198 THE MICHIGAN ACADEMY OP SCIENCE. 



types of Actiuiaiia — zoanthids and tetracocallids, liexactinians and 

 hexacorallids. 



As stated at the coniniencement the zoanthids flourish to-day only in 

 the tropical seas, nnder conditions similar to those favorable for the 

 growth of reef-building corals. A few species are met with in temperate 

 waters, in the same manner as we find a few scattered species of corals, 

 but they nowhere form the large incrusting colonies which they do in 

 warmer seas. Like the reef-building corals, the skeleton— forming repre- 

 sentatives of the zoanthids — the Eugosa — had in past geological times 

 a much wider geographical range. The zoanthid t3pe of coral flourished 

 in Palaeozoic times, the hexactinian types of coral assumed ])redominance 

 in Mesozoic times and has continued until to-day; the soft-bodied polyps 

 of both types still live in abundance in the warm tro])ical seas. 



Hitherto the study of fossil corals has shed no light u[)on the ancestry 

 of the third group of the Actiniaria, the Ceriantheae. It was hoped that 

 among the many Palaeozoic Actinozoa some forms might have been dis- 

 covered in which all the metasepta were added axially, at only one 

 region, as are the mesenteries in the cerianthids, but no such types have 

 been encountered. Therefore, so far as present researches go, it must 

 be assumed that the living cerianthids have no skeleton-producing repre- 

 sentatives, living or fossil, thus differing in this respect from ordinary 

 actinians and zoanthids which give us the hexacorallids and tetra- 

 corallids. 



In all probability, however, the Ceriantheae date back beyond the 

 Actiniae and Zoantheae, for in them the primary mesenteries (protoc- 

 nemes) are represented by only four bilateral pairs, while in the two 

 other groups there are six primary pairs. Of these six mesenterial pairs 

 the four pairs corresponding with those of cerianthids are early com- 

 plete, while the additional fifth and sixth pairs remain incomplete either 

 throughout life or become complete only in the later stages of develop- 

 ment. In any genealogical scheme of the Zoantharia one would place 

 the cerianthids with four protocnemic pairs as the lowest members, and 

 above these the actinians (hexacorallids) and zoanthids (tetracorallids) 

 with their six pairs of protocnemes, the two latter groups separating 

 with the commencement of the metacnemic stage. 

 J. E. DuERDEN, Ph. D., Assistant Professor of Zoology, 



Zoological Laboratory, University of Michigan, 



Ann Arbor. 



