A CAMPANULARLAN HYDROMEDUSAN 157 



Find a hydranth bud and study its structure. 

 Exercise 4. Make a semidiagrammatic sketch of it. 



Study the finer structure of an expanded hydranth. First 

 study the structure of a tentacle. It is made up of an axis 

 consisting of a single row of large endoderm cells, around which 

 is a layer of small ectoderm cells. Between these two cell layers 

 is the delicate non-cellular supporting layer. Find the highly 

 refractive nematocysts at the end of the tentacle. These are the 

 stinging organs with which the animal kills its prey. Each 

 one consists of a spiral, thread-like tube, with several barbs 

 at its base, which lie's coiled within the cavity of a cell called 

 the cnidoblast. The cavity is filled with a poisonous fluid; its 

 walls form an ovoid sac, of which the tube is the very much 

 elongated and invaginated outer end. A minute spine pro- 

 jects beyond the free surface of the cnidoblast into the water. 

 When the surface of the ectoderm is irritated the tube is 

 evaginated and violently shot out, and the poisonous fluid con- 

 tained in the cavity is injected into any animal that may be 

 struck. Look for nematocysts which have discharged their 

 spiral threads. 



Exercise 5. Draw the distal portion of a tentacle showing its 

 cellular structure; show the nematocysts at the end, 

 including several which have been discharged. 



Study the finer structure of the wall of the hydranth. It is 

 made up of an outer ectoderm and a much thicker inner endoderm, 

 each composed of a single layer of cells ; the inner ends of the 

 endoderm cells are amoeboid and often flagellate, the function 

 of the flagella being to maintain in circulation the fluids in the 

 gastro-vascular space ; between these two layers is the thin 

 non-cellular supporting layer. The hydrotheca encloses all, but it 

 is not in contact with the ectoderm. Study the structure of 

 the stem ; it has essentially the same structure as the hydranth ; 



