sponge body, but rather by the fact that the dermal membrane is that 
through which water enters to the interior, and the gastral that through 
which it leaves it. 
As modifications, the simplest is a simple elongation of the sac into a 
pouch, accompanied by a peculiar folding of the chamber-layer, which 
becomes evaginated towards the smooth ‘ dermal skin. In forms resem- 
bling a thick walled cup or bow), the folding of the chamber-layer has 
proceeded further, fresh evaginations arising, till, finally, branched ex- 
current canals of circular transverse section are produced, between which 
corresponding complicated branched incurrent canals extend from the 
subdermal trabecular space. The gastral membrane either extends over 
all the gastral openings of the excurrent canals as a smooth sieve net, 
and so forms the boundary of the simple gastral cavity, or it enters 
niche-like recesses of the wall, or finally it enters the excurrent canals 
through their wide gastral openings, and lines their surface up to the very 
end of their terminal branches. 
A further variation is produced by the closure of the simple mouth 
of the sponge-sac by a terminal sieve plate. 
The body-wall of the sac is frequently perforated by more or less 
regularly arranged gaps, which place the gastral cavity in direct commu- 
nication with the exterior. 
Striking alterations of form are not ‘seldom produced by the develop- 
ment of a stalk, as a direct process of the lower end of the sac. 
Through an expansion of the upper end and of the oscular opening 
many species acquire a funnel shape. Further widening out and flatten- 
ing convert this into a- flat disc ; or, if the growth is one-sided, into an 
ear shape; or, may be, in»,o a simple vertical plate. Again, by folding 
