$2 A GUIDE FOR THE STUDY OF ANIMALS 



Mesoglea, a jelly-like layer between the endoderm and 

 ectoderm layers. In the sponges this contains many wan- 

 dering cells, probably from the other layers. 



Porifera (pore bearers), animals with many more or less 

 independent cells, supported by solid skeletal parts and 

 penetrated by a system of canals which open on the sur- 

 face as pores. 



Directions. 



Study a simple sponge to see the shape, size, and point 

 of attachment. Identify the osculum. In a diagram of a 

 long section of a simple sponge identify the central cavity, 

 body walls, canals, inhalent pores, ostia, and osculum. In 

 a simple sponge cut like the diagram identify the same 

 structures. Do the same for the toilet sponge. 



Study a diagram of a portion of the body wall, consider- 

 ably enlarged. Identify the endoderm and ectoderm cells, 

 the spicules or fibers, and, among the spicules or fibers, 

 irregular amoeboid cells, sometimes called mesoderm cells. 



Examine a fragment or section of each kind of sponge 

 under the microscope. Notice the arrangement, shape, 

 and length of the spicules and of the fibers. 



Test both kinds of sponges by dropping a bit of each 

 into weak acid, and noting the results. Also burn a bit of 

 each and notice the odor. 



Questions. 



1. What is the shape of a simple sponge .' What enables 

 a mass of cells to retain such a definite shape .■' 



2. What seems to be the composition of the skeletons .' 

 Why is one type' of skeleton rigid and the other elastic } 



3. Since sponges are attached for most of their lives to 

 stationary objects, suggest means for obtaining food and 

 oxygen, and for getting rid of waste matter. 



