SOLITARY CORALS. 217 



showing the successive additions to the septum, and their relations to the rows of 

 trabecule, are well shown in the figure, as well as the way in which the outermost 

 trabecule in each row from place to place bend sharply towards the surface and end 

 in a spiniform granule. 



It will be observed that there is no dark central line and no dark " centres of 

 calcification." The centre of each trabecula is clearer than the surrounding calcareous 

 deposit, and the latter, which appears white by reflected but dark by transmitted 

 light, seems to have a very fine fibrous rather than a crystalline structure, but I 

 could not make out the details clearly in the thinnest sections that I was able to 

 prepare. In longitudinal sections each septum is clearly seen to be made up of a 

 number of trabecula? radiating like a fan from a centre, and each trabecula appears 

 to be made up of a number of growth segments joined end to end, each segment 

 formed by the fascicle of diverging fibres described by Pratz and Miss Ogilvie as 

 characteristic of the minute structure of the Madreporarian corallum. 



But if no definite crystalline structure can be discovered in the septa themselves, 

 or in the synapticula, the case is different for the " stereoplasm " in the interseptal 

 loculi. This stereoplasm, except where it is heavily charged with brown pigment, 

 appears dark by reflected and light by transmitted light, and, as is indicated in fig. 13, 

 it is clearly made up of coarse crystalline fibres. The orientation of these fibres should 

 be carefully studied, as it affords a proof that the stereoplasm is a secondary structure, 

 and not a simple addition to the thickness of the septa. At the thin innermost ends 

 of the septa the secondary fibro-crystals are disposed at right angles to the long axis 

 of the septum (as seen in transverse, i.e., horizontal section), and here we seem to have 

 the characteristic structure of the Madreporarian septum with a middle dark line or 

 " centre of calcification," &c. But where the stereoplasm is thick and completely fills 

 the interseptal loculus, it displays a number of curved lines of growth, generally 

 emphasized by the deposition of curved bands of dark brown pigment, and it should 

 be noted that, whereas the curved growth-lines of the septa have their convexities 

 directed outwards, the reverse is the case with the growth-lines of the stereoplasm. 

 The fibro-crystals of the latter, as is shown in fig. 13, are arranged in diverging 

 bundles, conformably to the curved lines of growth, in such a manner as to appear to 

 diverge from a stereoplasmic " centre of calcification " which is more apparent in 

 tangential than in horizontal sections. It is obvious that, after the septa and 

 synapticula have been formed, the soft tissues in the deeper parts of the corallum 

 shrink away from the septal and synapticular surfaces both externally and internally, 

 and as they shrink away the calicoblastic layer again enters into activity and deposits 

 the coarsely fibro-crystalline stereoplasm that eventually fills up the interseptal loculi 

 to a greater or less extent. 



Concerning the brown pigment, I have very little to say. It appears to be 

 deposited in the form of minute granules between the fibro-crystals, but I found it 

 impossible to make sections sufficiently thin to admit of an accurate study of it. 



i F 



