THE HYDROID CORALS. 



The simplest corals belong to the class 

 Hydrozoa. This class is divisible into 

 two well marked divisions or orders ; first, 

 the Hlydroidea, comprising the fresh- 

 water hydra and the marine hydroids, 

 and, second, the Sc)fphozoa or jelly 

 fishes. A few species of the first order 

 secrete a limy support, but the greater 

 majority are soft and jelly-like. 



The Hydroids have a more or less sac- 

 like body which is usually attached to 

 some object by a pedicel or stem of 

 greater or lesser length. They are colo- 

 nial animals, like true corals, and hun- 

 dreds or even thousands live together in 

 clusters, forming huge colonies. As in 

 the sea anemones, the mouth is placed at 

 the upper end and is surrounded by a row 

 of hollow tentacles, which are very con- 

 tractile and are seldom in the same posi- 

 tion for more than a few seconds. A few 

 of the Hydroids are capable of locomo- 

 tion and move about very slowly by a 

 gliding motion, and the fresh water hy- 

 dra has even been seen to move by stand- 

 ing on its head. The larger number, 

 however, are stationary and remain in 

 one place after leaving the young stage. 



There are three genera of these corals 

 which secrete a limy support or coral- 

 Hum, one of which, Millepora, has con- 

 tributed largely to the formation of 

 coral reefs. The pink coral, Stylaster 

 sanguineus, is a familiar example of the 

 stony Hydroids, and lives in the East 

 Indian seas. It is one of the most beau- 

 tiful of all the corals, its soft pink color 

 and delicate, branching form rivaling 

 even the precious coral, with which it 

 is, indeed, confounded by many people. 

 Another Hydroid,Distichopora nitida,is 

 an inhabitant of eastern seas, and varies 

 in color from violet or red to almost 

 black. It is more solid than the pink 

 coral, hence much easier to preserve. 

 The Millepora alcicornis is an American 

 species living in the waters of Florida 

 and the West India islands, and the 



corallium frequently grows to large di- 

 mensions. It is of a light yellow or 

 cream color, and the surface is thickly 

 covered with small holes, or pores, 

 whence the name Millepora, each one of 

 which was once the home of a little 

 polyp. Each little depression or cup is 

 shallow, but if a section be cut through 

 one of the depressions it is seen to be a 

 rather deep hole which has been succes- 

 sively partitioned ofif by the polyps, as 

 one generation succeeded another. In 

 other words, these cells are built in 

 stories or series. 



A very interesting and remarkable 

 fact concerning these stony hydroids is 

 that the animals or zooids are not all 

 alike nor do they perform like duties. 

 We have here an example of the division 

 of labor. For example, there is one kind 

 of zooid whose duty it is to digest the 

 food, another kind possesses tentacles 

 and secures the food, while a third kind 

 is reproductive and perpetuates the spe- 

 cies. This division of labor is very curi- 

 ous, and is a suggestion of that greater 

 division which we find in the higher ani- 

 mals, where each function is performed 

 by a distinct and separate organ. 



The larger number of Hydroids are 

 without a limestone support, their soft 

 bodies clinging to some foreign body, as 

 a shell, stone, or piling, either singly or 

 in clusters. A small Hydroid which in- 

 habits the coast of Massachusetts fre- 

 quently encrusts the shells of moUusks, 

 which in turn may be inhabited by a 

 hermit crab. 



The most familiar Hydroid to those 

 people who live away from: the sea shore 

 is the fresh water hydra. This animal 

 is very small, so small indeed that it can 

 barely be seen with the unaided eye. 

 When it is magnified it is found to have 

 a cylindrical body, attached by one end 

 to a submerged branch, leaf or stone. 

 The free end contains the mouth, which 

 is surrounded by a row of six or eight 



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