STRUCTURE OE A SPECIES OP MILLEPORA. 
119 
soft structures were either mounted entire for examination, or cut in the usual manner 
into fine vertical and horizontal sections. The sections were stained with carmine or 
magenta. The specimens hardened in osmic acid, and decalcified after subsequent 
immersion in absolute alcohol, yielded the best histological results. Those which had 
been hardened in absolute alcohol alone gave the best results as to the coarser anatomy. 
The specimens preserved directly in glycerine preserved most perfectly the forms of the 
several histological elements, and especially yielded good preparations of the thread- 
cells, preparations of which are best procured by grinding up between two glass slides 
a zooid and its immediately surrounding calcareous bed, removed with the point of a 
scalpel. A view of the structure unacted upon by acids is thus obtained. The speci- 
mens placed in chromic acid were of little service for sections, owing to a thick crystalline 
deposit of sulphate of lime which formed upon them in the solution ; but they showed 
best, on the under surface of the decalcified superficial film, the ramifications of the 
soft parts of the hydrophyton. 
Structure of the Corallum. 
The structure of the corallum is illustrated on Plate 2. The corallum has a widely 
spread encrusting base covering rocks, dead corals, &c., and at its surface presents a 
series of projecting, short, irregular tubercles and lobules, which never rise to any consi- 
derable height. Pig. 3 represents the appearance of two lobules of the corallum and 
a portion of a third, enlarged two diameters. The surface of the lobules is uneven and 
covered with slight rounded elevations. The calicles, or pores, of the zooids are dispersed 
over the entire surface both of the lobules and of the flatter encrusting portions of the 
corallum, being absent only at the tips of some of the lobules, which are possibly those 
that are in rapid growth. The calicles are disposed in irregularly circular groups, a 
larger calicle being in the centre of each group or system, with usually from five to 
eight smaller calicles arranged around it. These systems of calicles often occupy small 
rounded prominences on the surface of the corallum, and in parts of some specimens 
almost every system appears to have its separate small prominence. In some regions 
of the corallum the systems are scarcely defined, the calicles appearing irregularly 
placed ; but such an arrangement is only exceptional in the present species. An entire 
system of calicles has. been accurately drawn for me by Mr. J. J. Wild, and is repre- 
sented in Plate 2. fig. 4, enlarged eighty diameters. The outlines of the calicles are 
seen to be extremely irregular ; their cavities are encroached upon in all directions by 
projections of the contorted trabecular cceneiichymal tissue of the corallum. The 
larger central calicles of the systems measure about 1*5 millim. in diameter. 
The main mass of the corallum is composed of trabeculse of dense calcareous matter 
which forms a spongy-looking mass traversed in all directions by tortuous canals. In 
some species of Millepora the corallum is much more dense than in the Tahitian one, 
and in these might rather be described as a compact mass in which a series of tortuous 
channels are excavated for the reception of the soft structures^ In such species of 
