540 T. C. Brown — Development of the Mesenteries 



In such genera as Ithodactis, Halcampa, and Manicrna a 

 course of development is followed which is indicated in the 

 diagram, fig. 4. This differs from the preceding figure only 

 in the order of appearance of two pairs of the first eight 

 mesenteries. The asulcar directive mesenteries are the fourth 

 to appear instead of second as in the preceding examples. 



In fig. 5 is shown the order of appearance of the mesenteries 

 in Aiptasia diaphana. Here the eight first mesenteries appear 

 in exactly the same order as that illustrated in fig. 4, but the 

 muscle banners on the first pair of mesenteries to appear are 

 located on the ascular instead of on the sulcar side of these 

 mesenteries. Furthermore, the fifth and sixth pairs of mesen- 

 teries appear almost simultaneously in the space between the 

 first and second pairs ; as a result that pair which probably cor- 

 responds to the sixth appears on the ascular side of mesentery 

 number one instead of on the sulcar side. Additional couples 

 of mesenteries are introduced as indicated in the figure. 



In Gonactinia prolifera a condition occurs in which there 

 are sixteen mesenteries present as indicated in fig. 6. Eight 

 of these are complete and extend from the outer wall to the 

 stomodseum ; these correspond exactly with the eight mesen- 

 teries in Edwardsia. The other eight are all incomplete ; two 

 pairs of these correspond to the fifth and sixth pairs of mesen- 

 teries in the normal actinian and form couples with two of the 

 complete pairs ; the other two pairs form couples in the asulcar 

 exocoeles. 



A consideration of these figures shows very clearly that the 

 order of appearance of the eight primary mesenteries is not 

 constant ; that the position of the muscle banners on these pri- 

 mary mesenteries is not constant (e. g., position of muscle ban- 

 ners of first pair of fig. 5, compared with the corresponding 

 pair in figs. 2, 3, 4); and that the point at which the later 

 mesenteries are introduced is not constant (compare the sixth 

 pair in fig. 5 with the corresponding pair in figs. 2, 3, 4). 



Lacaze Duthiers pointed out long ago that the first two pairs 

 of mesenteries in modern Zoantharia appear soon after one 

 another, and that there is then a pause before the third pair 

 appear. The fourth pair succeeds the third pair rapidly and a 

 longer pause ensues. Finally the fifth and sixth pairs appear 

 almost simultaneously. Thus in the early development the 

 factor four is the dominant factor, not six as is generally 

 assumed. This prominence of the factor four in the early 

 development of typical hexameral forms, and the prominence 

 of the Edwardsian stage in the development of all of these 

 forms, have led the writer to believe that all Anthozoa are 

 derived from forms with an original tetrameral plan, and that 

 upon this original tetrameral plan a radially symmetrical hex- 

 ameral plan has been superposed in the case of the Mesozoic and 



