PHYLUM COELENTERATA 83 



2. General behavior (Hegner, pp. 127-33). On your own specimen or on 

 those in the general culture jar perform the following simple experiments. Use 

 a dean needle, touch the animals gently, and wait for the animal to expand 

 completely before stimulating it again or use a different specimen. Touch one 

 tentacle of a fully expanded individual gently. Does it contract? Do other 

 parts contract? Touch two or three tentacles simultaneously. Is the response 

 more marked than before? Try the comparative effect of a touch of the same 

 intensity on the foot, middle of the column, and hypostome. What parts of the 

 animal are the most sensitive? least sensitive? Try difference in response to 

 a weak and strong stimulus applied to the same region. Stir the water in the 

 culture and observe what happens. Do any of these experiments indicate that 

 the stimulus is conducted from the point of stimulation to other parts, as in the 

 frog? As in the frog the mechanism of irritability and perception of stimuli 

 is a nervous system and sensory cells and the mechanism of response consists 

 of muscle fibers. Owing, however, to the very simple structure of these systems, 

 there is but one response to all kinds of stimuli, namely, contraction. 



Make an outline drawing of a fully contracted Hydra. 



3. Cellular structure of the living animal. Mount a Hydra on a slide and 

 support the cover glass with small bits of broken glass or slivers of wood so that 

 the animal will not be crushed. The support must be thick enough to allow the 

 animal to extend itself freely under the cover glass but not so thick as to interfere 

 with the use of the high power. Small individuals are best for the purpose. 



a) Cellular structure of the column: The animal must be fully extended for 

 the following observations. Hydra consists of two layers of cells, an outer 

 ectoderm and an inner entoderm with a structureless gelatinous sheet, the meso- 

 gloea, between them. It is thus like two closely fitting cylinders, one within 

 the other. Hydra is therefore a diploUastic animal, that is, it consists of two 

 germ layers (Hegner, p. no). Examine the base of the column near the foot 

 with the high power and try to see these two layers. First focus on the surface 

 of the column and observe that it consists of a mosaic of elongated cells, with 

 pointed ends. These are called the epithelio-muscular cells of the ectoderm 

 because each has an epithelial and a muscular portion. A few nematocysts, oval 

 distinct bodies, also occur lying in the ectoderm cells, and there may sometimes 

 be observed between some of the epithelio-muscular cells greenish groups of 

 very small cells, the interstitial cells. Now focus slowly downward, and a new 

 layer of larger, more rounded, clear cells, the nutritive muscular cells of the ento- 

 derm, comes into view. Focus on the edge of the animal; the ectoderm then 

 appears in profile as a rather thin layer, separated from the thicker entoderm 

 by a dark line of uniform width, the mesogloea. 



b) Cellular structure of the tentacle: Examine the base of a fully extended 

 tentacle, and repeating the above-mentioned procedure observe first the ectoderm 

 cells; then by focusing on the optical center note the large clear rectangular 



