NATURE STUDY IN SCHOOLS. 



117 



Some sponges are made up of one of the hardest minerals we have. The 

 sspiculigenous and horny sponges constitute one group. Others are made up of 

 spicules of carbonate of lime. One gathers silica from the sea, and does not 

 gather carbonate of lime. The other does not gather silica, but gathers car- 

 bonate of lime. 



There lived, many years ago, a man by the name of Pliny, who used to 

 write books. He was not very wise, and after reading other people's books, 

 would write what he remembered of them. He was a Roman, and lived in 

 Pompeii. When this city was destrcyed, Pliny's father was killed, but he 

 managed to escape. Pliny said that sponges were animais, because when 

 they were touched they would shrink away. 



Not long after this, people began to think that they were vegetables, 

 but it has been decided that they were animals, because they have muscle 

 and nerve cells. Some plants shrink. 



Fig. 62. 



Fig. 63. 



Fig. 04. 



Green Cup Sponge. A, part 

 of B, life size. 



Giant Cup Sponge. 



Net Sponge. 



On an island off Key West there is a plant called the sensitive plant. 

 It grows very straight, and the leaves of the plant stand up very prettily. 

 If we were walking by these plants and should happen to touch the leaves, 

 they would begin to droop. After having passed them all, by looking back 

 we could see the path which we had taken by these drooping leaves hang- 

 ing down. 



The sense by which sponges perceive is called the sixth sense, which 

 we first studied about in the gorgonias. Some sponges depend upon rocks, 

 and spread all over them. These sponges have no fibres, but are classed 

 with the horny sponges. 



