PRINCIPLES OF ANIMAL BIOLOGY 25 



condition. The fertilized ova later give rise to new Volvox. How many 

 ova in the specimens studied? 



4c. Spermatozoa occur less frequently than ova. When present, they 

 are in bundles like sticks of wood in a rick. Several of these bundles may 

 sometimes be found together. If you do not find them, ask to have them 

 pointed out. 



Draw a specimen having parthenogonidia or daughter Volvox, repre- 

 senting the whole organism in outline, and the parthenogonidia or daugh- 

 ter Volvox more in detail. The outline should be at least 3 inches in 

 diameter. Show the somatic cells in a portion of the figure. Draw a 

 similar figure of a specimen containing ova and spermatozoa, representing 

 some of the germ cells in detail, and showing somatic cells in part of the 

 figure. May any advance in complexity of Volvox over Pleodorina be 

 observed? If so, in what respects? 



5. Hydra is a fresh-water animal found in lakes, ponds, and streams, 

 attached to the surface of dead leaves, aquatic plants, and other objects. 

 Two species are commonly found, the brown hydra (Hydra oligactis) 

 and the green hydra (Hydra viridissima) . Study a living specimen in a 

 salt cellar containing a small amount of water. Examine with the unaided 

 eye, with the dissecting microscope, and with the low power of the com- 

 pound microscope. 



5a. Somatic Cells. Focus on the margin of the body, and note a clear 

 outer layer of cells, the ectoderm. The darker part within is another layer 

 of cells, the endoderm. 



Mount a specimen on a slide, supporting the cover-glass so as not to 

 crush it. Focus on the margin. The serrations found there indicate 

 roughly the extent of the principal cells of the ectoderm. Among these, 

 find numerous round bodies smaller than the ectoderm cells, the nema- 

 tocysts, or stinging organs. The' nematocysts are lodged in cells 

 called cnidoblasts which may not be visible in the living animal. In 

 what part of the animal are the nematocysts most abundant? Do you 

 find groups of them anywhere? The structure of the nematocysts 

 should be studied from specimens prepared for this purpose. 



Examine mounted cross-sections of Hydra. Note the two layers 

 of cells, the ectoderm and the endoderm, surrounding the digestive or 

 g astro-vascular cavity. The bulk of the ectoderm is made up of the cells 

 previously observed as serrations at the surface, approximately rectangu- 

 lar in section and not very deeply stained. These are called epithelial 

 cells. Among the epithelial cells are pear-shaped or oval bodies, the 

 nematocysts. Look for the cnidoblasts in which the nematocysts are 

 contained. Numerous small deeply stained cells among the bases of the 

 epithelial cells are called sub-epithelial cells. From the sub-epithelial 

 cells are derived the cnidoblasts, and some other cells. 



Study also the endoderm. Are there distinct types of cells in this 



