142 POPULAR HISTORY OF THE AQUARIUM. 



with it, but that it is the instinct of these aniaials to work 

 upwards, so as to keep near the surface, for they do not 

 live in deep water. They also increase more vigorously at 

 the outward or seaward edge ; so that when the mountain 

 has sunk a little, and the Corals nearest to it have not risen 

 in proportion, tliere is formed a channel or lake between 

 the island and the outer edge of Corals ; and this outer edge 

 forms what is called a Coral-reef. As the island sinks lower 

 and lower, the lake or lagoon becomes wider and wider, and 

 in some cases, the island has sunk below the surface of the 

 sea, while the surrounding reef, keeping up to the surface, 

 and becoming in process of time covered with soil and vege- 

 tation, forms that circular kind of island which is called an 

 Atoll. 



Tun GiA.— (Plate Til. fig. 1.) 



While, on one hand, the simply formed Caryopliyllea 

 leads us to the branched masses of many-starred Madre- 

 pores, on the other hand, their rayed laminated surface pre- 

 sents a striking resemblance to the flat, mushroom-shaped 

 Coral, which seems to have so little attachment to sur- 

 rounding objects. Our Plate contains the representation of 

 a very small specimen of the Mushroom-Coral, which, al- 



