HYDROIDA. 197 



cavity is excavated in the substance of the body without any proper 

 lining membrane, and the reproductive organs are always external. 

 In the second order, the Asteroida, the polyps are always compound; 

 the mouth is surrounded by eight tentacles; the digestive cavity is 

 lined with a membrane, and the ovules are produced in the interior of 

 the animal. The polyps in this order are imbedded in a more or less 

 fleshy mass, which is generally supported on a horny or calcareous axis. 

 The polyps of the third order, the Hdiantlwida, are single, and either 

 possessed of a certain power of locomotion, or imbedded in a calcareous 

 polypidom. The mouth is generally surrounded by a great number of 

 tubular tentacles : the stomach is furnished with a distinct lining, and 

 the ovaries are internal. 



HYDROIDA HYDRA, OR FRESH-WATER POLYPS. 



It is in the polyps of this order that we find the nearest approach 

 to the preceding division. The body in these generally consists of a 

 homogeneous aggregation of visicular granules, held together by a sort 

 of glairy intercellular substance, and capable of great extension and 

 contraction ; so that the creature can at pleasure assume a great variety 

 of forms, extending its body and tentacles until the latter become so 

 fine as to be almost invisible, and again retracting itself until it acquires 

 the appearance of a small gelatinous mass. The tentacula which sur- 

 round the anterior extremity are irregular in number ; they are capable 

 of extension to a very great length when seeking for prey; and on 

 coming in contact with any object floating through the water, they 

 immediately twine round it, and convey it to the mouth. In some 

 genera the tentacles appear to be tubular, the internal cavity being 

 continuous with that of the stomach. To assist in the capture of living 

 prey, their surface is commonly roughened with a series of granules, 

 which in some cases contain a curious poisonous or urticating apparatus. 

 The mouth, which is situated in the centre of the circle of tentacles, 

 leads directly into a simple digestive cavity, which is not lined with 

 membrane. 



In ponds and rivulets, adhering to the leaves of aquatic plants, or 

 twigs and sticks that have fallen into the water, are found the Hydraa. 

 When stretched out, they resemble pieces of hair, from a quarter to 

 three quarters of an inch in length. Some are of a light-green colour, 

 and others brown or yellow; that is, the five varieties found in 

 England. It received its name from its several long arms being sup- 

 posed to resemble the fifty-headed water-serpent called Hydra, and 



