THE ANTIQUITY OP THE ZOANTHID ACTINIANS. 195 



THE ANTIQUITY OF THE ZOANTHID ACTINIANS. 



J. E. DUEUDEN. 



The zoiintliids are a distiuct group of actiniaus found in plenty only 

 in the warm tropical seas, where they are often an important constituent 

 of the luxuriant life of the shallower parts of coral-reefs. They are sep- 

 arated from the two other groups of the Actiniaria — ordinary actinians 

 and cerianthids — chiefly by their method of increase in number of the 

 mesenteries beyond the primary six pairs. In the Zoantheae the new 

 mesenteries arise as bilateral pairs at only two regions of the polyp, one 

 on eacb side of the ventral directives or axial pair of mesenteries; in the 

 Actiniae, as here restricted, new mesenteries are added in couples within 

 all the six primary interspaces ; in the Ceriantheae additional mesen- 

 teries arise at only one region of the polyp, which is axial. In addition 

 to this fundamental distinction the mesenteries in the zoanthids form 

 only a single cycle, and are disposed in couples, of which one moiety is 

 large (macrocnemes) while the other is small (microcnemes) ; further, 

 the tentacles are arranged in only two cycles, an inner and an outer ; the 

 polyps frequently give rise hy budding to incrusting colonies; a skeleton 

 is generally formed by the inclusion of foreign particles (sand-grains, 

 foraminifera, sponge-spicules, etc.) within the wall; and the stomodaeum 

 is provided with only a single stomodaeal groove or siphonoglyph. 



The adventitious skeleton of the zoanthids is constituted of separate 

 particles, and disappears as such when the polyp dies and its soft tissues 

 decay. Consequently there is the greatest improbability that any evi- 

 dence of the group will ever be found in the rocks, any more than in the 

 case of the altogether soft-bodied sea-anemones. Indeed, as regards the 

 Actiniaria as a whole, it is very unlikely that we shall ever be able to 

 determine their geological antiquity in a direct manner, that is, by find- 

 ing their actual remains or impressions in the rocks. Indirectly, how- 

 ever, we seem to have good grounds for certain conclusions regarding 

 their past history. 



As regards ordinary soft-bodied actinians we have very close repre- 

 sensatives in modern polycyclic hexameral corals. Aij extended study 

 of the polyps of a large number of species of both groups proves that 

 between the skeletonless actinians and the coral forming polyps there 

 are no morphological differences of any importance, except such as con- 

 cern the formation of the skeleton ; as regards the development and na- 

 ture of the mesenteries and tentacles the polyps show no divergence. 

 Actinians and hexameral corals have undoubtedly sprung from a com- 

 mon stock; we are justified in regarding them as members of a single 

 group, in the one case provided with a limy skeleton and in the other 

 case devoid of such. 



From such a close relationship we seem warranted in concluding that 

 whatever be the geological antiquity of the hexameral corals the hex- 

 actinians will have a like ancestry; skeletonless species have probably 

 always existed alongside of coral-producing forms and would be the first 

 to appear. 



