49° 
CCE LEJVTERA 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. 
A, Part of a stock witli the polyps withdrawn (slightly magnified); B, Five peripheral nutritive individuals 
round a central feeding individual (highly magnified). 
Fresh-Water Two other Hydroid polyps which live in fresh water, while 
Forms. a p the res t 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 
