A TUBULARIAN HYDROMEDUSAN 165 
the medusoid buds do not appear on the hydranths but on the 
stem; they become free-swimming meduse. 
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 entoderm 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 entoderm, each 
