CHAPTER XIX. 



BRANCH PORIFERA. 



THE SPONGES. 



The Simplest Sponge. The simplest sponge is a vase- 

 shaped body attached by the base. It is hollow, and has 

 an opening (osculum) at the free end. Through the wall 

 are many small holes, and water is constantly entering 

 these holes (inhalant pores) and passing out of the 

 mouth of the vase. The cause of 

 this current is the vibration of many 

 flagella, which project inward from 

 the cells lining the cavity. This 

 current of water brings oxygen and 

 food, consisting of minute plants 

 and animals, and remains of larger 

 animals and plants. The vaselike 

 body is supported by a large number 

 of three-rayed spicules, embedded 

 in the wall of the vase. These are 

 composed of carbonate of lime, and 

 they constitute the skeleton, making 

 the body fairly firm, and yet leaving 

 it flexible and elastic. An outer layer of cells constitutes 

 the ectoderm, the lining cells are the endoderm, while 

 between these is a thin layer called the mesoderm, in 

 which the spicules are embedded. (See Fig. 176.) 



More Complex Sponge. Somewhat higher is a sponge 

 that is vase-shaped, or cylindric, in which the inhalant 



307 



FIG. 175. SIMPLE SPONGE, 

 MARINE. 



Water enters minute holes in the 

 sides and passes out of the 

 opening at the top of the tube. 



