ORDERACTINOIDEA. 51 



2. Secretions, from the foot or base of the polyp, which are eitlier 

 calcareous or hormj, or of an intermediate nature, and rarely siliceous. 



The former may be called tissue-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 Alcyonaria — 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 lamellfe 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 lamellae 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. 



t See plates 10, 11, 12, and others ; also, figure 34, § 76. 



