262 



ZOANTHARIA. 



eluded in the Rugosa a tetrameral arrangement of the septa can- 

 not be clearly made out. 



Though showing a general radial symmetry, the majority of 

 the Zoantharians at the same time exhibit a distinct bilaterality of 

 their parts. This bilateral symmetry is shown by the presence at 

 each end of the longitudinal gullet (fig. 138, ce) of a pair of mesen- 

 teries which differ in the arrangement of their muscles from the 

 other mesenteries, and which are known as the " directive " mesen- 

 teries. In certain of the Madreporarians {e.g., Mussa, Euphyllia, 



Fig. 138.— Diagram of a cross-section of Caryo- 

 phyllia, the soft parts being unshaded and the 

 corallum black, ce, Gullet, with the "directive 

 mesenteries " (?;z) at each end ; iv, Body-wall ; 

 s, Septum ; t, Theca. (Slightly altered from von 

 Koch.) 



/ 

 Fig. 139.— Floor of the calice of a typi- 

 cal Rugose Coral (Zaphrentis Enniskill- 

 eni), of the natural size, showing distinct 

 bilaterality in the disposition of the septa. 

 y, The " fossula." From the Carbonifer- 

 ous Limestone. (Original.) 



and Lophohelid) there are, however, no " directive mesenteries," and 

 in such forms a complete radial symmetry obtains. On the other 

 hand, in the typical Rugose Corals very distinct bilateral symmetry 

 usually obtains, and is readily recognised by the general arrange- 

 ment of the septa, or by an exaggerated or reduced development of 

 certain of these structures. In many Rugose Corals this bilaterality 

 is rendered specially conspicuous by the presence of a shallower or 

 deeper groove or pit — the " fossula " — occupied by a limited num- 

 ber of septa of reduced size (fig. 139,/). Sometimes more than 

 one fossula may be present, and the position of the pit thus named 

 varies in different types of the Rugosa, being sometimes dorsal, 

 sometimes ventral, and sometimes lateral. 



There seems to be every ground for believing that the "fossula" of 

 the Rugose Corals is connected with the development of the reproduc- 

 tive organs. On this point the observations which have been made by 

 Moseley on the anatomy of the polypes of the recent genus Seriatopora 

 are particularly instructive. It has been shown, namely, that in Seriato- 

 pora it is only two of the twelve mesenteries (viz., the pair of ventro- 



