668 PROF. SCHULZE ON THE STRUCTURE AND ARRANGEMENT 
the sponge-tube : this is the cause of the groups already mentioned of smaller 
roundish excretory openings under each external boss, and which may be easily 
distinguished even by the naked eye from the large pores in the area under the 
ridge. 
The portions of the sponge-tube surrounding the wall-openings in the form 
of a flat circular margin differ essentially from the thicker parts, as they have 
no ciliated chambers, and are not traversed by a lacunar system. MARSHALL 
had already conjectured that the wall-openings may act like a sphincter by 
means of a membranous margin supported by a circle of siliceous spicules, and 
that in this way the openings may vary in size. These wall-spaces cannot, 
however, be compared with the oscula of other sponges, as the latter always 
serve as the exhalent openings for the water canal system. The cribriform 
operculum at the upper end of the sponge may be rather considered as 
- representing an oscular region, whilst the round wall-openings in Huplectella 
probably only serve for the free lateral entrance or exit of the surrounding 
water. 
The idea expressed by CLAus* that the Huplectella as a whole may be com- 
pared to a tube-shaped sycon, in which the separate boss-shaped projecting 
thickenings of the wall, as well as their corresponding ridges, may be con- 
sidered as homologous to the radial tubes; the circular wall-openings to the 
intercanals, and finally the upper cribriform opercular plate to the wider oscular 
opening of the sycon, is therefore inadmissible, as we can only possibly compare 
the single sac-shaped ciliated chambers of Huplectella to the single radial tube of 
a sycon, and not an entire system of such chambers with branched excretory 
passages such as we have in each projecting knob or ridge. MaArsHALL had 
already observed this, and, with more special reference to the relations of the 
spaces in the tube-wall of Huplectella, had attempted a more justifiable com- 
parison with a Leuconpersona. 
The whole structure of the cribriform opercular plate indicates that it is an 
oscular region serving as the last passage of exit for the discharged water, 
since the inner and side surfaces of the small lattice-beams are quite flattened, 
as if worn away by the ejected water, while the outer surface only is furnished 
with a small quantity of soft substance containing ciliated chambers, and is 
covered by the same skin which we find on the ridges and bosses of the 
tube-wall. 
After this explanation of the general arrangement of the soft parts of 
Euplectella aspergillum, I will now consider its histological structure more in 
detail. 
Here, as in all sponges which I have closely examined hitherto, we can 
* Ueber Luplectella aspergullum, 1868, 
