68 THE BIOLOGY OF THE ANIMAL 



sponge skeleton and notice that it is construct- 

 ed of fibres united together. Can you detect 

 with or without a magnifying - glass whether 

 these fibres differ in size, color, arrangement, 

 etc., in different parts of the skeleton ? If yoa 

 find any differences, how do you account for 

 them? Tear off a small scrap of the skeleton 

 and examine under the low power of the com- 

 pound microscope. What arrangement of the 

 fibres do you find ? Does this give you any 

 clew to the reason why a toilet sponge can 

 absorb so much water? Look again for dif- 

 ferences in the shape and size of fibres. How 

 are the fibres held together ? Do you find any 

 traces of joints ? Do you find any foreign 

 bodies, as grains of sand or pieces of shell 

 firmly attached to the fibres or incorporated 

 in them ? If so, how do you account for their 

 presence ? 

 Draw a magnified piece of sponge to show the shape 

 and arrangement of the fibres. 



Example 3.— Fresli--water Sponge {Spmgilla 8p.) 



Material. — This sponge is common in many lakes 

 and rivers throughout the country. It forms dark- 

 green patches on the surfaces of submerged rocks, 

 logs, pieces of bark, etc. It frequently grows in 

 large, branched clusters from a few inches to a foot 

 or more in length, the branches sometimes being as 

 large around as the thumb. Owing to the color and 

 shape of these branched forms they are often mis- 

 taken for water -weeds or masses of algse. The dif- 

 ference can be told at once even by an inexperienced 



