40 GUIDE TO THE COEAL GALLERY. 



on being touched, causes tlie 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 structm'e 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. Hydrocorallinae (Coral-like Hydrozoa). 



III. Medusa? 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-Hke Hydroid colony would be the result. BougainviUea 

 fruficosa* (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 ; Avhile 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 



