88 CORAL FORMATIONS. 
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kind is the Tubipora. Others secrete only scattered granules of lime 
through the tissues: and still others are fleshy throughout. Many of 
them, besides forming granular calcareous secretions within the body 
of the polyp, give origin to a horny secretion at base, analogous to the 
epidermic secretions (hair, nails) of other animals; and this secretion 
receiving constant additions from the polyps as they are successively 
budded out, forms the axis of the growing branch. Of this character 
is the horny axis of the Gorgonia or sea-fan, which was long taken 
for a vegetable production. ‘The crust which covers the axis consists 
of united polyps, which expand over its surface; and when expanded, 
each branch becomes a spike of flowers. 
Tue Hyprorea. 
The Hydroidea constitute the second grand division of zoophytes, 
corresponding in rank with Actinoidea. While the Actinoidea have 
a radiated interior cavity with internal organs of reproduction, and 
eject their ovules through the mouth, the Hydroidea have greater 
simplicity of structure—the internal cavity being a simple tube, with- 
out organs of reproduction, and the ovules pullulating (or growing 
out) singly or in clusters from the sides of ananimal. ‘The polyps are 
with few exceptions quite minute, and the zoophytes act no important 
part in reef-making. A coronet of tentacles surrounds the mouth, as 
in the Actinie, though somewhat different in character. 
This order includes the Hydre, the Sertulariz, and the Tubularie. 
Some species form thready tufts and plumes of extreme delicacy, 
and others (the Hydre) are simple polyps. The fine branchlets of 
the feathery species consist, when dead, of one or two series of micro- 
scopic cells: and when alive each cell is the site of a minute flower- 
animal. The Hydra, an animal a line or less in length, consists of a 
tubular body, with a mouth at one extremity surrounded by a circle 
of tentacles; and the structure of the animal is so simple that it may 
be turned inside out, and still live and eat; it may be cut into forty _ 
or more parts, and from the dissected body, will grow as many distinct 
Hydre. 
Tue Bryozoa.—The Bryozoa are other coral-making species; but 
they are related to certain molluscs called Ascidie rather than to 
zoophytes. In habit and size they much resemble the Hydroidea. 
From a minute cabin-like cell, they extend a circlet of slender arms 
or tentacles, and expand into a delicate goblet-shape flower, seldom 
over a line in diameter. These polyps differ both from the Actinoidea 
and Hydroidea, in having two extremities to the alimentary canal— 
