EOW CORALS GROW, 
31 
posited. Fig. 32, f7, shows the twelve rudimentary parti- 
tions. These, after the young polyp has become stationary, 
finally enlarge and become joined to the external walls of 
the coral now in course of formation (Fig. 32, C), form- 
ing a groundwork or pedestal on which the polyp rests. 
D represents the young polyp resting on the limestone ped- 
estal, with the tentacles well developed. 
But little is positively known as to the rate of growth of 
corals. A common brain coral {Mceandrina laiyrinthica), 
measuring a foot in diameter and four inches thick in the 
most convex part, attained its growth in twenty years. 
Fig. 32.— Development of a coral polyp, Astroides calycularis. A, ciliated 
f^astrula; B, young polyp with 12 septa; C, D, young polyp farther advanced, 
with 12 tentacles; C, the corallum and limestone septa beginning to form. 
Magnified. 
To the order of alcyonoid corals, which have but eight 
tentacles, belong the sea-fans," sea-pens," etc. In the 
family of sea-fans {GorgonidcB) the coral-stock is horny 
or calcareous, branching tree-like, or forming a flat net- 
work. Gorgonia flaiellum is red or yellow, and abundant 
on the Florida reefs. In the Arctic seas and the deeper, 
colder waters of the Newfoundland Banks and St. George's 
Banks, Primnoa reseda and Paragorgia ariorea grow; the 
latter being of great size, the stem as thick through as one's 
wrist, and the whole corallum over five feet in height. 
