On the Structure of a Species of Millepova. 179 



to eight in number. In histological structure, as also in chemi- 

 cal composition *, the coralla of the genus Millqwra seem to 

 show no marked differences from Anthozoan coralla. 



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

 the larger centi'al calicles of the systems, has from four to six short 

 knobbed tentacles, and is provided with a mouth and certain gas- 

 ti'ic cells, closely resembling those figured by AUman as occurring 

 in Gemtnnria implexa t. The other kind occupies the smaller cali- 

 cles, is longer and more slender than the mouthed zooid, has from 

 five to twenty tentacles, and no trace of a moutli. The usual num- 

 ber 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 

 striatiou, or apparent septa, so characteristic of the tentacles of hy- 

 droids. They have spheroidal heads composed of masses of thread- 

 cells. 



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

 tudinal muscular fibres, which are disposed in bundles, and are 

 attached inferiorly to the vessels of the hydrophyton which join 

 the somatic caviiy at the base of the zooids. Circular muscular fibres 

 are possibly also present. As in Hellipora, only a thin layer at 

 the surface of the coral is living. 



The soft parts of the hydrophyton consist of a network of canals 

 and vessels occupying the corresponding canals in the corallum. 

 The canals are composed of an ectoderm and an endoderm. The 

 ectoderm rests on a thin layer of membrane. It is mainly composed 

 of fusiform finely granular cells with an oval nucleus, but is much 

 modified in certain regions. In the upper part of the living layer 

 its cells are abundantly converted into the parent cells of thread- 

 cells, and on the actual surface into a layer of prismatic cells 

 showing at the very surface hexagonal outlines. This layer is 

 believed to be continuous over the whole outer surface of the coral. 

 It is continued down into the calicidar cavities, and in the con- 

 tracted condition almost closes their orifices. The endoderm con- 

 sists of two elements— yellow pigmented cells closely similar to 

 those of other hydroids, and small transparent highly refracting 

 globules. The pigmented cells are abundant in the somatic cavi- 

 ties of the zooids, and in the canals and vessels of the hydrophyton. 



They impart a bright yellow colour to the tips of the tubercles 

 of the li«ng coral. The canal-system of the hydrophyton anasto- 

 moses most freely with the somatic cavities of the zooids, and 

 establishes a free communication between them. Two kinds of 

 thread-cells are present. The one is of the peculiar form occurring 

 only in Hydrozoa, viz. that which has in the expanded condition a 

 short, wide, bladder-like structure at the base of the thread next 



# Structure and Classification of Zoophrtes, by J. D. Dana (Philadelphia, 

 1846), Appendix, p. 130. Corals and Coral Islands, ejusd. (London, 1872), 

 p. 105. 



t Gymnoblastic and Tubularian Hydroids, pi. viii. fig. b. 



