s 



30 THE VOYAGE OF H.M.S. CHALLENGER. 



the axis of Isis elongate/,, Esper, and other forms, and surround their bases with a black 

 horny mass which becomes separated from a portion of the ectoderm and is homologous 

 with the axial skeleton of Antipatharia. The polyps of a colony are united to each 

 other by basal processes, but have no true ccenenchyma. Koch admits that this species 

 appears at first glance to belong to the true Actiniae, but believes it to differ on account 

 of the presence of a chitinous base secreted by the polyps, which is not distinguishable 

 from that of the Antipatharia. In his discussion of the phylogenetic relations of the 

 Antipatharia, Koch calls attention to the fact that Lacaze Duthiers first showed their 

 relationship to the Hexacoralla through Gerardia, but that on account of the peculiarity 

 of the skeleton of the Antipathidae, the latter still remained an isolated group. Koch 

 thinks he has added a link in Gephyra, which by its skeleton unites the Antipathidae 

 with the Hexacoralla, and proposes to derive Antipathes from skeletonless Hexacoralla 

 similar to the Actiniae of the present day. The polyps in their anatomical and histological 

 structure are quite like many small Actinias, but in Gephyra and Antipathes there are 

 simplifications, particularly in the musculature, to be attributed to a reduction in the size 

 of the polyps, and their union into colonies. The tentacles for the same reasons have 

 become fewer, but scarcely altered. The mouth and oesophagus show no marked 

 variation in Gephyra, Gerardia, and Antipathes. In Antipathes only two mesenteries 

 are fully developed, the remaining eight being more or less rudimentary, but by their 

 position and kind of degeneration we may judge that the ancestor of Antipathes had six 

 tentacles and the same number of antimeres. Gerardia has twenty-four mesenteries and 

 the same number of tentacles, whilst Gephyra approaches the Actiniae closely in the 

 number of tentacles and mesenteries. Gerardia stands alone in having a network of 

 canals uniting the polyps, but Gephyra would in this respect approach it more closely 

 were it shown that the colony results from budding. 



Koch suggests the following as the most probable stages in the phylogenetic 

 history : — 



1. Soft Actiniae which secrete for their support a horny substance by means of 



the basal ectoderm. 



2. Those situated on a slender cylindrical base surround it and enclose it with 



horny matter. 



3. The polyps, by budding, form a colony. Axis ceases to be solely secreted around 



some foreign substance, and now grows independently beyond the limits of 

 supporting substance. 



4. Degeneration sets in in certain parts. 



5. The axis becomes entirely independent. With increase in number, the polyps 



become reduced in size, and connected with this is a reduction in the number 

 of mesenteries and tentacles. 



