40 HYDROIDS AND JELLY-FISH 



Comb-bearing Jellies, the lovely Sea Anemones, Alcyonarians, and 

 wonderful stony Corals. The Coelenterata are divided into four 

 classes : (i) the Hydrozoa (the Sertularians or Sea-firs and the 

 Pond Hydra) ; (2) the Scyphozoa (Jelly-fish) ; (3) the Ctenophora 

 (Comb-bearing Jellies) ; and (4) the Anthoza (Sea Anemones and 

 Corals). 



If on a fine summer afternoon we should visit a pond, the 

 surface of which is partly covered by a green mantle of duck- 

 weed, we should probably have little difficulty in obtaining several 

 specimens of the interesting little Hydra or Fresh-water Polyp. 

 A glass jar should be partly filled with clear pond- water, and some 

 of the duck-weed carefully transferred to it. Then if we closely 

 examine the slender stems of the weed as they hang down in the 

 water, we shall see that attached to some of them are minute 

 green objects about J inch in length specimens of the common 

 Green Hydra (Hydra viridis). If now we examine one of these 

 little objects through a pocket magnifying glass, we shall see that 

 it has a very slender cylindrical body, and that the free end is 

 crowned by a circlet of waving feelers or tentacles, from six to ten 

 in number ; while the base of the body is slightly expanded, disk- 

 shaped, and acts as a sucker, grasping the stem of the duck-weed 

 for support. On continuing to watch the little Hydra, we shall 

 see that the body slowly elongates and sways from side to side, 

 while the tentacles expand and contract and wave about in all 

 directions. 



Along with the Hydra, we have probably captured a number 

 of very minute crustaceans, the so-called "water-fleas," which are 

 now swimming about with their peculiar jerky motion. One of 

 these, in its progress through the water, touches against a waving 

 tentacle of the Hydra and is held fast. The other tentacles 

 bend towards it, and the little creature soon disappears down 

 the mouth of the Hydra. Now, each tentacle of the Hydra is 

 provided with most remarkable stinging cells, called " thread- 

 cells," or nematocysts, for the capture of prey. Each of these 

 peculiar cells, which are embedded in the outer skin, or ectoderm, 

 contains a cyst with a very fine hollow thread coiled up inside ; 

 while a pointed process projecting from the outer surface of the 

 cell acts as a trigger, so sensitive that at the least touch it causes 

 the coiled stinging thread to be discharged and to sting and 



