THE METAZOA — DIVISION OF LABOR 



201 



up of apparently several individuals, 

 fourth natural size. 



One 



The Structure of a Sponge. — The simplest kind of a sponge 

 has the form of an urn, attached at the lower end. A common 

 sponge living in Long Island Sound is a tiny urn-shaped animal less 

 than an inch in length. It 

 has a skeleton made up of 

 very tiny spicules of lime, 

 of different shapes. Cut 

 lengthwise, such an animal 

 is seen to be hollow, its 

 body wall being pierced 

 with many tiny pores or 

 holes. The bath sponge, 

 the skeleton of which is 

 made up of fibers of horn, 



or a variety known as the a horny fiber sponge : IP, the incurreut pores ; 

 finger sponge shows the '^' osculum. Notice that this sponge is made 



pores even better than the 

 smaller limy sponge. In a 

 bath sponge, however, we probably have a colony of sponges living 

 together. Each sponge has a large number of pores opening into 



a central cavity, which in turn 

 opens by a larger hole, called the 

 osculum, to the surrounding '\\'ater. 

 A microscopic examination 

 shows the pores of the sponge 

 to be hned on the inside ^^\i\\ 

 cells having a collar of living 

 matter surrounding a single long 

 cilium or flageUuni. The flagella, 

 lashing in one direction, set up 

 a current of water toward the 

 large inner cavity. This current 

 bears food particles, tiny plants 

 and animals, which are seized 

 and digested by the collared cells, these cells probably passing 

 on the food to the other cells of the body. The jellylikc middle 

 layer of the body is composed of cells which sccicle lime to 

 form the spicules and the reproductive cells, eggs, and sperms. 



Diagram of a simple sponge : /, inhalant 

 openings ; 0, exhalant opening or 

 osculum. 



