ORDER ACTINOIDEA. aM 
2. Secretions, from the foot or base of the polyp, which are either 
calcareous or horny, or of an intermediate nature, and rarely siliceous. 
The former may be called tisswe-secretions, the latter foot-secretions. 
46. Tissue-secretions. ‘These secretions take place from the tissues 
of the sides and the base of the polyp. In a few species—the coral- 
ligenous Aleyonaria—even the skin often adds to these secretions by 
depositions of lime in its texture; but in the other Actinoidea, the 
exterior of the polyp remains soft and fleshy, so that every portion of 
the Corallum, even to each spine and lamella, is entirely concealed 
within the polyp, as completely as the skull of an animal beneath 
its fleshy covering. All corals are more or less cellular, and through 
these cellules the animal tissues extend, forming, together with the 
exterior, a complete animal structure, corresponding closely with the 
coral structure. Even the most solid plates of the latter are more or 
less penetrated by fibres of animal tissue.* By comparing the 
radiated cell of a coral,t with the radiated visceral cavity of the 
Actinia or Palythoa, as described in §§ 25 and 32, the relations of the 
two will be as apparent as they can be made by any explanations. 
The radiated calcareous plates of the one alternate with the radiated 
visceral lamelle of the other. 
These secretions do not take place from all parts of a polyp. 
The disk, the stomach, and the upper portions generally of the ani- 
mal, remain fleshy, as well as the interior of the visceral cavity, in 
order that the polyp may be free to expand or contract, and perform 
the various functions essential to life. The tentacles, however, may 
secrete lime, and not unfrequently the calcareous lamelle of a cell 
project by this means into these organs; and, in the same way, some 
corals are covered throughout with short spines. 
The corallum has a close correspondence, therefore, to an internal 
skeleton. It is not a collection of cells containing polyps, like the 
cells of a bee-hive, but is contained itself wholly within the polyps. 
* This has been shown by Hatchett, and also by Milne Edwards and Bowerbank, 
and may be easily verified by dropping a piece of compact coral into a dilute acid. These 
tissues may be distinctly seen on examining with a high magnifying power, thin frag- 
ments polished down, till they admit the passage of light. A minutely reticulated struc- 
ture may be distinguished, though much irregular; and it appears probable that the 
tissues consist in part of the animal cellules within which the lime was secreted. The 
results of some microscopic examinations by the author upon different species of corals, 
will be given in the Appendix to this volume. 
} See plates 10, 11, 12, and others ; also, figure 34, § 76. 
