No. i.] MORPHOLOGY OF THE ACTINOZOA. 1 49 



men, of which a section is represented in Fig. 13, possessed 

 seven tentacles (Figs. 10 and 12); the six corresponding to those 

 present in the preceding stage {t x -t z ), and the odd ventral ten- 

 tacle (/ 4 ). This form, as will be seen from Fig. 13, had ten 

 mesenteries, the fifth pair (5) arising in the space between the 

 dorsal directives IV, which are undergoing a separation from 

 each other in the same manner as was seen in the case of the 

 second pair in the preceding stage. It seems evident that in 

 the Ceriantheae a region of special growth occurs at the dorsal 

 surface, localized at the time of the formation of the dorsal 

 directives, and persisting, perhaps, throughout the entire life of 

 the individual. 



VI. The Phylogeny of the Actinozoa. 



It is now generally conceded by those authors who have had 

 an opportunity of comparing the morphological characteristics 

 of a number of the various groups of forms, for convenience 

 associated under the term Actiniaria, that they have descended 

 from ancestors which possessed an arrangement of the mesen- 

 teries similar to that occurring in existing Edwardsias. In the 

 preceding pages I have endeavored to set forth certain facts 

 which tend to confirm this phylogenetic deduction, and to sug- 

 gest explanations of a few points which do not, at first, seem to 

 be in harmony with it. 



It is necessary now to discuss the facts at our disposal with 

 special reference to their bearing upon the affinities of the vari- 

 ous groups of Actinozoa to each other. As regards the rela- 

 tions of the Actinozoa to other Ccelenterates, there is little to be 

 said ; the majority of authors who have committed themselves 

 upon the subject, agree in tracing the Actinozoan stem back to 

 a form similar to the Scyphistoma. The evidence we have seems 

 to point in that direction ; but it must be acknowledged that it 

 is exceedingly scanty, and there are many points of difference 

 between any Scyphistoma of which we have a description and 

 the simplest Actinozoa. It seems probable, however, that the 

 Actinozoa are to be traced back to an ancestor possessing only 

 four mesenteries. The occurrence of an octameral symmetry 

 in the simplest Actinozoa seems to point in that direction, as 

 well as the fact that, in the development of the Hexactinioe, 



