GENERAL BIOLOGY 



are hollow processes of the body wall. How many are there? Com- 

 pare the number of tentacles in brown and in green hydras. Is 

 there more than one circle of tentacles? Observe the knob-like 

 swellings on the tentacles. Measure the length of the tentacles 

 when expanded ; when fully contracted. For what purpose are the 

 tentacles used? Buds: Young hydras of various sizes and stages 

 of development may be attached to the sides of the parent. Are 

 colonies formed by budding? Why? 



Draw an entire animal, with all the parts named above. 



II. STRUCTURE. 



1. The body wall of the animal is composed of two layers of 

 cells, one within the other, (a) The Ectoderm is the outer 

 layer. What is its color? How much of the thickness of the body 

 wall does this layer form? (b) The Endoderm is the inner lining 

 of the body cavity (digestive cavity). In the green species (Hydra 

 viridis) it contains chlorophyll bodies; in the brown species, H. 

 fusca, it contains "sooty corpuscles." Which layer is the thicker? 

 (c) The supporting layer or Mesoglea is a thin gelatinous layer 

 between the ectoderm and the endoderm. 



2. The tentacles. Examine a tentacle with the high power. Of 

 how many layers is it composed? Focus up and down so as to 

 obtain views (optical sections) at various levels. Is the tentacle 

 hollow or solid? The elements of the two layers can 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 when the tentacle is extended or contracted? 



(b) The interstitial cells are small rounded cells placed be- 

 tween 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 

 containing an oval, highly refractive capsule, the nematocyst. The 

 capsule is filled with fluid and contains a spirally wound filament 

 formed by the doubling in of the wall of the capsule at one pole. 

 The nematocysts form knob-like swellings on the tentacles. They 

 are of two 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 



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