378 NOTES ON PLYMOUTH SPONGES. 



(d) Held under a stream of fresh water some minutes, tissues all 

 destroyed. 



(c) One minute lying in fresh water, of which for thirty seconds it 

 was entirely submerged; a few flagella were found moving, in most 

 parts they were not visible, in most places the cells had thrown out 

 strings of protoplasm, and were in other ways altered. 



Though improbable, it was logically conceivable that the comparative 

 immunity to rain and other fresh water was due to modification of the 

 naked protoplasm itself. While (c) was in full action, therefore, 

 a drop of fresh water was introduced under the cover-slip. For a 

 moment the flagella quickened, almost instantly stopped, and within 

 a few seconds the cells successively became transparent, then ovoid, 

 then disappeared. 



It appears to be fairly certain, therefore, that this apparently fragile 

 member of a singularly delicate group of animals must have some 

 exceptional provisions, (1) to resist evaporation, (2) to withstand injury 

 from such evaporation as still takes place, (3) to resist the entrance of 

 noxious fluids, videlicet pure water. 



To meet (1) and (3) I propose at once the spicules. Dendy, in his 

 masterly review of the Heteroca3la,* pointed out the anomalous 

 position of >S^. compressum, in having a highly developed cortex and yet 

 retaining what may be called the "conal acerates," that is, the centrifugal 

 bunch of unbranched spicules which surmounts the end of each radial 

 tube. The first, as he shows, is a Grantiad character ; but the second 

 is typically Syconid. 



Now I suggest that the thick, continuous, cortex, set with its dense 

 mass of club-shaped radial spicules, enables the sponge to pursue its 

 daring existence ; clothing it with a deep armour of calcareous mosaic 

 through which, when the skin is contracted on its pores, a minimum 

 amount of permeation or evaporation can take place. The shillelagh- 

 like outer ends of the spicules serve, like the heads of iron nails set in 

 a pile at sea, to cover and protect the surface of the substance in which 

 their points are embedded. 



As to (2). Tlie danger to a sponge from evaporation of the contained 

 water comes at a stage short of desiccation. If we suppose a rigid, 

 cylindrical, Sycon to be exposed to the sun and wind for two or three 

 hours between tides, the returning water would find it — though perhaps 

 damp, and still in cellular life — with its gastral cavity empty; 

 evaporation having replaced the liquid reservoir by a bubble. Such 

 a sponge is doomed. It has occurred to me again and again, when 

 measuring the oscular currents, to be surprised at the sudden quietude 



J. M. S., vol. 35. 



{ 



