CCELENTERA TES. 
m 
498 
tlie central axis. Monoxenia and similar animals are considered simple, because the 
repeated organs develop similarly and simultaneously, and are comparatively few 
number. The mouth, too, is circular. In many other polyps, however, the 
regularly radiate type is slightly departed 
8 from; the mouth, instead of being round, 
forming a long slit, while there is a tend¬ 
ency for the originally radiate animal 
to become bilateral. From this account of 
a simple polyp, it is easy to understand 
what kind of animal it is which makes 
coral; and our readers, if they have not 
already done so, will give up speaking of 
“ insects building up the coral - reefs.” 
It is, however, by no means all such 
polyps that form coral, nor do those 
which form it produce it always in the 
same way. Numbers of polyps, such as 
the beautiful sea-anemones, never produce 
any hard substance, but remain soft and 
delicate, though dangerous, at least to 
small animals, because of their stinging- 
cells. Many of these soft sea-anemones 
are highly specialised creatures, as may 
be seen from the coloured Plate; but 
those which secrete coral are generally 
simpler, and smaller, and grow in vast 
colonies. It is the accumulation of all 
their little contributions of coral which, 
in the process of time, build up islands or 
even continents. In regarding coral 
animals as reef-builders, we may leave 
out of account, as unimportant, those 
which form hard spicules within their 
bodies, and consider only those which 
perform most of the work. Imagine a 
crowd of small animals like sea-anemones 
fixed to a rock, each one secreting a layer 
of carbonate of lime between itself and 
the rock, and this layer becoming thicker 
and thicker till each polyp rises on a 
little pedestal. There is probably a race 
between them, as there is among trees in a forest, which shall reach the highest 
to get most of the food as it passes by on the currents in the water. Now, it is 
obvious that a crowded colony of polyps like this would in a short time add a 
thick layer of solid carbonate of lime to the rock on which they first settled, and 
this is, in brief, the principle of reef-building. As a matter of fact, however, it is 
Monoxenia danoini (magnified). 
L, Longitudinal section, on the left, through one of the 
interseptal cavities, on the right through one of the 
partition walls ; M, Transverse section through the 
line m n; N, Transverse section through the line 
sit\ O, The eight-lipped mouth-aperture, with the 
bases of the arms ; a b c 0 , Principal axis ; p, 
Pharyngeal cavity ; g, Digestive cavity; k, Divisions 
of the digestive cavity; to, Radial septa or walls 
dividing up the digestive cavity; e, Masses of eggs ; 
u, Mesenterial filaments; /, Masses of muscle and 
connective tissue. 
