24 CORALS AND CORAL LSLANDS. 



is often ribbed, and the ribs are ordinarily only an outer ex- 

 tension of the interior septa ; so that surface spines are in fact 

 but the outer margins of septa. 



The first of the preceding figures, representing Thecocyathus 

 cylindraceus, Sourt, exhibits another of the forms of these 

 simple corals. It is described by Pourtales, from specimens 

 collected by him at a depth of loo to 200 fathoms off the 

 Florida reef The actual size was one-third that of the figure. 

 The second figure represents a living species, the Flabellutii 

 pavonifiuf/i, described and figured by the author from specimens 

 obtained at Singapore. 



The bottom of the calide, or polyp-cell, in the corallum is 

 sometimes made simply by the meeting of the radiating septa ; 

 occasionally by the same, with the addition of a point or 

 columella at the centre ; often by a twisting together of this 

 part of the radiated septa. Very often, also, it is a mere 

 porous mass. Sometimes there is a circle of prominent points 

 about the centre, as seen in the figure of a Caryophyllia on 

 page 22, which are the extremities of narrow vertical strips 

 (called /(?//) lying in the planes of the septa. Similar points 

 exist in the Thecocyathus on the preceding page, though not 

 in sight in the figure. 



In many cases the bottom is quite solid ; and this may be 

 so either (i) because the coral secretions fill up all the pores 

 as the polyp increases in age, and thus make the interior of 

 the corallum solid or nearly so; or (2) because there are 

 formed periodically, as the polyp grows upward, solid horizon- 

 tal plates across the bottom, so that beneath, in the interior of 

 the corallum, there is a series of plates or tables with spaces 

 between. The Pocilloporae, among recent corals (page 70),, 

 and the Favosites among ancient, are examples. Increasing 

 solidity with the increasing age of the polyps is also produced 

 at times by additions to the exterior of a corallum. In many 

 species, the skin, over part or all of the exterior, gradually 

 disappears or dies away and leaves the corallum bare, while 

 all is living within ; and in such cases the skin, before disap- 

 pearing, often adds a layer of stony material to the exterior. 



