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



may perhaps be symbiotic Algae, which have contracted under the influence of hardening 

 reagents. They are uniformly distributed throughout the hyaline cells of the surface 

 ectoderm, but have not been observed in the stomodseum nor in any portion of the 

 entoderm. This is the only instance known to me of the probable occurrence of symbiotic 

 Algae in the tissues of Antipathinse. 



All the specimens examined contained a moderate number of pear-shaped spermatic 

 capsules imbedded in the tissues of the transverse mesenteries. The stomodseum appears 

 to be continued laterally along the free border of the transverse mesenteries, which then 

 becomes evoluted at certain points into mesenterial filaments. The rounded free 

 margin of each mesenterial filament has, like that of other genera, the same structure 

 as the stomodaeal ectoderm. 



Mesenterial Filaments. 



Considerable diversity of opinion has been expressed on the origin of the mesenterial 

 filaments of the Anthozoa, and it wdl be well to review the position taken up by various 

 investigators in view of the possible origin of these structures in Antipatharia, although 

 embryological data are necessary before a positive conclusion can be arrived at. 



The mesenterial filaments of Hexactiniae contain a dilation at the free margin, 

 which consists of three lobes. The median lobe (Nesseldrusenstreif) consists chiefly of 

 glandular cells and nematocysts, whilst the two lateral lobes (Flimmerstreifen) contain a 

 large number of elongate and ciliated epithelial cells, which are supposed to be concerned 

 in circulation. 



In 1879 von Heider, from a study of the structure of Cerianthus membranaceus, came 

 to the conclusion that the mesenterial filaments are derived from the ectoderm of the 

 stomodaeuni, which becomes invaginated along the free margin of a mesentery. This 

 view was based on the fact that the epithelial cells of the mesenterial filaments have the 

 same character as those of the stomodaeum, into which they pass without any delimitation. 

 A year later the brothers Hertwig combated this view, and adhered to their former 

 opinion that the mesenterial filaments are entirely of entodermal origin. They point out 

 that the structure of the mesenterial filaments is the same in the incomplete as in the 

 complete mesenteries, and that in the former case the filaments never come in contact 

 with the stomodaeum, and thus could not derive cells from it. They point to the fact 

 that histological evidence is insufficient to decide a developmental point, and their 

 observations tend to show that the ectoderm and entoderm of Actiniaria are, histo- 

 logically, almost indistinguishable from each other. Wilson has shown that in 

 Alcyonaria there are two distinct types of mesenterial filaments, which differ in struc- 

 ture, in development, and also in function. The filaments attached to the " dorsal " 

 pair of mesenteries consist chiefly of elongate narrow epithelial cells, which bear strong 



