POSTLARVAL DEVELOPMENT. log 



4. 



its radial plan before the next cycle commences to form, when the additions 

 take place in such a manner as to again confer bilaterality upon the polyp 

 as a whole. Thus the first two cycles of septa become perfectly radial 

 before an additional cycle commences, and the growth of this is then con- 

 tinued in a bilateral manner ; likewise, the new second and third cycles 

 assume their radial stage before the members of the fourth cycle make their 

 appearance, proceeding from one border to the other. In like manner the 

 first-cycle mesenteries are nearly radial before those of the second cycle arise 

 and introduce a conspicuous bilateral symmetry ; and on these assuming the 

 radial plan the third-cycle mesenteries begin to appear, again in a bilateral 

 manner. 



The successive dorso-ventral growth followed by the constituent mesen- 

 teries and septa of each cycle may also be regarded as conferring a certain 

 individuality upon the cycle. The different cycles, arising independently, seem 

 to represent so many distinct recurring phases of growth in the life of the 

 polyp, not a continuous addition from one aspect to the other, as is usual in 

 permanently bilateral animals, particularly segmented forms. The members 

 of a cycle appear in a dorso-ventral sequence, and may retain their differences 

 in size for a long time, but in the end they become equal and thereby confer 

 radial symmetry upon the polyp. Then another cycle commences to form 

 in somewhat the same bilateral dorso-ventral succession, displays for a time 

 its consecutive origin, and afterwards attains radiality. 



The conception of recurring phases of growth in cyclic coral polyps is 

 best realized when comparison is made with the mesenterial increase char- 

 acteristic of the Ceriantheae. Here the mesenteries beyond the protocnemes 

 always develop in a regular bilateral successive manner, from the dorsal 

 (anterior, sulcar) to the ventral (posterior, asulcar) aspect, the oldest being 

 dorsal or anterior and the youngest ventral or posterior, recalling more the 

 method of growth of segmented animals ; in cerianthids there is never a 

 reversal of growth to the anterior end, followed by a successive series to the 

 other, such as occurs in ordinary hexactinians. Employing the term "band 

 of proliferation," introduced by Van Beneden in 1897, we ma y sa Y there is 

 only one median band of proliferation in ceriauthids, while in hexactinians 

 there are many such bands, the number increasing with age at first 6, then 

 12, 24, etc. 



In the Zoantheae also mesenterial growth is alwa} T s in the same succes- 

 sion after the protocnemic stage. The increase takes place within the two 

 exoccelic chambers on each side of the ventral directives ; there are only two 

 bands of proliferation or zones of growth. In this case, however, the order 



