in the Zooids of Anthozoa. 537 



When skeletal structures are formed these are composed of 

 plates (sclerites) of carbonate of lime, and the septa within the 

 corallites always bear a direct relation to the intermesenterial 

 spaces. 



There is at least one modern genus which seems to almost 

 bridge the gap between these two widely separated divisions 

 of the Anthozoa, namely Heliopora. Moseley* has clearly 

 shown that this is an Alcyonarian with eight complete 

 mesenteries, and Bourne has further shown that in this type 

 the calcareous skeleton is built up in precisely the same way 

 as in the scleractinian or stony corals by a layer of cells derived 

 from the ectoderm and calleda clicoblasts. In all the Alcyo- 

 naria except Heliopora the calcareous skeleton consists of 

 spicules, each spicule forming as the entoplastic product of a 

 cell, f Hence in so far as the origin and minute structure 

 of the skeleton is concerned there is no great unbridged gap 

 between Alcyonaria and Zoantharia. 



A section across the stomodseal region of an Alcyonarian 

 zooid shows that the arrangement of internal structures is 

 simple and definite. These are shown diagrammatically in 

 fig. 1. The zooid is clearly bilaterally symmetrical with the 

 axis of symmetry coinciding with the long axis of the stomodseum. 

 A ciliated stomodseal groove (the sulcus) is located at one side of 

 the stomodseum. Eight complete mesenteries extend from the 

 outer wall to the stomodseal wall and each of these bears a muscle 

 "banner" (cross section of a longitudinal muscle band) on that 

 side which is toward the sulcar side of the zooid. This is the 

 simplest type of complete Anthozoan zooid ; occasionally 

 imperfect or modified individuals may show fewer parts but 

 this is the typical plan in all complete and perfect individuals. 

 Attention is called to the fact that this zooid is built on the 

 plan of eight or two times four, not on the plan of six. 



Among the Zoantharia or typical hexameral zooids the 

 simplest form is represented by the modern genus Edwardsia, 

 a type which does not build a skeleton. Figure 2 shows that 

 this form is bilaterally symmetrical ; a ciliated groove is located 

 at either side of the stomodseum, the sulcus at the lower (ventral) 

 and the sulculus at the upper (dorsal) side. The muscle 

 banners on the four axial mesenteries (the directive mesentric 

 couples) are turned away from one another and away from the 

 axis of symmetry ; those on the other four mesenteries are 

 turned toward the sulcar side of the zooid. The cross section 

 of this zooid differs in only two particulars from that of an 

 Alcyonarian ; there are two ciliated grooves in the stomodseum 



* H. N. Moseley, Phil. Trans. London, vol. clxvi, p. 91 fie, 1876. 

 f G-. C. Bourne in "A Treatise on Zoology," Lankester ed., Anthozoa, 

 pp. 79-80. 



