164 ELEMENTARY ZOOLOGY. 



cles of food. Apparently the food is not digested in 

 this general cavity of the animal, but is taken up by 

 the individual cells and digested in the cells, much as in 

 the case of the Paramecium and other Protozoa. 



182. A Simple Sponge Grantia: a Practical Exercise. This 

 small cylindrical sponge is found on our own coasts, at slight depths. 

 It occurs in clusters, attached to various kinds of objects. While 

 it is not the simplest type of sponge, it is good to give a general idea of 

 the structure of sponges. Specimens may be preserved in alcohol. 

 Some should be allowed to dry for comparison. 



After studying the general shape and size of the body, and any 

 differences that can be seen between the two ends, and the arrange- 

 ment of the pores and spicules as they appear from the outside, split 

 the body longitudinally with a sharp scalpel. Examine these hemi- 

 sections carefully, within and without, with a hand lens and identify 

 and describe the following: 



The general body wall: its thickness and character. Is it solid 

 or perforated? Notice the shape or the cavity (cloaca). Does the 

 cavity have any communications with the outside world? Do you 

 find any signs of the pores on the inside? 



With a razor, make some cross sections just as thin as you can, 

 and study these with the low power of the microscope. Study the 

 pores and the canals and the spicules, and see if you can make a 

 diagram to show your idea of the conditions. How does the food 

 get into the animal? 



183. The Sponge of Commerce. This bath sponge is merely the 

 horny skeleton of a large ocean sponge which is much more complex 

 than Grantia. This skeleton is developed in a similar way to that 

 of Grantia and corresponds to it, though it is a much more perfect 

 skeleton. The soft, living parts have been destroyed in the process 

 of preparing the sponge for market. Just as in Grantia, the skeleton 

 was developed in the middle layer of cells (mesoderm) and there 

 was on the outside a layer known as ectoderm. Lining the interior 

 of the canals and cloaca was the entoderm. 



184. The Essential Structure of Sponges. It will be seen, from the 

 two examples that have been described, that while the sponge is 

 much more complex than the Protozoa, it is very much simpler than 

 the ordinary animals. There is no heart, no brain, no lungs, no 

 organs for locomotion or for special sense. The body consists merely 

 of a body wall surrounding the cloacal cavity (Fig. 42), lined with 



