110 THE VOYAGE OF H.M.S. CHALLENGER. 
of hydrochloric acid, the colourmg matter is set free, and remaining suspended in the 
fluid gives it a blue tint. It is, however, not in solution, but can be observed under the 
microscope to exist in the fluid in the form of small, amorphous, intensely blue masses 
adhering to small shreds of tissue, &c., and in this condition may be proved to he 
insoluble in strong hydrochloric acid. If the coloured solution formed by hydrochloric 
acid be filtered, the blue colouring matter remains on the filter, and the filtrate has only 
a very slight greenish tint. 
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 the position of the G line of the solar 
spectrum, that of the less refrangible end to a little short of the E line. 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 acid again. 
Dana states that the blue colour of Heliopora is of animal origin, and is lost on 
immersion of the coral in nitric acid. The colouring matter was not analysed by Mr 
Silliman (U. 8. Expl. Exped., vol. vii., Zoophytes, J. D. Dana, Philad., 1846, p. 537). 
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, a mesoderm, and an endoderm. The general arrangement of these three layers 
of tissue will be seen in Plate I. fig. 1. 
Ectoderm.—The ectoderm consists of a layer of cells, which invests the whole external 
surface of the coral with a uniform covering. Its structure is shown in Plate IL. fig. 4. 
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 external layer is 
viewed from above, the ends of the cells present a series of polygonal areas. The cells 
are about ‘02 mm. in length. Between the contracted bases of these cells are other 
nregularly shaped cells with similar nuclei and contents, and also scattered throughout 
