160 INTRODUCTION TO ZOOLOGY. 



The sponge is usually colored more or less green. 

 The color is due to the presence of the same coloring 

 matter that is found in leaves. It is chlorophyl, and 

 uses up carbon dioxide, setting free the oxygen. It 

 is probable, however, that the chlorophyl is not a 

 part of the sponge itself, but is due to some vegetable 

 organism that finds a lodgment among the sponge 

 cells. 



All fresh-water sponges die in the winter and 

 nearly all disintegrate. Myenia fluviatilis, however, 

 is firm enough to withstand disintegration, and in the 

 spring the old sponge furnishes a substratum for a 

 new growth from gemmules. 



PROTOZOA. 



One of the characteristics of the Protozoa is that 

 they are too small to be studied without a microscope. 

 However, some of them are large enough to be recog- 

 nized by the unaided eye or by the use of the simple 

 lens. 



The bell animalcule is exceedingly common 

 in ponds and ditches. The body is bell shaped 

 and is attached by a slender stalk to some sup- 

 port. The rim of the bell is surrounded by a 

 fringe of cilia which by their vibration produce 

 currents of water that bring particles of food to the 

 mouth. 



The cilia are borne on a ridge which constitutes 

 the rim of the bell. This ridge is called a peristome. 

 The mouth is within the peristome, but at one 

 side. 



When a number of food particles have beeii accu- 

 mulated by the vibration of the cilia, the bell and the 

 stalk are suddenly and strongly contracted. The 

 stalk is very much shortened by being thrown into 

 five or six spiral coils. The cilia fold inward, the 



