THE ANTHOZOA 



number of mesenterial couples, each couple consisting of a macro- 

 mesentery and a micromesentery, their muscle banners vis a vis. 

 In the youngest observed stages of Zoanthidea there are six macro- 

 mesenteries and six micromesenteries, whose disposition is shown 

 in Fig. XXIII. 3. They are numbered according to the prob- 

 able order of their development. (M'Murrich's (75) account is 

 followed in preference to that of van Beneden.) It is obvious that 

 the sequence is the same as that observed in Hexactinian larvae, 



FIG. XXIII. 



1. Polythoa, sp. ? showing expanded zooids. 



2. Zoanthus, sp. ? growing on a piece of sponge showing retracted zooids springing from 

 a membranaceous stolon. 



3. Diagram showing the microtypal arrangement of mesenteries in a young Zoanthid. 



4. Diagram showing the macrotypal arrangement. Numerals in 3 and 4 as in Fig. 

 XXI. In both microtype and macrotype all the mesenteries succeeding the first twelve are 

 developed in the sulco lateral exocoeles, shaded in the diagrams. SD, sulcar directive 

 macromesenteries ; Asd, asulcar directive micromesenteries. 



the difference being that the fourth, fifth, and sixth pairs, instead 

 of completing the cycle of twelve equal and complete mesenteries, 

 remain incomplete and are micromesenteries. There is no doubt 

 that the micromesenteries 4, 4, are homologous with the sulcular 

 directives of Edwardsia or Actinia, but their arrested develop- 

 ment as well as the absence of a sulculus suggests that the 

 Zoanthidea have branched off from a parent form common to the 

 Edwardsiidea and themselves, and are not descended from an 

 Edwardsia form, as are the groups hitherto considered. Be this 

 as it may, the subsequent development is peculiar. New mesen- 



