40 
GUIDE TO THE CORAL GALLERY. 
on being touched, causes the barbed thread to be everted, thereby 
stinging and poisoning the prey. Thread-cells are characteristic of 
the Coelentera. Hydra reproduces itself sexually by means of eggs, 
which form in little wart-like swellings on the surface ; or asexually, 
by forming buds which grow out from the wall, develop mouth and 
tentacles, and normally become detached. Hydra , which is named 
after the monster of the fable, can be cut into pieces, and, condition- 
ally on containing a portion of the two cell layers, each fragment 
will develop into a complete animal. 
Classification. 
Having given a brief outline of the structure of one of the 
simplest forms, an account will now be given of the groups of 
Hydrozoa, which, for convenience of description, will be referred to 
under the three headings : — 
I. Hydroida (Hydroid Zoophytes). 
II. Hydrocorallinm (Coral-like Hydrozoa). 
III. Medusae and other allied free-swimming forms (Jelly-Fish, 
Siphonophora, and Ctenophora or Comb- Jellies). 
I. HYDROIDA (HYDROID ZOOPHYTES). 
The horny plant-like growths in Case 3 A, b, have fundamentally 
the same structure as the Hydra. If the little sac were to form a 
horny protective cover on its surface, to become longer, to give off 
buds, which likewise budded, all the buds remaining in connection 
with each other, and each surmounted by its crown of tentacles — a 
plant-like Hydroid colony would be the result. Bougainvillea 
fruticosa * (Figs. 2, 3. and specimen in Case 3) is a branching Hydroid 
colony, every branch terminating in a polyp, as each individual of a 
colony is termed. All the polyps are vitally connected with each other 
by the common living tissues inside the stems. The polyps are of two 
kinds, one kind being in the form of an elongated sac or tube with a 
crown of tentacles, that is to say, like Hydra ; while the other, when 
mature, resembles a small Medusa or Jelly-Fish. The Medusa-like 
polyp (Fig. 4) ultimately becomes detached and swims away. The 
little free-swimming polyp, which we must now call a Medusa, is bell- 
shaped ; the true mouth, which leads into the stomach, is at the end 
of the clapper (manubrium) hanging down from the centre of the 
