HYDRA 161 
is a hollow projection of the body-wall and has the same 
structure. 
Among the ectoderm cells of the distal portion of the body, 
and especially of the tentacles, notice the oval, highly refractive 
stinging organs or nematocysts. 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 a 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 
nematocyst is injected into any animal that may be struck. 
Exercise 2. Make a large semidiagrammatic drawing of the 
animal showing the details of its structure; label all 
carefully. 
Methods of reproduction. Hydra reproduces both sexually and 
asexually. Well-fed polyps will soon begin to bud off new 
individuals. The bud makes its appearance first as a projec- 
tion of the body-wall, and soon becomes a distinct branch. 
Tentacles and a mouth make their appearance at the extremity 
of the branch, and the young polyp is complete. It remains 
attached to the parent for a while, then detaches itself and 
begins an independent life. 
Besides this asexual method of reproduction, which may go 
on as long as the animal is well fed and vigorous, reproduction 
by sexual methods also occurs at more or less irregular inter- 
vals. Sexual organs appear in the form of projections of the 
ectoderm of the body-wall. Two classes of these projections 
appear, smaller ones, which are testes, and larger ones, which 
are ovaries. The former of these organs, which lie near the 
distal end of the animal, produce spermatozoa; the latter are 
