90 



COELENTEEATA— ANTHOZOA 



PHYLUM II 



succession 



The twelve pairs of third cycle mesenteries are found to develop in a 

 'which is altogether unexpected. They follow the same dorso- 

 ventral order as the second cycle pairs, but in two series. A primary series 

 of six pairs — one pair within each sextant — appears within the exocoele on 

 the dorsal aspect of each of the second cycle mesenteries, one pair following 

 upon another, and then another series of six pairs arises on the ventral 

 aspect of the second cycle mesenteries in the same order (Fig. 128). In the 

 later stages of development the regularity of the mesenterial succession is not 



3E Ts: 



re E 



m 



m 



Fio. 126. 



GrowUi stages of coral polyp in Mnrandni {" Manicina") areolata. Diagrammatic figures showing orfler of 

 appearance of the six primary pairs of mesenteries. In n only two i)airs of mesenteries are present, of which one 

 pair (i) is united with the stomodaeum, while the other (ii) is free ; in /) the second pair of mesenteries has become 

 complete, and a third pair(!ii) has appeared on the ventral border ; in c another pair(iv) is found within the dorsal 

 chamber ; in </ the first four pairs of mesenteries to arise have all become complete, and the fifth and sixth 

 pairs (v, vi) have appeared, but remain incomplete for a long period, the secondary mesenteries ap]jearing in 

 the meantime (cf. Fig. 127). The actual stages given are taken from Maeandra areolata, but a like sequence is 

 presented by other species whose development has been followed (after Duerden). 



always maintained ;■ one region may be somewhat in advance of, or may lag 

 behind its normal development. 



The sequence thus outlined in the briefest manner is sufficient to show 

 that the develoi^ment of the mesenteries in coral polyps is bilateral, and 

 takes place in stages from one extremity to the other. The radial symmetry, 

 characteristic of the adult polyp, is thus derived from primitively bilateral 

 organs, which appear in an antero-posterior succession. Moreover, each cycle 

 represents a separate period of development, as compared Avith the successive 

 growth in one direction of ordinary segmented animals. 



The first two cycles of tentacles (prototentacles) generally arise a cycle at 

 a time, either simultaneously or one following the other. The later tentacles 



