PEOTOZOA — EADIOLAEIA. 



29 



Ordovician cherts from Cornwall and the south of Scotland. Gallery X. 



Drawings of the species found in these rocks by Dr. G. J. 



Hinde are exhibited alongside. Other specimens of Eadio- Wall-case 



larian rock from foreign localities are also shown. Eadiolaria ^ 



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. 



POKIFEKA (Sponges). 



'Next the Protozoa come the fossil remains of sponges, the Wall-eases 

 foreign ones being displayed in the Wall-cases, and the 7, 8. 

 British ones in the Table-cases. Among the latter will be Table-eases 

 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 various 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 microscope 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. From 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 



