234 THE OCEAN WORLD, 
aid of which it can swim. Sometimes one of its extremities becomes 
enlarged, the other remaining slender and pointed. Upon this an 
opening is formed communicating with the interior cavity: this is 
the mouth. The larva swim backwards; that is to say, with the 
mouth behind. — 
It is only at a certain period after birth that the coral polyp fixes 
itself and commences its metamorphoses, which consist essentially 
in a change of form and proportions. The buccal extremity is 
diminished, and tapers off, whilst the base swells, and is enlarged—it 
becomes discoid; the posterior surface of this disc is a plane, the 
-Fig. 91.—Very young Polpys, attached to a Fig. 92.—A young Coral Polyp fixed upon a Rock. 
Bryozoon. (Lacaze-Duthiers. ) : 
front representing the mouth, at the bottom of a depression edged 
with a great cushion. Eight protuberances or swellings now appear, 
corresponding to the chambers which divide the interior of the disc ; 
the larva has taken its radiate form. Finally, the protuberances 
become elongated and transformed into tentacula. In Fig. 91 a 
young coral polyp is represented fixed upon a‘bryozoon. It forms a 
small disc, the fortieth part of an inch in diameter, and having its 
spicula already coloured red. Fig. 92 shows the successive forms of. 
the young polyps in the progressive phases of their development— 
being a young coral polyp fixed upon a rock still contracted. Fig. 93 
is a similar coral polyp attached to a rock and expanding its tentacula, 
Fig. 94: represents a small pointed. rock covered with polyps and 
polypidoms of the natural size and of different shapes, but all young, 
and indicating the definite form of development which the collective, 
beings are to assume. 
