PRESH-WATER SPONGE. 



159 



T]ie Avater goes through numerous small holes in 

 the epidermis, into a subepidermal chamber. It then 

 filters throughout the sponge, bathing every cell, and 

 finally passing out through the excurrent canals. It is 

 caused to move in the sponge by cilia, one of which is 

 found in every sponge cell that lines one of the canals. 

 By this movement of the water, food and oxygen are 

 brought to each cell. 



Tlie gemmule consists of a 

 mass of granular protoplasm 

 surrounded by a thick wall in 

 which are embedded many 

 spicules arranged radially. 

 These gemmule spicules are 

 often somewhat sjoool shaped, 

 or like an a.xle with a wheel 

 at each end. There is always 

 an opening from the inside 

 of the gemmule through the 

 wall. 



CROSS SECTION OF GEMMULE OF FRESH- 

 WATER SP(.1NGE 

 (HETEROMYENI-i AKGVKOSPEKM.i). 



The spicules are siliceous ; that is, they ai'e com- 

 posed of about the same material that glass is made 

 of. Consequently, acids do not affect them. Most 

 marine sponges liave calcareous spicules, but in all 

 fresh-water sponges the spicules are siliceous. 



Additional Facts About the Sponge. 



The fresh-water sponge has no definite shape. Its 

 shape is determined by its rapidity of growth in any 

 given direction. It has no mouth, no body cavity, 

 no stomach. It can scarcely be called an individual, 

 but is a mere agjri'egation of cells. Each cell breathes 

 for itself, takes its own food, and does just what every 

 other cell does. 



