1907.] AND SUPPOSED SPECIES IN CORALS. 53i^ 



demands of the physical conditions, and modify its inherited 

 growth tendency, — or perish. Totally different forms are pro- 

 duced in totally different envii'onments, but these forms must not 

 be regarded as " species," for they are mere variations of vegetative 

 growths in response to the necessities of the life sui-roundings 

 of the colony. 



The type of vegetative growth is affected mostly by the 

 physical conditions of the w^ater in which it lives, but the actual 

 structure of the coral depends greatly upon the presence, or 

 absence, of sediment. Some very strange results are produced 

 by waters in which sediment is held suspended, and from which 

 it is being deposited ; and sediment will alter the appearance of a 

 coral more strikingly than any other influence. The deposition 

 of sediment is the greatest agent in causing coral death : corals 

 are very easily killed by even comparatively little sediment, and 

 are profoundly altered by it, if they are to successfully resist its 

 influence. 



The extent of silt formation at the surface of an atoll in mid- 

 ocean is hard to imagine, and it has certainly not been appreciated 

 by those experimenters who have attempted to estimate the age 

 of an atoll by catching the silt in a net, as it passes into the 

 lagoon. Adown the submarine slopes of the atoll, for a hundred 

 miles east and west, the bottom was found by the cable soundings 

 to be finely triturated coral- sand, and it is only an vincertain and 

 inconstant fraction that passes into the lagoon. Silt is one of 

 the shifting influences of the atoll, and so may visit the coral 

 colonies for only a brief portion of their lives, and then partial 

 death and strange repair-growths result. In the lagoon, and in 

 some portions of the barrier pools, silt is a constant factor, and 

 here shows to the greatest advantage the modifications that it is 

 cajDable of producing in coral-growths. Speaking generally, silt 

 alters the vegetative growth-form of colonies only in as far as it 

 produces flat-topped rock masses by killing the uppermost zooids, 

 and cavises amorphous and irregular growths by partly killing 

 the uppermost growing cells of the growing points. But in the 

 surface structure of the coral, it produces great and wonderful 

 changes. Its effects are best studied by comj)aring the upper and 

 lower surfaces of partial plates. In these plates, the upper flat 

 surface is alone exposed to the action of the deposition of sediment, 

 and here the corallites tend to be small, and to be raised from the 

 general surface, and the intervening spaces themselves tend to 

 be sculptured and complicated in various ways (PL XXYIII. 

 fig. 2). Below, the corallites are larger and are flush with the 

 general surface, and the intervening spaces are flat and plain 

 (PL XXYIII. fig. 1). 



Now this condition is entirely the result of the attempt of the 

 uppermost zooids to build a silt-resisting structure. 



The corallites are smaller, and are raised from the general surface 

 in order to minimise the chance of silt dropping in and choking 

 the zooid. The intervening coral body is variously seulptui-ed 



