1876.] Structure of a Species o/Millepora. 449 



No Millepora appears to have been hitherto known to occur at Tahiti. 

 The name of the species of the one the structure of which is described 

 in the paper was not ascertained. It resembles M. tuberculosa, as 

 described by Milne-Edwards *, in outward form, but differs from it in 

 having the calicles of two kinds disposed on the surface of the corallmn 

 in regular separate systems, in this respect resembling more closely 

 M. plicata, M. foliata, and M. Ehrenbergii as described by the same 

 author. 



The coral was examined in the fresh condition, and also preserved 

 in alcohol, chromic acid, and glycerine, and treated with osmic acid. 

 Hardened specimens were decalcified and examined by means of sections. 

 The corallum is a spongy mass composed of more or less contorted 

 trabecule of calcareous matter, which is disposed in a series of thin 

 layers following the contours of the surface, and representing successive 

 additions by growth. Within these layers ramify a series of canals 

 which give off branches and subbranches, the whole ramifications being 

 intimately connected with one another, and with the calicular cavities, by 

 a network of smaller channels. The main canals are sometimes large 

 enough to be easily seen by the naked eye, and run for as great a 

 distance on the surface of the corallum as 1| inch. This system of 

 branching canals is held to be characteristic of the coralla formed by the 

 hydroid genus Millepora, distinguishing it from all other coralla. The 

 calicles are of two kinds, small and large. They are disposed on the 

 surface of the corallum in irregularly circular systems. A large calicle 

 occupies the centre of each system, and is surrounded by a ring of 

 smaller calicles, usually from five to eight in number. In histological 

 structure, as also in chemical composition t, the coralla of the genus 

 Millepora seem to show no marked differences from Anthozoan coralla. 



The zooids are of two kinds. The one, short and stout, occupies the 

 larger central calicles of the systems, has from four to six short knobbed 

 tentacles, and is provided with a mouth and certain gastric cells, closely 

 resembling those figured by Allman as occurring in Gemmaria implexa %. 

 The other kind occupies the smaller calicles, is longer and more slender 

 than the mouthed zooid, has from five to twenty tentacles, and no trace 

 of a mouth. The usual number of tentacles in the mouthless zooid is 

 about twelve to fifteen. The tentacles are larger than in the mouthed 

 zooid, and disposed at irregular intervals along the body. They show 

 the transverse striation, or apparent septa, so characteristic of the tenta- 

 cles of hydroids. They have spheroidal heads composed of masses of 

 thread-cells. 



The zooids of both kinds are provided with well-marked longitudinal 



* Hist. Nat. des Corallaires, pi. F3. figs, la, lb. 



t Structure and Classification of Zoophytes, by J. D. Dana (Philadelphia, 1846), 

 Appendix, p. 130. Corals and Coral Islands, ejuscl. (London, 1872), p. 105. 

 \ Gymnoblastic and Tubularian Hydroids, pi. viii. fig. 5. 



