STRUCTURE AND RELATIONS OE CERTAIN CORALS. 
10 . 
The colouring-matter is dissolved at once off the filter by alcohol, and an intensely 
blue solution, very like that of sulphate of copper in colour, is thus obtained. The colour, 
however, is not dissolved out of the corallum by the action of alcohol alone. The 
deep blue and alcoholic solution gives a spectrum in which all the violet, red, and 
yellow are totally absorbed, and the green and blue alone transmitted. The absorption 
of the violet end of the spectrum extends to G, that of the less-refrangible end to a 
little short of E. Addition of potash or ammonia solutions to the blue solution changes 
the colour to a dirty green, which is contained in a flocculent precipitate. The blue 
colour reappears on the solution being rendered again acid. 
In Bronn’s ‘Klassen und Ordnungen,’ 1860, “ Actinozoa,” p. 22, is an account of the 
analysis made by the younger Silliman, at the request of Dana, of various coralla. 
Amongst the species analyzed was Heliopora. Mr. Silliman therefore probably inves- 
tigated the colouring-matter of this coral at the time when he submitted it to analysis. 
On the Structure of the Soft Tissues of Heliopora. 
The arrangement of the structures constituting the general superficial layers of 
Heliopora , and of those common to the ccenenchymal tubes and calicles, will be con- 
sidered in the first place, and in the second the structure of the polyps themselves. 
As in other Alcyonarians the various structures are to be classed as belonging to an 
ectoderm, mesoderm, and entoderm. The general arrangement of these three layers of 
tissue will be seen in Plate 8. fig. 1. 
The ectoderm consists of a layer of epithelial cells, which invests the whole external 
surface of the coral with a uniform covering. Its structure is shown in Plate 9. fig. 10. 
The cells composing it are elongate and club-shaped, with wide rounded summits and 
pointed lower extremities, which run out into fine threads which can be traced some 
way into the layer beneath them. The cells contain a nucleus and nucleolus, and their 
general contents are finely granular ; they are closely packed side by side, placed parallel 
to one another, and vertically to the surface of the coral. When the epithelial layer is 
viewed from above, the ends of the cells present a series of polygonal areas. The cells 
are about *02 millim. in length. Between the contracted bases of these cells are other 
irregularly shaped cells wfith similar nuclei and contents, and also scattered throughout 
the layer are to be found nematocysts. The ectoderm is prolonged to form the lining 
of the stomachs of the polyp ; otherwise it is superficial only. 
The mesoderm consists of three different histological structures : — (1) A nearly homo- 
geneous transparent connective-tissue; (2) layers of connective-tissue cells; and (3) 
masses of finely fibrillar tissue. 
(1) Beneath the ectoderm is a thick layer, of a mean thickness of about ‘07 millim., 
likewise extending over the whole surface of the coral, which consists of a highly 
transparent connective-tissue, which is almost homogeneous, but in which faint lines 
indicating slight fibrillation may here and there be seen. 
Extensions of this homogeneous layer form the central layers of the membranes 
