3 68 



COELENTERATA ANTHOZOA 



but a variable number of other rudimentary and incomplete 

 mesenteries have recently been discovered by Faurot. 1 In the 

 Zoanthidea the mesenteries are numerous, but the order is 

 remarkable for the fact that the dorsal directives are incomplete, 

 and that, of the pairs of metacnemes that are added, one mesentery 

 becomes complete and the other remains incomplete. In most of 

 the genera of the Antipathidea there are only ten mesenteries, 

 but in Leiojpathes there are twelve, and as they bear no bands 



DI). 



VD 



DD 



Fia. 163. Diagrams of transverse sections of 1, Alcyonarian ; 2, Edwardsia ; 3, Ceri- 

 anthus ; 4, Zoantlms ; 5, Favia ; 6, Madrepora. DD, the dorsal directive mesen- 

 teries ; VD, the ventral directives ; /- VI, the protocnemes in order of sequence. 



of retractor muscles it is difficult to determine accurately their 

 true relation to the mesenteries of other Zoantharia. 



In the Cerianthidea the mesenteries are very numerous, and 

 increase in numbers by the addition of single mesenteries alter- 

 nately right and left in the ventral inter -mesenteric chamber 

 throughout the life of the individual. These mesenteries do 

 not bear retractor muscles. 



In the Actiniaria and Madreporaria, with the exception of 

 the genera Madrepora, Porites, and a few others, there are also 

 very many mesenteries. The two pairs of directives are usually 

 present, but they may not occur in those zooids that are produced 



1 Faurot, Arch. Zool. Exper. 3rd ser. iii. 1895, p. 71. 



