ANIMAL LIFE IN PONDS & STREAMS 



not run away with the idea that they in any way 

 resemble our familiar idea of a mouth. These large 

 holes are called oscula by scientists, but we wish to 

 avoid scientific words as much as possible. The 

 simplest sponge of all, consists of a little bag, which 

 remains affixed by its base to a seaweed. All over 

 its sides there are many pores and at its tip there 

 is a single osculum ; it is known as the Purse Sponge 

 and is common round our coasts. The inside of the 

 purse sponge is lined with cells, each one of which 

 is tipped with a little whip which waves about un- 

 ceasingly. The waving of the whip causes water 

 to flow through all the pores into the hollow bag 

 and out again by way of the osculum. Although 

 most sponges, including our fresh-water forms, are 

 much more complicated than the purse sponge, the 

 same thing happens in them all, water is drawn 

 in by way of the pores and forced out by way of 

 the oscula. 



The best place to seek for the fresh water sponges 

 is on the under sides of floating wood, broken tree 

 branches and the like. Their appearance depends 

 upon whether they have been growing in a well- 

 lighted spot or in darkness ; they contain chlorophyl 

 as do the higher plants, and sponges grown in the 

 light are green, those which the light has not reached 

 are buff-coloured or corn-yellow. The pond sponge 

 is brighter green than its river frequenting relative 

 and is a coarser creature altogether. It often forms 

 little finger-like outgrowths, whereas the river 

 sponge is more leaf like. 



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