PORIFERA. 157 



5. Place a bit of the sponge on slide and allow weak acetic or hydro- 

 chloric acid to pass under the cover. Note and interpret results. 



200. 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 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 out- 

 line prepared for Grantia, noting the points of contrast. Is there any- 

 thing like the osculum? the cloaca? Gemmules or reproductive 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 microscope. Test as before (for calcic 

 carbonate) with dilute acid. Is the skeleton elastic? Why? 



DESCRIPTIVE TEXT. 



201. 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 52). 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 53). 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 Crelenterata, and 

 the individual cells are very much more independent of each 

 other in consequence. 



202. General Characters. 



i. Porifera possess a system of internal chambers through 

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



