512 T. C. Brown — Development of the Mesenteries. 



has been drawn to show the probable condition at this stage in 

 the development of such a type as Streptelasma prqfundum 

 from the Middle Ordovicic. Additional mesenteries were evi- 

 dently added in both the counter and cardinal quadrants of the 

 Rugosa as they are in the sulcar lateral exocoeles of the Zoan- 

 thids as suggested by Duerdem Fig. 10 is a view looking 

 down into the cup of an adult specimen of Streptelasma pro- 

 fundum, while fig. 11 shows the hypothetical arrangement of 

 the mesenteries in the stomodseal region of an adult zooid of 

 this species. The apparently perfect radial symmetry pos- 

 sessed by many adult forms among the Rugosa (e. g., Strepte- 

 lasma corniculum and S. rusticum) is a character acquired 

 late in life and is in no way connected with the early develop- 

 ment of the individual. 



As a result of the studies here briefly summarized, and of 

 others not yet completed, the writer has arrived at the follow- 

 ing conclusions : 



1. All Anthozoa, Paleozoic, Mesozoic, and Modern, are 

 derived from one common stem form in which the zooids 

 w r ere bilaterally symmetrical and probably had eight mesen- 

 teries.* 



2. One branch from this common stem, arising early in the 

 Ordovicic, leads up to the modern Alcyonaria (Octocoralla). 

 To this grand subdivision probably belong all such genera as 

 Columnaria, Favosites, Heliolites, Syringopora, etc., which 

 are either without septa, or have apparent septa (pseudosepta) 

 or septal spines which bear no direct relation to the soft parts 

 of the zooid in either number or position. 



3. Another branch from this common stem, likewise arising 

 early in the Ordovicic, embraces the typical tetrameral corals 

 of the Paleozoic — the Rugosa, having eight primary mesen- 

 teries and four primary septa, with secondary septa inserted in 

 the four primary exocoeles in a unilateral pinnate manner, and 

 with tertiary septa inserted late in life in the secondary exo- 

 coeles. This branch of the Zoantharia became extinct, in 

 North America at least, at the close of the Paleozoic. 



4. Another branch from the common stem gave rise to the 

 Mesozoic and later . Zoantharians — Actinians, Scleractinians, 

 Zoanthids, and Cerianthids. All of these pass through an 

 eight-mesentery (Edwardsian) stage in their development, but 

 the great majority of them acquire a later secondary radial 

 symmetry. The particular mode of arriving at this six-fold 

 symmetry varies greatly in the different subgroups. 



Bryn Mawr College, 

 Bryn Mawr, Pa. 



* Compare G. C. Bourne, il A Treatise on Zoology," E. Eay Lankester 

 editor, Anthozoa, pp. 55-56. 



