I 4 2 LAGOON CORALS. PART in. 



sink to 68 Fahr. till a depth of 100 fathoms, but light 

 and abundance of uncombined air are essential, and 

 these decrease as the depth increases. The polypes 

 perish if exposed directly to the sun even for a short 

 time, so they build horizontally between these limits. 

 The actinian polypes in the corals, which live at diffe- 

 rent depths in the crevices of the atolls, have the same 

 general structure ; their disks and tentacles are some- 

 times tinted with brilliant colours ; some sting, others 

 have a considerable diversity of individual character. 



On the margin of the atolls, close within the line 

 where the coral is washed by the tide, three species of 

 Nullipores flourish ; they are beautiful little plants, very 

 common in the coral islands. One species grows in 

 thin spreading sheets, like a lichen ; the second in stony 

 knobs as thick as a man's finger, radiating from a com- 

 mon centre ; and the third species, which has the colour 

 of peach blossom, is a reticulated mass of stiff branches 

 about the thickness of a crow's quill. The three species 

 either grow mixed or separately, and, although they can 

 exist above the line of the corals, they require to be 

 bathed the greater part of each tide : hence a layer two 

 or three feet thick, and about twenty yards broad, 

 formed by the growth of the Nullipores, fringes the 

 circlet of the atolls and protects the coral below. 



The lagoon in the centre of these islands is supplied 

 with water from the exterior by openings in the lee side 

 of the ring, but as the water has been deprived of the 

 greater part of its nutritious particles and inorganic 

 matter by the corals on the outside, the hardier kinds 

 are no longer produced, and species of more delicate 

 forms take their place. The depths of the lagoon varies 

 in different atolls from fifty to twenty fathoms or less, the 

 bottom being partly detritus, partly live coral. In these 

 calm and limpid waters the corals are of the most varied 

 and delicate structure, of the most charming and daz- 



