166 ZOOPHYTES OF GUERNSEY. 



into a mouth surroimded by a crown of slender contractile 

 tentacles. One of the commonest representatives of the 

 Hydroida is the Fresh-water Polyp, or Hydra, which may be 

 met with in stagnant pools, attached to leaves and stems of 

 aquatic plants, or floating pieces of wood. It consists of a 

 tiny lump of soft jelly-like substance, brown or green in 

 coloTU', capable of changing its shape in the most protean 

 manner. When extended it bears a circle of long slender 

 tentacles, and when contracted it is rolled up into a little ball. 



Like many of its allies, the Hydra possesses wonderful 

 recuperative powers. It may be turned inside out, like a 

 glove, without any apparent discomfort or derangement of its 

 functions ; and if cut into any number of small pieces, each 

 fragment will grow into a new and perfect animal. Its mode 

 of increase is simple in the extreme, and resembles the bud- 

 ding of plants. A bud is pushed out on one side of the body, 

 like a minute tubercle, and this grows until it develops a 

 circle of delicate tentacles from its free extremity ; and thus 

 it remains for a time attached to its parent, until it finally 

 drops off and assumes an independent existence. Occasionally 

 individuals are found bearing several young ones budding 

 from the sides like a miniature tree. This moJe of reproduc- 

 tion, called (jemmation^ takes place only during the summer 

 months ; in the autumn egg-shaped granules are produced 

 which lie dormant in the water, like seeds. Embedded in the 

 skin of the Hydra there are a number of urticating organs or 

 stinging-cells, by means of which the creature paralyses its 

 prey. These cells, called nematocysts^ or cnidcB^ are peculiar 

 to the subkingdom Coelenterata, to which the Hydroida 

 belong, and are highly develojjed in the .[elly-fishes, or 

 Medusa3. 



The Freshwater Polyp (^Hydra) not having a polypary 

 or horny skeleton, entirely perishes and disappears when it 

 dies ; but in the Sertularians every zooid, or individual polyp, 

 leaves its horny integument attached to the general community, 

 and thus in time there grows up an elaborately branched tree- 

 like stem, the complexity of which increases with the age of 

 the colony. These compound Hydroids, of which abundant 

 examples may be found on all our shores among cast up sea- 

 weed, may be likened to a Hydra whose buds, instead of 

 dropping off, remain permanently attached to the parent's 

 body, while they in turn develop new buds, and so on inde- 

 finitely ; so that gradually a more or less shrub-like structure 

 is formed ; and this is termed a pulypnry. In all instances 

 the entire cluster is produced by continuous growth from a 



