THE FLOWER-ANIMALS, 143 
and is more especially represented in the warm seas by an 
abundance of beautiful and highly-coloured forms like 
flowers. Hence they are also called Flower-animals 
(Anthozoa). Most of them are attached to the bottom 
of the sea, and contain an internal calcareous skeleton. 
Many of them by continued growth produce such im- 
mense stocks that their calcareous skeletons have formed 
the foundation of whole islands, as is the case with the 
celebrated coral reefs and atolls of the South Seas, the re- 
markable forms of which were first explained by Darwin.” 
In corals the counterparts, or antimera—that is, the cor- 
responding divisions of the body which radiate from and 
surround the central main axis of the body—exist some- 
times to the number of four, sometimes to the number of 
six or eight. According to this we distinguish three legions, 
the Fourfold (Tetracoralla), Sixfold (Hexacoralla), and Hight- 
fold corals (Octocoralla), The fourfold corals form the 
common primary group of the class, out of which the six- 
fold and eightfold have developed as two diverging branches. 
The second class of Sea-nettles is formed by the Hood- — 
jellies (Medusze) or Polyp-jellies (Hydromedusz). While 
most corals form stocks like plants, and are attached to 
the bottom of the sea, the Hood-jellies generally swim about 
freely in the form of gelatinous bells. There are, however, 
numbers of them, especially the lower forms, which adhere 
to the bottom of the sea, and resemble pretty little trees. 
The lowest and simplest members of this class are the 
little fresh-water polyps (Hydra and Cordylophora). We 
may look upon them as but little changed descendants of 
those Primeval polyps (Archydre), from which, during the 
primordial period, the whole division of the Sea-nettles 
