GROWTH OF ZOOPHYTES. 101 
The Nullipore zone along the reef-line between low and high tide, 
is clearly made out by Mr. Darwin, and is one of the interesting 
results of his investigations. It performs a very important part, by the 
protection it gives the reef from abrasion. ‘The exposed reef is thus 
gathering lime from the waters, and extending itself, when, if devoid 
of this protection, it would be constantly yielding to the sea. On the 
inner reefs, where the protection is not needed, it is not given. Some 
species of Nullipore, however, occur in these regions, and others are 
found at various depths. 
As the Caryophyllia family extend into deeper waters than most 
other reef-corals, it might be inferred that these at least may constitute 
a lower bed, or substratum. But this is by no means the case. ‘The 
Caryophyllie are but sparingly distributed; the species are few, and 
mostly small; and not a dozen different kinds were detected in the 
Pacific. ‘Their contributions to reefs are, therefore, inconsiderable. 
4. RATE OF GROWTH OF ZOOPHYTES. 
The rate of growth of zoophytes, is a subject but little under- 
stood. We do not refer here to the progress of a reef in formation, 
which is another question complicated by many co-operating causes ; 
but simply to the rapidity with which particular species of coral- 
zoophytes increase in size. ‘There is no doubt that the rate is differ- 
ent for different species. It is moreover probable that it corresponds 
with the rate of growth of other allied polyps that do not secrete lime. 
The rate of growth of Actinie might give us an approximation to the 
rate of growth in a Mussa, which are coral animals of like size and 
general characters ; for the additional function of secreting lime 
would not retard necessarily the maturing of the polyp; and from 
the rate of growth of the same animals in the young state, we might 
perhaps draw some inferences as to the rate in polyps of corresponding 
size. But no observations on this point were made by us while 
abroad. 
Although the rapidity is undoubtedly far less than was formerly 
stated, the following facts from different sources seem to show that the 
rate is still greater than has been of late believed. Mr. Darwin, citing 
from a manuscript by Dr. Allen of Forres, some experiments made on the 
east coast of Madagascar, states that, in December, 1830, twenty corals 
were placed by this gentleman apart on a sandbank, in three feet 
water, (low tide,) and in the July following, each had nearly reached 
26 
