CORAL REEFS SEA GOOSEBERRY. 



711 



with the rich verdure of the tropics, the wind and birds having supplied it 

 with seeds as soon as it rose above the waves. The enclosed lake is often 

 fringed with graceful palms, and the whole island, the product of minute 

 coral polyps, becomes a fruitful habitation for man. 



These different kinds of reef formations were all traced back by Darwin 

 to fringing reefs, and accounted for by the subsidence of the land round 

 which these reefs originally formed. A fringing reef would gradually become 

 a barrier reef as the land on which it stood sank, and in the same way, a reef 

 surrounding an island would be transformed, after subsidence of the island, 

 into a circle of coral rock enclosing a lake. As long as the highest points of 

 the land remained above water, the lagoon would contain islands, but, when 

 the original land had entirely sunk beneath the waves, an atoll would result. 

 In the case of a very large island, surrounded, as New Caledonia now is, by 

 a barrier reef, the gradual sinking of the land would lead to the formation of 

 a complicated archipelago of islands, like that of the Maladives to the west 

 of Ceylon, where 12,000 small islands appear to represent a huge reef which 

 must once have surrounded a vast island long since sunk beneath the waves. 

 Darwin's theory of subsidence has, however, recently been disputed, and 

 cannot be considered as fully established. 



The Ctenophora, which constitute the last division of the Cnidaria, are 

 beautiful, almost transparent, marine creatures, either more or less round in 

 form, or else flattened out in the shape of ribbons. 

 Their relationship to the other groups is still a matter of The Ctenophora, 

 uncertainty; they are often placed near the Hydrozoa. 

 The animals belonging to this group move through the water, mouth 

 downward, by means of swimming plates or "combs," from which they take 

 the name of Ctenophora or Comb-bearers. Eight 

 rows of these swimming plates run from pole to pole 

 of the body, each plate consisting of a row of fine 

 hairs or cilia connected at their bases, but capable 

 of independent movement. The animal can, at will, 

 move the plates either separately or collectively, and 

 thus bring about slow locomotion in the direction 

 opposite to the mouth, and, by the help of tentacles 

 and other appendages, twisting and swaying move- 

 ments can also take place. In the ribbon-like forms 

 muscular contractions bring about serpentine move- 

 ments. 



The mouth leads into a stomach, which, again, 

 opens into a funnel that sends off eight vessels, which 

 run, in meridians, down the sides of the body, just 

 below the eight rows of swimming plates. The animal 

 is usually provided with tentacles and with long 

 "capturing filaments" (Fig. 20). These are not 

 armed, like the tentacles of other Cnidaria, with 

 stinging cells, but with small knobs on coiled stalks, 

 which are called ** adhesive" cells. These are not 

 poisonous, but simply sticky, and when shot out at 

 prey entangle it and prevent its escape. They differ from stinging cells also 

 in the fact that they are not finally discharged from the body, but, when 

 the prey is mastered, can be drawn in again, and are capable of functioning 

 any number of times. 



Fig. 20. THH SKA Gooac- 

 BKRRY (Cydippe). 



