I 68 ELEMENTARY ZOOLOGY. 



gradually dissolve passages in the material and build their 

 living cells along these passages to large erect individuals 

 a foot or more in height and supported by well-developed 

 skeletons. 



The sponges have interesting life relations to many 

 other forms. While they have no power of attacking 

 animals other than microscopic ones and few or no ani- 

 mals use the sponges directly as food, their thicket-like 

 masses furnish a hiding place for many species of minute 

 worms, Crustacea, and the young of many species. They 

 are protected from attack on the part of the higher ani- 

 mals by the toughness of the skeleton and probably by 

 unpleasant odor and taste. 



187. Definition.of the Porifera. The Porifera are to be distinguished 

 by the following facts: 



1. They are multicellular in the adult, and are made up of three 

 different layers of cells ectoderm, entoderm, mesoderm. 



2. They have numerous openings by means of which water, 

 bearing their microscopic particles of food, is swept in by the action 

 of ciliated cells. 



3. They are wholly aquatic. 



4. Sponges develop a skeleton in the middle layer which supports 

 the body and furnishes a framework that determines the shape the 

 animal will have. 



1 88. Classification of Sponges. The classification is based on the 

 character of the skeleton, and on the nature of the entodermal 

 canals in the body wall. 



Sponges with calcareous or limy skeleton Calcarea. 



Example: Grantia. 

 Sponges not having calcareous skeleton Non-calcarea. 



Siliceous skeleton. Examples: Fresh-water sponge, "Venus 

 Flower Basket." 



Horny skeleton. Examples: The bath sponges. 



189. Topics for Themes in Zoology and English. 



i. Economic value of sponges. Sponge fisheries and the 

 manner in which they are conducted. Sponge " farming " 



