SPONGES. 91 



Spongia ?— Plate XXIV. 



Perhaps tliis may ultimately prove of the same as I have already had 

 in two specimens. It is rooted on a small shell of the Venus Islandica. 



Height ten lines : gi'eatest diameter about seven lines. Form ex- 

 actly resembling a pear, affixed by the stalk, which diminishes to about 

 a line in diameter where smallest. 



The whole surface is rough, dingy white or greyish, like fine woollen 

 cloth. 



When inactive it stands erect ; the summit obtuse, round, and even. 



But, when active, the skin opens and recedes from the centre. This 

 exposes the yellowish substance below it, which is penetrated by a deep 

 orifice immediately in the centre, and four large orifices in the yellowish 

 substance around it. 



The skin can close again, however, when the whole parts exposed 

 are entirely covered up. I have not seen the skin recede so far as to 

 show the exterior edge of the four orifices next to it. 



These orifices are of rather irregular shape : they tend to circular. 

 All are not of equal size : the central orifice is the smallest. Neither do 

 they dilate equally : for two may be larger, and two of the four smaller; 

 or only one may be smaller. 



This product forms a vortex, which whirls around the light sub- 

 stances on the surface of the water. Their motion is quite evident, 

 shewing the vortex stronger at one time than at another. 



Expulsion of internal air or water would have such an effect. If 

 there is expulsion there must be absorption, for which no separate chan- 



