A TUBULARIAN HYDROMEDUSAN 151 



the medusoid buds do not appear on the hydranths but on the 

 stem ; they become free-swimming medusae. 



Exercise 2. Make a semidiagrammatic sketch of a large hydranth 

 and a portion of the stem on a large scale ; label the 

 different parts. 



Exercise 3. Make a sketch on a large scale of the smallest 

 hydranth or hydranth bud you can find. 



Mount a hydranth and a part of the stem on a slide in dilute 

 glycerine, and study their finer structure. First study the 

 structure of a tentacle. It is not hollow as is the tentacle of 

 Hydra, but 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 stinging 

 organs or nematocysts at the end of the tentacle. These are the 

 organs with which the animal kills its prey. Each one consists 

 of a spiral thread-like tube, with several barbs at its base, which 

 lies 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 projects beyond the free surface 

 of the cnidoblast into the water. When the surface of the 

 ectoderm is irritated the tube is evaginated and shot violently 

 out, and the poisonous fluid contained in the cavity of the nema- 

 tocyst is injected into any animal that may be struck. Look 

 for nematocysts which have discharged their spiral threads. 



Exercise 4. 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 structure of the wall of the hydranth. It is made 

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



