224 



HOW DO WE CLASSIFY ANIMALS? 



A microscopic examination shows the pores of the sponge to be 

 lined on the inside with cells having collars of living matter sur- 

 rounding a single long cilium called 

 — mo«tk. flagellum (fla-jeTum). The flagella, 

 .ectoccerm lashing in one direction, set up a cur- 

 rent of water, bearing food particles, 

 ---fiatfelicc toward the large inner cavity where 

 they are digested. The digested food 

 w<t ** then passes by osmosis to the other 

 cells of the body. From some of the 

 H°St rrent cells in the jelly-like middle layer of 

 the body lime is secreted to form the 

 spicules, and the reproductive cells, 

 Longitudinal section of a simple eggs, and sperms occur. The spicules 



sponge. How does this animal get -. , , , , , » . , 



its food? form the skeleton ol the sponge. 



Practical Exercise 3. Make a diagram of a simple sponge showing how food 

 is taken in and waste given out from the body. How does a sponge breathe ? 



Coelenterates. The Coe- 

 lenterates are a large group 

 of animals, practically all of -™ 



which are found in salt 

 water. They include the 

 beautiful sea anemones, 

 jellyfish, and corals. outer layers 



The Hydra. This little itmer ^"jgi. 

 creature is shaped like a cc^ty^. 

 hollow cylinder with a circle 

 of arms or tentacles at the vva. 

 free end. It is found at- 

 tached to dead leaves, sticks, 

 stones, or water weeds in 

 fresh-water ponds. When 

 disturbed, it contracts into 

 a tiny whitish ball, a little f 00 ^ — 



larger than the head Of a Longitudinal section of Hydra. How does food 



. m, , /. . i get into the body ? How many layers of cells are 



pin. The outer layer of the found here? 



