REPORT ON CORALS—HYDROCORALLIN &. 21 
Soft structures of Millepora. 
Structure of the Zooids.—The pores of Millepora are occupied by two kinds of 
zooids. In each system of calicles the central larger one is occupied by a short and 
broad gastrozooid provided with a mouth, whilst the surrounding smaller calicles lodge 
longer and more slender dactylozooids which have no trace of mouth. A system 
of expanded zooids is shown (Pl. XIV. fig. 1), one of the dactylozooids being omitted 
in the drawing in order to show the central gastrozooid more clearly. 
Gastrozooids.—The gastrozooids are much shorter and broader than the dactylo- 
zooids. They were not directly measured but were estimated to be about *5 mm. in 
height. They are cylindrical in form, with a short conical hypostome, and four, five, or 
six tentacles arranged equidistantly in one whorl just below the hypostome. The tentacles 
consist of a short, stout, cylindrical stem, with a spheroidal knob-like tip composed almost 
entirely of nematocysts. At the summit of the hypostome is the mouth, which in the 
living expanded animal has a conspicuous glistening white appearance—no doubt because 
light is strongly reflected by the large gastric cells which surround the aperture. 
The mouth-area is circular in outline (in Millepora alcicornis quadrangular some- 
times), Pl. XIV. fig. 2, GZ. The circular area is occupied by a series of large, elongate, 
transparent gastric cells, which are disposed in a radiating manner around the centre of 
the area. The actual mouth-orifice takes the form either of a threefold or cruciform slit 
between the gastric cells. The gastric cells (Pl. XIV. fig. 7) are elongate, irregularly 
cylindrical in form, and transparent and bladder-like in appearance, and without any 
trace of a nucleus. They line the internal cavity of the gastrozooid for at least one-third 
of its length, but to what extent exactly was not ascertained. They are figured by 
Professor Agassiz in a figure of a gastrozooid of Millepora alcicornis (1c. Pl. XVL fig. 6). 
They appear to be closely similar to the piriform cells described by Allman, and figured 
by him as occurring in Gemmaria impleca.' They are here termed gastric, because 
the fact that they occur only in the gastrozooids seems to render it probable that they 
exercise a digestive function. 
Dactylozooids.—The dactylozooids are long and slender in comparison with the gastro- 
zooid. They differ very much in length, as will be seen from the figure; the longest of 
them measure about 13 mm. in length. They are cylindrical in form, tapering towards the 
upper extremity. They have no trace of a mouth, nor of any of the gastric cells of the 
gastrozooids in their body-cavity. They bear tentacles at irregular intervals from near 
the bases to the summits of their bodies. The tentacles are very variable in number; 
some zooids have only five tentacles, whilst all numbers from five to twenty (and possibly, 
in exceptional cases, a slightly greater number) occur in others. From twelve to fifteen 
is the most usual number. The tentacles consist of a cylindrical stem, longer and more 
1 Allman, Gymnoblastic and Tubularian Hydroids, pl. viii. fig. 5. 
