66 THE STORY OF ANIMAL LIFE 



filter does with its food when it gets it. The 

 ciliated cells of the internal lining take in solid 

 particles just as Amoeba does; and from these 

 they may be passed on to the cells of the middle 

 layer, amoeboid cells, which can move about. 

 These cells are considered to be derived from the 

 primary layers of the body, especially the inner 

 one, and to have wandered into a cellless middle 

 layer, comparable in nature with that of some 

 Ccelenterates. 



The sponge is full of firm or gritty particles, 

 which form its skeleton, and remain when the 

 sponge is dead, and the softer parts decayed. 

 These, when magnified, often present beautiful 

 and curious shapes. The use of them is not only 

 to support the body, but also to prevent the 

 sponge from being eaten by other animals. 



There is found in the English canals and 

 rivers a small, fresh-water sponge, usually green- 

 ish in colour. This is named Spongilla fluviatilis, 

 the River-sponge, and affords an exception to the 

 usual marine distribution of sponges. In the 

 winter it dies gradually away, at the same time 

 forming asexual buds, or "gemmules," in the 

 interior of its substance, which are liberated in 

 the spring, and become young sponges. 



Some of the marine sponges are parasitic. 

 Most people have doubtless found on the sea- 

 shore now and then a dead oyster-shell, com- 

 pletely riddled with small round holes, very 

 similar in appearance to those seen in " worm- 

 eaten " wood. These are the work of Clione, a 

 parasitic sponge which is very fatal to the oyster. 

 At first sight it seems a puzzle how the sponge 

 made its way into the hard shell ; it has no 

 mouth to bite or suck its way into the soliq 1 



