Hydractinia, Parkeria, and Strom atopora. 57 



number and slightly in size outwards ; so that while they 

 average transversely about l-300th inch in diameter near the 

 centre, their cavity is about l-125t]i inch in transverse dia- 

 meter at the circumference of a specimen of Parkeria \\ inch 

 in thickness. On progressing outwards, these cylindrical 

 pillars, for the most part, lose their individuality from the 

 increase in quantity of the tissue-fibre, which involves those 

 in its course as the latter assumes a columnar disposition, 

 increasing in size outwards. The columns so produced thus 

 radiate from the centre to the circumference, and, arching 

 towards each other in all directions as they arrive at each 

 lamina, appear to divide the " interval," in the vertical section, 

 into a number of chambers. These are the " chamberlets " of 

 Dr. Carpenter. 



So long as the vertical tubes retain their individuality — that 

 is, in the first three or four intervals, where they are not ob- 

 scured by the additional growth of the tissue-fibre (fig. 17, h) — 

 they, with the lamhite of Parkeria, closely resemble the laminae, 

 intervals, and tubes respectively of Tubipora musica, especially 

 as the whole structure of the latter is elaborated out of a 

 similar tissue ; but besides being almost incomparably larger 

 (that is, while the laminae, intervals, and tubes in Parkeria 

 are at the part mentioned respectively l-900th, l-200th, and 

 l-900th inch across, those of T. musica are l-24th, l-4th, and 

 1-lOth inch across, the cross diameter of the interval indicating 

 the length of the tube in each instance), the tissue of T. musica is 

 not reticulated but sieve-like and laminiform, all the holes being 

 on the same plane and of all kinds of sizes, precisely like the 

 structure of the calcareous tissues in the Echinodermata. 

 The radiating tubes of T. musica, too, are for the most part 

 opposite each other, so as to appear vertically continuous for 

 a long distance, although internally their cavity is frequently 

 interrupted by a diaphragm of the same sieve-like tissue, 

 which is for the most part just below the lamina ; and it is 

 worth noticing that while the openings in a vertical section 

 of the lamina of T. musica resemble those in the lamina of 

 Parkeria, they are also present in a ring-like form inside the 

 tube of T. musica opposite the lamina — that is, just above the 

 diaphragm ; so that the radial tubes, as in Parkeria, w^ere in 

 communication with the passages in the centre of the lamina, 

 and not so continuously hollow as at first sight they would 

 appear to be. 



Having now described Parkeria from the vertical section, 

 let us turn our attention to the surfaces of the lamina (that is, 

 the outer and inner surfaces), concentrically — an examination 

 which the same wninfiltrated specimen renders comparatively 



