THE BRITISH SPONGES. 15 



are seen to be carried out from the sponge in the current 

 at short and irregular intervals. It is impossible to witness 

 the scene without being at once satisfied that it must flow 

 from some cause connected with the vitality of the sponge 

 itself; nor does it seem possible that a circulation so uni- 

 form in its course, and continuous and turbulent, can be 

 maintained by the breathing of any insect, worm or mol- 

 lusk fortuitously nestling in the sponge, and which, more- 

 over, were in vain sought for. 



By this circulation, sponges operate the usual changes 

 of organized beings on the water around and within them, 

 for they soon die unless this is often renewed ; and they 

 render it equally unfit for the support of other life. On 

 what power the circulation depends is unascertained. That 

 it cannot be ascribed to any contraction or irritability in 

 the sponge, or in its pores and canals, is very certain ; but 

 other two explanations of it have been oftered. Dr Grant 

 considers it to be very probable that the pores and canals are 

 lined with minute vibratile cilia, by whose well ordered and 

 regulated play the cm'rent is originated and maintained ; 

 but he admits that he had not been able to perceive the 

 cilia after the most careful search for them, and after he 

 had become familiar with the best modes of detecting their 

 presence with the microscope. Dutrochet, on the contrary, 

 considers the phenomena as coming under his law of endos- 

 mosis and exosmcmx ; * and to this supposition I am well 



• To tliose of my readers wlio are not familiar with Dutrocliet's 

 tbeory, the following illustrations of it may be necessary. " When the 

 ccpcum of a chicken was half-filled with milk, tied, and then immersed in 

 rain-water, he found that it became gradually fuller and fuller, and at 

 length very turgid, having, in 30 hours, increased in weight from 196 

 to 313 grains. When a denser tiuid was substituted fur the milk, such 



