Hydrozoa is a class of animals of the sub-kingdom Coelenterata, in which the walls of the body inclose a simple undivided cavity which acts both as a body cavity and a digestive cavity. The body is essentially composed of two layers, an outer layer or ectoderm and an inner or entoderm. Reproductive organs are developed as external processes of the body-wall, but reproduction also takes place by fission.
The Hydrozoa are all aquatic and almost all marine. The fresh-water hydra is a very good type of the class. The body is quite soft, and when fully contracted appears like a particle of matter resting on the surface of a plant or stone; but when expanded it shows a long slender body of a bright green or light-brown colour. One end of the body develops into a number of long slender tentacles, within which, near their bases, the mouth of the animal is found. This is the distal or free-growing end.
The other and more slowly growing end is known as the proximal, and ends in a kind of disc or foot by which the hydra attaches itself to objects. The body is hollow from one end to the other. It is mainly found in semi-stagnant waters, where, hanging from its foot-disc, with its long tentacles expanded, it seizes on the small crustaceans or other suitable prey which comes in contact with it. Its tentacles have a stinging power which soon paralyses its prey. Under favourable conditions one or more hydras are usually found attached to the parent form. Such are produced by a process of budding from the parent. Each of these ultimately separates from the parent stem and becomes an independent hydra.
The Hydrozoa were divided by Professor Nicholson into six sub-classes: the Hydroida, the Siphonophora, the Lucernarida, the Graptolitoidea, the Hydrocorallinse, and the Stromatoporoidea. Research Hydrozoa
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Abbreviations
Nature
Research Hydrozoa
