8 A MANUAL OF ZOOLOGY 



with the low power of the microscope. Note the multi- 

 tude of small needle shaped bodies, the spicules. How 

 many distinct forms appear? In what plane do all the 

 prongs of the spicules extend? By careful manipulation 

 a fourth prong on some of the spicules will be seen to 

 project into the cloacal cavity. 



Remove the specimen from the stain, wash in acid 

 alcohol, 95 per cent, and absolute alcohol, clear, and mount 

 in balsam, with the cloacal surface uppermost. Do not 

 press the cover glass down. Use the low, and the high 

 power of the microscope. A layer of cells whose nuclei 

 are very prominent is above (inside) the spicules, the 

 entoderm. Note the extent of this layer of cells. During 

 life these cells are supplied with flagella and collars. The 

 activity of these flagella keeps a current of water moving" 

 from the outside through minute pores in the wall into the 

 cloacal chamber and out through the osculum. The pores 

 may be seen as small clear spaces between the spicules. 

 The mesodermal cells are located outside of the entodermal 

 layer. This layer is no't continuous, the cells are scattered 

 somewhat. The ectoderm is not easily seen but is located 

 outside the mesoderm. 



Make a drawing representing all the points mentioned 

 above. 



Many of the larger specimens prepared as above will 

 be found to bear reproductive cells. They will be seen 

 as large, deeply stained cells in the mesodermic layer. 

 Two kinds will be noticed, the male and the female. The 

 former, spermatozoa, occur in spheres, each sphere con- 

 taining hundreds of sperm cells. The nuclei of the sperm 

 cells are m'uch smaller than those of the mesoc^ermal cells 

 which give the spheres a dotted appearance. The spheres 

 themselves are quite large. The ova, or egg-cells, are pro- 

 vided with but a single nucleus. Sometimes a nucleolus 

 will be found imbedded in the nucleus. The egg cells are 

 amoeboid, seldom regular in shape. Make a drawing rep- 

 resenting the two forms of reproductive cells. 



Prepared slides of cross, and longitudinal sections will 

 be provided. Make a drawing to represent the crosa 

 section. 



