148 THE ANATOMY OF INVERTEBRATED ANIMALS. 



acterized by being traversed by numerous tubular cavities, 

 which open at the surface, and the deeper parts of which are 

 divided by numerous close-set transverse partitions, or tabular 

 cUsse2nments, while vertical septa are rudimentary or alto- 

 gether absent. These were regarded as Anthozoa, and 

 classed together in the division of Tabidata^ until the elder 

 Agassiz * published his observations on the living Millepora 

 alcicornis, which led him to the conclusion that the Tabulata 

 are Hydrozoa allied to Hydractinia, and that the extinct Mu- 

 gosa were probably of the same nature. 



The evidence adduced by Agassiz, however, was insuffi- 

 cient to prove his conclusions ; and the subsequent discovery 

 by Verrill that another tabulate coral, Pocillopora, is a true 

 Hexacorallan, while Moseley ^ has proved that Heliopora 

 coeridea is an Octocorallan, gave further justification to those 

 who hesitated to accept Agassiz's views. 



The recent very thorough and careful investigation of a 

 species of Millepora occurring at Tahiti,^ by Mr. Moseley, 

 although it still leaves us in ignorance of one important 

 point, namely, the characters of the reproductive organs, yet 

 permits no doubt that Millepora is a true Hydrozoon allied to 

 Hydractinia^ as Agassiz maintained. The surface of the 

 living Millepora presents short, broad hydranths, the mouth 

 of which is surrounded by four short tentacles. Around each 

 of these alimentary zooids is disposed a zone of from five to 

 twenty or more, much longer, mouthless zooids, over the bod- 

 ies of which numerous short tentacles are scattered. Each 

 of these zooids expands at its base into a dilatation, whence 

 tubular processes proceed, which ramify and anastomose, giv- 

 ing rise to a thin expanded hydrosoma. The calcareous mat- 

 ter (composed as usual of carbonate, with a small proportion 

 of phosphate of lime) forms a dense continuous crust upon 

 the ectoderm of the ramifications of the hydrosoma, that part 

 of it which underlies the dilatations of the zooids constituting 

 the septa. As the first formed hydrosomal expansion is com- 

 pleted, another is formed on its outer surface, and it dies. 

 The " thecal " canals of the coral arise from the correspond- 

 ence in position of the dilatations of the zooids of successive 

 hydrosomal layers, and the tabulte are their supporting plates. 



Thus the grou|) of the Tabulata ceases to exist, and its 



» " Natural History of the United States," vols. iii. and iv., 1860-'62. 

 2 Moseley, " The Structure and Relations of the Alcyonaruxn, Heliopora, 

 carulea^'' etc. (" Proceedinijs of the Roval Society," November, 1875.) 

 8 " Proceedin^rs of the Pvoval Societv," 1876. 



