PROTOZOA — EADIOLAEIA. 
29 
Drawings of the species found in these rocks by Dr. G. J. .Gallery X. 
Hinde are exhibited alongside. Other specimens of lladio- Wall-ease 
larian rock from foreign localities are also shown. liadiolaria 
are occasionally found scattered through rocks other than 
cherts, but as a rule their delicate skeletons have been 
dissolved away. 
See further Haeckel’s “ Eeport on the Eadiolaria collected 
by H.M.S. ‘ Challenger,’ ” 1887, in which the fossil species 
are also dealt with. 
PORIFERA (Sponges). 
Next the Protozoa come the fossil remains of sponges, the Wall-cases 
foreign ones being displayed in the Wall-cases, and the 
British ones in the Table-cases. Among the latter \rill be 11 - 15 . 
seen many of those preserved in flint, and familiar either to 
collectors in the chalk-pits or the searchers for pebbles on the 
beach. To explain the structure of these a rather long 
description is necessary. 
Sponges are animals most of which live in the sea at all 
depths, while one family alone is found in fresh water. 
Since they have no organs for locomotion, sense, or reproduc- 
tion, they are usually indefinite in shape as well as very 
variable in size. The only organs readily seen are several 
holes, often mounted on slight projections at vai-ious parts of 
the surface ; from these holes, which are called “ oscula,” a 
current of water is always issuing. Closer examination 
discovers scattered over the whole surface a far larger number 
of small openings through which the water is as constantly 
being drawn in ; these are called “ ostia.” Study of thin 
sections of a sponge under a micro.scope shows how this flow 
of water through the sponge is brought about. The water 
that enters the ostia is led by irregular winding canals into the 
deeper parts of the sponge. These incurrent canals open by 
very minute pores into a number of small round chambers, 
whose walls are furnished with little lashes (or “ flagella ”) in 
constant motion. Prom each such flagellated chamber the 
lashes are continually driving the water through a wide open- 
ing into another set of canals, the excurrent canals, which 
lead it out to the oscula. The sponge, unlike the animals 
hitherto considered, is a combination of many cells, modified 
for various services. Thus the outer surface and the walls of 
the canals are coated with flattened cells ; the cells lining the 
flagellated chambers bear each a lash surrounded by a collar 
