in the Zooids of Ant/tozoa. 539 



instead of one, and the muscle banners on the snlcar couple of 

 mesenteries are turned away from the axis of symmetry instead 

 of toward it. The tentacles in this type are simple as in other 

 Zoantharians, not branched or pinnate as in the Alcyonaria. 

 Nevertheless, the underlying eight-fold structure is very 

 evident ; furthermore complex Zoantharian zooids with a few 

 exceptions pass through a stage in their development which 

 corresponds to the adult condition in Edwardsia. This has 

 been called the Edwardsian stage. 



Great emphasis has been laid upon the fact that the muscle 

 banners on the sulcar directive mesenteries in the Zoantharia 

 are turned away from the axis of symmetry while in the 

 Alcyonaria they are directed toward this axis. A study of 

 the early stages in Zoantharian zooids, and of the order of 

 introduction of the mesenteries in these, indicates that there 

 are always eight primary mesenteries corresponding to the 

 eight mesenteries of Edwardsia^ but that the position of the 

 muscle banners upon these mesenteries, the particular order of 

 their introduction, and their final position in the zooid, 

 are features which are not constant. A few examples will 

 illustrate these points. 



In such genera as Actina, Sagartia and Bunodes, investi- 

 gations of the early stages of development have shown that the 

 first eight mesenteries to develop correspond to the Edwardsian 

 mesenteries, and that they have the muscle banners similarly 

 placed. The order of appearance of these mesenteries is 

 indicated by the numerals in fig. 3. The first pair appear on 

 either side of the stomodseum and divide the cavity of the 

 zooid into two unequal parts, a smaller sulcar and a larger 

 asulcar space. The second pair is introduced, one on either 

 side of the asulcar end of the axis of symmetry ; these are the 

 asulcar (dorsal) directives. The third pair is introduced in a 

 corresponding position, at the sulcar end of the axis ; these are 

 the sulcar (ventral) directives. The fourth pair is introduced, 

 one on either side of the stomodseum between the first and 

 second pairs. After a short pause the fifth and sixth 

 pairs are added, one in each of the sulcar and middle 

 lateral chambers, making the typical twelve of the hexameral 

 forms. These twelve mesenteries form six couples; two 

 of these, the sulcar and asulcar directives, have the muscle 

 banners turned away from each other. In the other four 

 couples formed by the fourth and fifth, and first and sixth 

 mesenteries on either side of the stomodaeum, the muscle 

 banners are turned toward one another. The spaces enclosed 

 within any one of these couples is called an entocoele ; the 

 spaces between the couples are called exocoeles. It can be 

 clearly seen that these genera pass through an eight-mesenteried 

 stage in the course of their development, and the mesenteries 

 of this stage correspond exactly with those of Edwardsia. 



