210 A. FE. Verrill—The Bermuda Islands; Coral Reefs. 
abundant than any other coral. The hydroid coral (Millepora alci- 
cornis, pl. XxxA) is, perhaps, more generally diffused and more 
abundant than any of the others, for it lives under a great variety 
of conditions and seems to grow very rapidly. The ‘rose-corals” 
(Mussa fragilis, and other species of that genus, figs. 76-84) are 
also very common, both on the reefs and close to the shores, and 
are notable on account of the large size and bright colors of their 
living polyps. The branching Oculinas are chiefly found in shel- 
tered places and at some depth; also in the sounds, especially in 
Harrington Sound. 
The green, olive, yellow, and yellowish brown colors, so prevalent 
in the colors of the reef corals and actinians, are in most cases 
chiefly due to the presence of unicellular algoid plants (Zodxanthelle) 
living as parasites or symbiotically in the tissues of the polyps, 
chiefly in the cells of the endoderm. These plants give out oxygen, 
when exposed to sunlight, which may be utilized by the tissues of 
the coral and thus serve as a sort of respiration for them. Corals 
well supplied with such oxygen-producing plants can live a long 
time in aquaria without change of water, if exposed to sunlight part 
of the time. 
The varying colors depend partly upon the relative abundance of 
the Zodxanthelle and partly upon the colors of the latter, for there 
seem to be several kinds, with different tints. 
Meandra labyrinthiformis (Linn.) Oken. Brain Coral. Figs. 71-71e. 
Diploria cerebriformis Edw. and Haime, and of many later writers. 
Meeandra labyrinthiformis Verrill, these Trans., xi, pp. 70-73. Pl. x, figs. 
1-3; pl. xii, fig. 5, 1901. 
This species, when living, is usually lemon-yellow or orange- 
yellow in color. When it grows under favorable conditions, with 
plenty of room, it forms large hemispherical or dome-shaped masses, 
sometimes 5 to 8 feet or more in diameter. Such specimens must be 
several hundreds of years old,* for colonies of this and, related 
species, of known age, in the Yale Museum and elsewhere, which 
were measured from time to time while living, grew about } to + 
of an inch annually, in the warmer waters of Key West and the 
Tortugas.t 
But such large examples are seldom perfectly regular, owing to 
various injuries received from storms, etc. Those that are 1 to 2 
* See above, p. 149, note. 
+ See also Dana, Corals and Coral Islands, ed. 3, pp. 253, 418. 
