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 



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Fig. 91. — Very young Polpys, attached to a Fig. 92. — A young Coral Polyp fixed upan 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 incli 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. 



