I 66 ZOOLOGY 



205. Comparison Demonstrations. 



1. Fresh-water Sponge. — In portions of the country where the streams are clear, 

 swift, and with rocky bottoms, a fresh-water sponge may often be found which will 

 be valuable to compare with Grantia or to substitute for it. It grows attached to 

 submerged objects and is commonly of a dirty greenish color, though this may 

 vary. This sponge is firm and gritty to the touch, and may be either compact or 

 branched. Use the general outline prepared for Grantia, noting the points of 

 contrast. Is there anything like the osculum? the cloaca? Gemmules or re- 

 productive bodies may occur imbedded in the flesh, especially at the base. 



2. The Sponge of Commerce. — This is merely the skeleton of a sponge from which 

 all the cellular part has been removed. Select a small rounded specimen. Do you 

 find any signs of the attached end? of an osculum? Split the sponge with scissors, 

 beginning with an osculum. Are there any canals as in Grantia? If so, what is 

 their arrangement? Examine a small portion of the skeleton under the micro- 

 scope. Test as before (for calcium carbonate) with dilute acid. Is the skeleton 

 elastic? Why? 



Descriptive Text 



206. The Protozoa are unicellular animals, or at most, 

 masses of similar cells in a more or less globular form. This 

 condition is comparable to the morula stage of the embryos of 

 higher animals (see §54). In all the other groups (Metazoa) 

 the cells at some stage in development are in at least two 

 layers, an inner and an outer or superficial layer, a structural 

 condition which we have seen at its simplest in the gastrula 

 (see §ss). The exact position of the Porifera in the animal 

 series has long been a matter of debate, but the great majority 

 of zoologists agree that they stand below all the other Meta- 

 zoa, presenting transitional features between the Protozoa 

 and Metazoa. For this reason they are especially interesting. 

 Some authors include them with the next phylum — the Coelen- 

 terata. They possess two cell-layers, but the division of labor 

 among the cells is not so decided as in the Coelenterata, and 

 the individual cells are very much more independent of each 

 other in consequence. 



207. General Characters. 



I. Porifera possess a system of internal chambers through 

 which the water flows. The water enters by means of many 

 minute pores at the surface, passes along radiating tubes (incur- 

 rent and radial canals) to the central cavity (cloaca) and escapes 

 through one or more larger openings (oscula) at the unat- 

 tached end. There is no true coelom (see §58). 



