The royal natural history . latter case thelower side of the stock is quite smooth as if polished, and reproduces exactly thesurface of the glass with all its markings. There are two families of these Hydrocorallia, as they are called, namely, theMUleporidai and the Stylasteridce. They are of great interest as illustrating themarvellous adaptability of living forms. While the true corals, which are polyp-colonies somewhat differently organised from these hydropolyps, secrete greatmasses of solid rock, we find two small families of minute hydropolyps also 49° CCELENTERA TES. building up hard co
The royal natural history . latter case thelower side of the stock is quite smooth as if polished, and reproduces exactly thesurface of the glass with all its markings. There are two families of these Hydrocorallia, as they are called, namely, theMUleporidai and the Stylasteridce. They are of great interest as illustrating themarvellous adaptability of living forms. While the true corals, which are polyp-colonies somewhat differently organised from these hydropolyps, secrete greatmasses of solid rock, we find two small families of minute hydropolyps also 49° CCELENTERA TES. building up hard coral-stocks. This phenomenon is called convergence; twodifferent kinds of animals, starting from different points, become adapted tosimilar conditions of life, and eventually come superficially to resemble oneanother. Just as these hydropolyps forming coral were long thought to be truecorals, so many other animals have, on account of their resemblance, been classedtogether which are now known to belong to different MILLEPORA. A, Part of a stock with the polyps withdrawn (slightly magnified); Ii, Five peripheral nutritive individualsround a central feeding individual (highly magnified). Fresh-Water Two other Hydroid polyps which live in fresh water, while Forms. &\\ ^he rest, are marine, deserve mention. Of these, Cordilophoralacustris forms branched trees from one to three inches high, rising from a net-work of roots attached to stones, wood, mussel-shells, etc. The whole stock—exceptthe club-shaped heads of the individual polyps, which are provided with proboscis-like mouths and irregularly-branched thread-like arms—is covered with a delicatehorny envelope. In these stocks, which are of a red-grey colour, the sexes areseparate. Until the middle of the present century, the Cordilophora had onlybeen met with in brackish water on the coasts of Europe and of North that it appeared from time to time in the lower courses of rivers, such asthe Thames, the E
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
