No. i.] MORPHOLOGY OF THE ACTINOZOA. 1 53 



ing fact that all the forms so far mentioned are strictly bilateral, 

 both in their structure and in the formation of their secondary 

 mesenteries. The Hexactiniae are in reality bilateral also, so 

 far as their structure is concerned, as was demonstrated by the 

 discovery of the directive mesenteries by Schneider ('71), and 

 in the early stage of their development bilaterality also prevails. 

 At the conclusion, however, of the development of the mesen- 

 teries V and VI, a radial arrangement appears to step in, the 

 new secondary mesenteries arising in pairs in all the mesente- 

 rial spaces. It is difficult to understand why a radial symmetry 

 should be thus suddenly superimposed upon previous bilaterality, 

 and I believe that such an idea is erroneous. The three forms, 

 Scytopliorns, Gojiactinia, and Oractis seem to me to indicate 

 that there has been a gradual development of the second cycle 

 mesenteries of the Hexactiniae, upon a bilateral plan. A single 

 secondary mesentery first formed in each dorso-lateral inter- 

 mesenterial space, the tendency to develop mesenteries singly 

 and bilaterally, one on each side of the so-called dorso-ventral 

 axis, which exists in the Edwardsiae, Ceriantheae, and in the 

 larval twelve-mesenteried stage, still persisting. In the next 

 stage a second mesentery is added in the same space on each 

 side, forming a pair with that already present, the original single 

 bilateral method of mesentery formation still persisting. With 

 the formation of this pair, however, this tendency is replaced 

 by one which leads to the formation of all future mesenteries in 

 pairs, the bilateral arrangement, however, still persisting, and 

 so the condition found in Oractis is reached. Finally, a third 

 pair of mesenteries is added in each ventro-lateral intermesen- 

 terial space and the Hexactinian condition is reached. 



If this scheme of development be correct, it is evident that 

 the Hexactinian condition with twelve pairs of mesenteries is 

 truly radial neither in structure nor in its phylogenetic develop- 

 ment, and this latter fact explains the interesting observation 

 of the Messrs. Dixon upon the embryo of Bunodes tJiallia already 

 referred to (p. 133). 



There is one point, however, in their account which needs 

 correction, — that is, the comparison they make between a sec- 

 tion taken about the middle of the body of the Bunodes embryo 

 and a section of a Pcacliia. In both cases ten pairs of mesen- 

 teries are present, but in the Bunodes embryo it is the dorso- 



