490 



CCRLENTERA TES. 



building up hard coral -stocks. This phenomenon is called convergence; two 

 different 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. 



MILLEPORA. 



A, Part of a stock with the polyps withdrawn (slightly magnified) ; 7>, Five peripheral nutritive individuals 

 round a central feeding individual (highly magnified). 



Fresh-Water 

 Forms. 



Two other Hydroid polyps which live in fresh water, while 

 ^he rest are marine, deserve mention. Of these, Cordilophora 

 lacustris 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 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-grey colour, the sexes are 

 separate. Until the middle of the present century, the Cordilophora had only 

 been met with in brackish water on the coasts of Europe and of North America. 

 After that it appeared from time to time in the lower courses of rivers, such as 

 the Thames, the Elbe, etc., and now it has found its way far inland both in the 

 Old and New Worlds. It occurs in the Saale, near Halle, and is specially plentiful 

 in the slightly brackish lake of Eisleben. In Hamburg it has in some places 

 invaded the water-pipes supplying the city, developing in them to such a degree 

 as actually to stop them up. This history of the migration of Cordilophora is 

 instructive in helping us to understand the rise of at least a part of the fresh- 

 water fauna. In this case, within our own experience, an animal inhabiting 

 brackish water has in a few years become so adapted for living in fresh water 

 as to be considered altogether a fresh-water form, without the least apparent 

 change in its organisation. Whether a change in organisation would not 



