NOTES ON PLYMOUTH SPONGES. 379 



of a Leucandra or other sponge employed, after being transferred from 

 one basin to another ; a quietude at once replaced by the accustomed 

 stream when the obstructing bubble was removed. This was 

 accomplished in very wide-mouthed sponges by merely raising the 

 osculum, but in most of those dealt with, the bubble could not escape 

 without the aid of pressure. 



Hence the flat form of »S'. compressum. In its cloaca a bubble is never 

 formed. The tide leaves it with rounded outline, so that in a sponge a 

 centimetre wide, its shorter axis may be nearly half as much. As 

 there occurs evaporation, even from its protected surface, into the air 

 round it, and the fine capillaries of its walls suck in fresh supplies of 

 water from the central drop, the sides gradually come nearer, like the 

 capsule surfaces of an aneroid barometer, until the cloacal cavity may 

 attain itself such capillary dimensions that only very dry air can further 

 extract the moisture.* 



As a matter of observation, above the rising tide it is easy to observe 

 on every side flat, yellowish, sponges, like oval pieces of whitey-brown 

 paper, which swell out at once in water to their natural rounded form ; 

 and if a sponge in the rounded form be taken from the water and laid 

 on blotting paper, it becomes flat. On the other hand, in tide-pools 

 which never dry I have found several specimens of S. com2Jrcsswm 

 with the oscular part of the cloaca cylindrical, and this observation 

 only corroborates one noted long ago by Grant. 



The cylindrical form is never met with on exposed sites. One such 

 specimen was found, not in a pool, but hanging under a large rock, down 

 wliich, from weeds and growth of all sorts, a trickling of sea-water kept 

 up through the whole period between tides. There was a constantly 

 renewed drop falling from the open cylindrical mouth, and when this 

 was dried away with a handkerchief the sponge could not flatten, like 

 those accustomed to be dry in every ebb, but its stiff round tube 

 remained open and empty. 



Among all the Calcarea, the only sponge that I know described of 

 absolutely comparable shape is Sycortis lingua, Haeckel (Newfoundland), 

 which appears to me a near connection of >S'. ciliatum. Haeckel notes 

 that only two sponges have dermal spicules at all comparable with 

 *S^. compressiim for size and arrangement ; the one is Leucandra lumdata 

 (Cape of Good Hope), which takes the form of " plattgedriickte langliche 

 Schlaiiche " ; the other is Ascandra falcata (Adriatic), apparently cylin- 

 drical. 



As to the siliceous companions of S. compression, their complicated, 



* In the drying of marine organisms the external deposit of salt, and internal concentra- 

 tion of brine, must considerably retard ultimate desiccation ; though probably with injurious 

 results to organisms whose protoplasm is not adapted to withstand such salinity. 



