■REPORT ON CORALS — HYDROCORALLIISLE. 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 (PI. 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 arc cylindrical in form, with a short conical hypostome, and four, five, or 

 six tentacles arranged ecmidistantly in one whorl just below the hypostome. The tentacles 

 consist of a short, stout, cylindrical stem, with a spheroidal knobdike 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, G Z. 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 (I.e. PL XVI. fig. 6). 

 They appear to be closely similar to the piriform cells described by Allman, and figured 

 by him as occurring in Gemma ria implexa. 1 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 li 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, Gyninoblastic and Tutmlariau Hydroids, pi. viii. fig. 5. 



