The Skeleton and Classification of Calcareous Sponges. 61 



The incompleteness of the observations is recognised, but on the 

 whole it would appear that their suggestiveness justified the observer 

 in devoting 15 seconds to making them. 



The observed intensity of the bands, attributed to sky polarisation, 

 is evidence of the quantity of light reflected by solid particles in the 

 atmosphere. It seems not improbable that the unexpected brightness 

 of the general sky and landscape during totality may have been 

 connected with the amount of light reflected from the dust suspended 

 in the atmosphere and illuminated by the sun-lit plains outside the 

 moon's shadow. The light colouring of the plains, due to the dried 

 herbage at that time of year, is very marked at Pulgaon, but it must 

 not be forgotten that the " sun-lit plains " were in the moon's penum- 

 bral shadow for more than an hour both before and after totality. 



The observations occupied the 15 seconds, ending 15 seconds before 

 the end of totality. 



About 30 seconds after the end of totality, polarisation was again 

 looked for, but no trace could be detected near the sun in any position 

 of the instrument. 



" The Skeleton and Classification of Calcareous Sponges." By 

 G. P. BIDDER. Communicated by ADAM SEDGWICK, F.R.3. 

 Received May 6, Head May 26, 1898. 



I. Skeleton. 



An element which seems to have been too little regarded in 

 the physiology of sponges is the permanent tension of their walls.* 

 The contours of the surfaces, particularly where they rise over 

 projecting spicules, are alone sufficient to demonstrate that there is 

 surface-tension between the protoplasm of the sponge and the sea- 

 water. Both the outer and the inner surfaces of a cylindrical or of 

 a spherical sponge unite, therefore, in exerting a force which tends 

 to contract its diameter. In many sponges there would appear to be 

 also some form of elastic matter in the tissue immediately underlying 

 the collar-cells ; since in teased preparations of the living sponge, 

 fragments of the chamber-wall turn inside out and swim about like 

 ciliate -larva?. While the collar-cells are active, these united tensions 

 are resisted by the pressure of the water in the cavities they line. A 

 broad generalisation of the mechanism of a sponga's currents shows 

 that the velocity in the oscular stream (of comparatively narrow 



* I have to thank Mr. a. T. Walker, of Trinity College, Cambridge, for rescu- 

 ing me from some fallacies with regard to the effects of this tension, and Professor 

 Lewis for most kind patience in mitigating my ignorance of crystallography, and 

 much valuable information. 



VOL. LX1V. B ' 



