A CAMPANULARIAN HYDROMEDUSAN ial 
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 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 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 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 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 entoderm, 
each composed of a single layer of cells; the inner ends of the 
entoderm cells are amceboid 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; 
