58 CORALS AND CORAL ISLANDS. 
which grew, and multiplied its polyps as it grew, by this meth- 
od. In such species some of the disks of the polyps will be 
found to have two mouths. This is the first step in the pro- 
cess In others, the two mouths will be found to be partly 
divided from one another by new-tormed tentacles ; and finally 
each will have its own circle complete and all else in polyp 
perfection. 
Many of the Astrea hemispheres of the Pacific, grown by 
this method, have a diameter of ten to fifteen feet. 
In other Astrzea-like species, this spontaneous fission ends 
in a complete separation of the two polyps formed ; and conse- 
quently in a forking of an old branch. The figure of a Cau- 
lastreea, on Plate IV. (figure 1), illustrates this mode of 
branching. In the left hand polyp there are already two 
mouths, and the work of subdivision is consequently begun ; 
while in those to the right, which have a single mouth, the 
subdivision has just been completed, and also the forking of 
the old branch. Thus spontaneous fission goes forward, and 
branches accordingly multiply. By this method some of the 
most magnificent clumps of coral zodphytes found in tropical 
seas have been, and are being, developed each from a single 
germ. Many of them have the perfect hemispherical sym- 
metry of the solid Astraas. 
Sometimes, when a new mouth forms in an enlarging disk, 
there is not at once a separation of the two, but the disk con- 
tinues to enlarge in one direction and another, and then an- 
other mouth opens, and so on until a string of mouths exists 
in one elongated disk; and finally, a separation occurs, but 
only to commence or carry forward another long series. In 
this way the corals with meeandrine furrows are made, some 
kinds of which are popularly called Brain coral, and pertain 
to the Meeandrina family (figure on page 65). The same may 
