434 TJie Structure and Life-History of a Sponge. [Sess. 



the iucurrent and excurrent streams of water in a sponge, 

 and his account of what he then observed has been often 

 quoted. The apertures on the surface of a bath sponge, for 

 example, may be readily seen not to be all of the same size, 

 the smaller being the pores opening into the inhalant canals, 

 and the larger the " oscula," or mouths, carrying off the 

 water after it has penetrated the sponge mass, and had 

 extracted from it what was necessary to build up the living 

 structure and repair waste. The oscula are often raised 

 above the general surface of the sponge, as if to prevent any 

 of the expelled water, with its waste products, finding its 

 way back through the pores. The mechanism by which 

 this constant circulation is kept up in the interior of a 

 sponge was for long a mystery, but has now, thanks to the 

 aid of the microscope, been clearly established. Professor Grant, 

 indeed, had, by a kind of " happy guessing," arrived at some 

 knowledge of the true solution, when he said that he considered 

 it " very probable that the pores and canals are lined with 

 minute vibratile cilia " ; but though he made diligent search 

 for these, with the help of the best objectives then obtain- 

 able, he never was able to detect the presence of such cilia, 

 though still of the belief that they were present. Dr John- 

 ston, however, entirely missed the explanation of this won- 

 derful phenomenon, ascribing it to Dutrochet's law of osmosis, 

 then promulgated for the first time. In accordance with 

 this law, Dr Johnston argued that, " from the unequal densities 

 of the mucilaginous secretion of the sponge and the circum- 

 fluent water, there must be unceasingly going on an oozing 

 out of the one and an entrance inwards of the other." One 

 cannot help feeling sorry, somehow, that these two enthusi- 

 astic early workers among the sponges never witnessed the 

 exceedingly interesting sight of the monad-like cells clustered 

 in the flagellated chambers of a sponge, by whose movements 

 this unceasing ebb and flow is carried on. When speaking 

 of the endoderm, it was remarked that it exhibited the same 

 structure of polygonal epithelial cells as the ectoderm, with 

 one exception. The exception consists of these cells of the 

 inner tissue, known as the " flagellated chambers." The 

 exhalant canals where they begin to widen out are lined with 

 these flagellated cells, seated on the fundamental tissue in 



