Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/284

208 Both the kinds (α and β) are usually filled, except in the axis, by a dark granular matter; and the tubes therefore present the appearance of dark edges with a longitudinal central clear line. The length, in some instances, extended over more than one field of the microscope. Each ends in a cul-de-sac, which in this instance is not swollen out; and often where there are no cell-contents for a little space the absence of special walls can be readily determined. Usually no origin or ending of the tubes can be seen; but in a few instances their commencement in dark spots at the margin of the coral-wall can be readily seen (fig. 6). These spots are either not much larger than the tube, or are very much bigger, and are filled with a mass of globules with whose exterior the tubes seem to be continuous. Occasionally small dark pigment-masses with a definite globular shape are in contact with one end of the tubes (fig. 14). The large masses are probably the remains of resting spores or oospores; and the others are conidia (fig. 3). With regard to the numbers of the tubes β there appears to be no regularity; in some places they are very widely apart, and in others crowded; but they never appear to inosculate with others, but simply branch.

In one or two tubes in the specimens examined there are dark spots; and in one the calibre is swollen out at one spot.

γ. There are here and there very minute tubes which ramify frequently and in a short space, so as to be very dendritic in appearance; they are densely black and opaque, and their diameter is about one half that of the other tubes.

2. More or less globular conidium-like masses are either separate or crowded, and in this last instance often are in linear series, (α) They constitute moniliform bodies (fig. 3), sometimes with tubular projections. (β) They are in evident linear series, but are disconnected; nevertheless traces of excavations, which probably are relics of old tubes which once contained them, are occasionally visible.

3. Tubes having a calibre twice as large as the others, or even more, and whose contents are discontinuous, dark and often in the form of the conidium-globule (fig. 8).

There is a piece of a Brachiopod shell in the matrix within the calicular fossa of the coral; and it shows tubes β to perfection; and they look like so many straight and curved wires (fig. 4).

The tubes mentioned under section γ, and the more or less irregular black spots with which they are continuous, readily receive explanation after the study of the Algæ parasitic in the Thamnastræa from Tasmania (fig. 1), and of Calceola sandalina (fig. 11).

In the Thamnastræa the enormous multitude of tubes simulate radiating spicula in appearance, and here and there one or two can readily be traced running into a black mass. This irregular shape produced by the growth of the Alga depends on the special molecular structure of the coral. If the tubes were obliterated by fossilization, and the black spaces, not unlike the lacunæ of bone, remained, the appearance would greatly resemble that of some parts of Goniophyllum.