
No. 426.] MADREPORARIAN CORALS. 469 
The development of the colonial polyps was not followed far 
beyond the stage represented in Fig. 3. Growth was very 
slow compared with that of the isolated individuals, and most 
were preserved for further study. There was no indication, 
however, but that under favorable natural conditions the 
aggregations would have grown into fully mature colonies. In 
the later stages the primary irregularities due to crowding 
would probably have been outgrown, and the groups would 
then present all the characters of normal colonies arising by 
budding. 
In one feature the polyps constituting an ordinary colony of 
S. radians differ from those in a colony formed of aggregated, 
primarily independent units. Polyps arising by gemmation 
are, as a rule, only incompletely cut off from the parent; their 
internal cavities are from the beginning in communication with 
those of the adult polyps, so that the nutrient fluid can circu- 
late from one individual to another. Such would not be the 
case at first with aggregated colonies, and at the very early 
stage at which the observations on Siderastrea were made I 
could not assure myself that intercommunication was estab- 
lished. The intervening external epitheca would for some 
time interfere with such a possibility, but no doubt this struc- 
ture, along with the basal plate, would be left behind in the 
upward growth of all except the marginal polyps. For, except 
a most rudimentary formation at the margin, the epitheca is 
absent from mature colonies. 
It does not seem probable that the phenomenon of aggre- 
gated colonies is a prominent feature of coral growth. Still, 
it is necessary that its possibility should be recognized, for 
wherever it occurs the resulting colony will probably differ 
somewhat from one produced by normal budding or fission, 
while when occurring in usually simple corals its non-recogni- 
tion might lead to disastrous results. 
All students of both fossil and recent corals are familiar with 
the fact that very often simple coralla are found adhering to the 
dead portions of colonies, undoubtedly the young of these. 
Among West Indian corals the occurrence of young polyps 
under such conditions is very frequent in Manicina areolata 
