324 



CHAPTER XX. 



GENERAL CHARACTERS AND DIVISIONS OF 

 THE ALCYONARIA. 



The second great division of the Actinozoa is that of the Alcyonaria 

 or Octactinice, defined by the possession of polypes with eight pin- 

 nate ly fringed tentacles, the mesenteries and inter?nesenteric chambers 

 being also eight in number. The corallum is usually sclerobasic, or 



spicular, or formed of both an 

 axial sclerobasis and detached 

 spicules. In other types, the pol- 

 ypes composing the colony may be 

 provided with separate " thecce." 

 The Alcyonaria are essen- 

 tially distinguished from the 

 Zoantharia by the possession 

 of eight unpaired mesenteries 

 and eight tentacles (reduced in 

 some rare cases to six or four). 



The mesenteries (fig. 209) are 

 symmetrically grouped round the 

 oesophagus, so that there is a 

 dorsal intermesenteric space, and 

 a ventral one, together with three 

 lateral compartments on each 

 side. The " directive " mesen- 

 teries of the ventral side (fig. 209, 

 Nos. 1, 1), have the longitudinal 



/;// .. 



Fit;. 209. — Transverse section of a polype of A I- 

 cyonium, enlarged. (After O. and R. Hertwig.) 

 The numbers indicate the four pairs of mesenteries. 

 s, CEsophagus transversely divided ; Im, One of 

 the eight longitudinal "retractor" muscles of the 

 mesenteries ; ve, Ventral side of polype ; do, Dorsal 

 side. Nos. i i, and 44, are the "directive" mes- 

 enteries. 



retractor muscles attached to 

 their opposed faces ; whilst the opposed sides of the dorsal directive 

 mesenteries have the transverse muscles. The mesenteries are not in 

 pairs ; and the order in which they appear has not been precisely in- 

 vestigated, the numbering given in the annexed figure not being certainly 

 known to express the order in which these structures are developed. 



