3464 



CCELENTERA TES 



ner of reproduction of the Hydrocorallia which are tropical animals and assist 

 in building up the coral islands is not yet known. They grow upon rocks or 

 dead corals, often covering the skeletons of sea fans (Gorgoniidce) , and are even 

 found in the Bermudas on old bottles thrown into the sea. In the latter case the 

 lower side of the stock is quite smooth as if polished, and reproduces exactly the 

 surface of the glass with all its markings. 



There are two families of these Hydrocorallia, as they are called, namely, the 

 Milleporidce and the Stylasteridce. They are of great interest as illustrating the 

 marvelous adaptability of living forms. While the true corals, which are polyp 

 colonies somewhat differently organized from these hydropolyps, secrete great 

 masses of solid rock, we find two small families of minute hydropolyps also build- 

 ing up hard coral stocks. This phenomenon is called convergence; two different 



MII.I.EPORA. 



A. Part of a stock with the polyps withdrawn (slightly magnified) ; B. Five peripheral nutritive individuals 

 round a central feeding individual (highly magnified). 



kinds of animals, starting from different points, become adapted to similar 

 conditions of life, and eventually come superficially to resemble one another. Just 

 as these hydropolyps forming coral were long thought to be true corals, so many 

 other animals have, on account of their resemblance, been classed together which 

 are now known to belong to different groups. 



Fresh- Water Two other hydroid polyps which live in fresh water, while all 



Forms tne rest are marine, deserve mention. Of these, Cordilophora lacustris 



forms branched trees from one to three inches high, rising from a 



network of roots attached to stones, wood, mussel shells, etc. The whole stock 



except the club-shaped heads of the individual polyps, which are provided with 



proboscis-like mouths and irregularly-branched thread-like arms is covered with 



a delicate horny envelope. In these stocks, which are of a red-gray color, the sexes 



are separate. Until the middle of the present century, the Cordilo-bhora had only 



