hollow or solid ? The elements of the two layers c^a be most easily 

 seen in the tentacles. Observe the following : 



(a) The ectoderm cells are large and conical with their apices 

 directed inward. The boundaries of the outer ends form a mosaic, 

 their inner ends rest directly on the supporting lamella. Do these 

 cells vary in shape \vhen the tentacle is extended or contracted ? 



(b) The interstitial cells are small rounded cells placed between 

 the inner ends of the large ectodermal cells. 



(c) The cnidoblasts or "thread cells" are modified interstitial 

 cells prolonged at the outer end into a cnidocil or "trigger" and con- 

 taining an oval, highly refractive capsule, the nematocyst. The cap- 

 sule is filled with fluid and contains a spirally wound filament formed 

 by the doubling in of the w r all of the capsule at one pole. The nema- 

 tocysts form knob-like swellings on the tentacles. They are of tw r o 

 kinds : ( i ) smaller, more numerous ones situated at the bases of 

 the longer cnidocils and containing short stout threads; (2) larger 

 ones lying near the middle of each knob-like swelling, globular in 

 shape when seen from the face, flask-shaped when seen from the 

 side ; they contain long slender filaments armed with barbs at the 

 basal end. Run in a little iodine and observe the ejection of the 

 threads of the nematocysts. Note that the threads are turned in- 

 side out in the process of discharge, the basal portion being dis- 

 charged first. AYhat is the use of the barbs? The hollow thread? 

 The fluid in the cysts ? Do nematocysts occur anywhere else than 

 on the tentacles ? 



(d) The endoderm cells line the cavity of the tentacles. They 

 are large and some of them bear flagella by which currents are 

 caused. Focus on the middle of the thickness of a tentacle and 

 observe the flagella on the endoderm cells and the nutrient particles 

 streaming up and down the cavity of the tentacle. What difference 

 can you detect in the relative numbers of these elements (cells) in 

 the various parts of the body? 



Make drawings to show the characteristic layers, and cellular 

 elements. 



III. REPRODUCTION. 



I. Asexual reproduction occurs by the formation of hollow out- 

 growths from the sides of the body wall. Each of these acquires a 

 mouth and tentacles at the distal end of its body and finally, con- 

 stricting at the base, separates from the parent animal. Look for 

 such buds in various stages of development. 



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