in the Zooids of Anthozoa. 541 



later Zoantharians. This original tetrameral, or, more cor- 

 rectly, bilateral symmetry has been retained unmodified by the 

 Alcyonaria and the Paleozoic Rugosa. 



The importance of this underlying bilateral symmetry 

 becomes more evident when such aberrant types as the Zoan- 

 thids and Cerianthids are taken into consideration. 



The Zoanthids are forms which do not build skeletons ; the 

 order of development of their mesenteries is shown in fig. 7. 

 The first eight or Edwardsian mesenteries develop exactly as 

 they did in those forms illustrated by fig. 4 ; they differ only 

 in that the fourth pair remains incomplete, i. e. , does not 

 reach the wall of the stomodseum. The fifth pair always and 

 the sixth pair generally remain incomplete. Additional mesen- 

 teries are added in couples at only two points, in the sulco- 

 lateral exocoeles adjacent to the third mesentery on either 

 side. Eech couple consists of a complete or macromesentery 

 and an incomplete or micromesentery. The complete mesen- 

 tery is always located on the sulcar side and the incomplete 

 mesentery on the ascular side of the couple. 



In the Cerianthids a very different course of development is 

 found ; this is shown in fig. 8. The first four pairs of mesen- 

 teries that appear have been compared with the first four or 

 Edwardsian mesenteries of Halcampa, but the ciliated groove 

 between the third pair seems to be a sulculus, not a sulcus ; 

 hence the axial direction is really reversed. The remaining 

 mesenteries are added in a peculiar way ; the fifth pair is intro- 

 duced in the space between the fourth ; the sixth pair in the 

 space between the fifth, and so on. Therefore, after the first 

 four pairs, mesenteries are added in pairs and at only one 

 point in the zooid.* Duerden has shown that the mesenteries 

 are introduced in a similar manner in certain of the zooids of 

 Porites. 



The foregoing discussion shows that in a great many (per- 

 haps most) of the Zoantharia, a resting stage occurs when 

 only eight mesenteries have been formed. If at this stage of 

 the development a corallite or skeleton should be formed it 

 evidently would have only four septa and later septa would be 

 added in accordance with the mode of addition of the mesen- 

 teries, since in all Zoantharia with skeletons the septa are 

 known to arise in the intermesenterial spaces. 



It has already been shown that a stage does occur in the 

 Paleozoic Rugosa when only four septa are present, f Fig. 9 



*In fig. 8, and the description here given I have followed the figures of 

 larvae by Boveri and description of development by Bourne. This does not 

 agree with S. J. Hickson, Camb. Nat. Hist. , vol. i, p. 368, fig. 163 (3), 1909. 



fC. E. Gordon, this Journal, (4), vol. xxi, pp. 109-27, 1906. E. G. Car- 

 ruthers, Ann. Mag. Nat. Hist., (7), vol. xviii, pp. 356-63, 1906. T. C. 

 Brown, this Journal, (4). vol. xxiii, pp. 277-84, 1907 ; Ann. N. Y. Acad. Sci., 

 vol. xix, pp. 45-97, 1909. 



