450 On the Structure of a Species of Millepora. [Apr. 6, 



muscular fibres, which are disposed iu bundles, and are attached inte- 

 riorly to the vessels of the hydrophyton which join the somatic cavity at 

 the base of the zooids. Circular muscular fibres are possibly also 

 present. As in Heliopora, 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 endoderm. The ectoderm rests on a 

 thin layer of membrane. It is mainly composed of fusiform fine granu- 

 lar 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 calicular cavities, and in the 

 contracted condition almost closes their orifices. The endoderm consists 

 of two elements — yellow pigmented cells closely similar to those of other 

 hydroids, and small transparent highly refracting globules. The pig- 

 mented cells are abundant in the somatic cavities 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 living coral. The canal-system 

 of the hydrophyton anastomoses 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 occur- 

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

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

 which bladder is armed with three spines set in one whorl. In Mille- 

 pora the spines are unusually long and set at right angles to the thread. 

 This kind of thread-cell alone occurs in the tentacles; it occurs also 

 more sparingly in the hydrophyton. 



The other kind of thread-cell is larger and ovoidal in form, closely 

 resembling that figured by Allman as occurring in Gemmaria impleoca. 

 These thread-cells are confined to the hydrophyton. They form densely 

 set zones around the bases of the zooids. 



The other species of Millepora examined appear to agree in all essen- 

 tial particulars with that occurring at Tahiti. They have mouthed and 

 mouthless zooids, but these are not arranged in regular systems. They 

 have the same two kinds of thread-cells, with a similar distribution. 

 The Tahitian Millepora, like the others examined, is infested by a para- 

 sitic fungus, which exists in the soft superficial tissues, as well as in the 

 substance of the corallum, and has a decided green tint. 



