CCELENTERA TA . 



thick subepithelial sheet of fibres and ganglion cells in the pori- 

 stome, the muscles of the ectoderm are weakly developed and are 

 confined to the peristome and the tentacles. The entodermal 

 musculature is much stronger. At the oral end of the column is 

 usually a strong circular (sphincter) muscle which by its contrac- 

 tion can draw the top of the column over the peristome. The 

 septa also bear muscles, on one side running transversely, on the 

 other longitudinally, the latter alone being strongly developed and 

 producing marked ridges (fig. 202) on the septa. 



In the Hexacoralla the septa are arranged in pairs, not only in being 

 close to each other, but in having similar faces turned towards each other. 

 The rule is (fig. 203) that in each pair the sides bearing muscle ridges are 

 turned towards each other, but in two pairs lying m the sagittal axis these 

 muscles are turned outward. From these relations these septa are called 

 directives. It is however to be noted that in our common anemone, Me- 

 tridium, occasionally three, more frequently but one pair of directives 

 occur. The paired condition of the septa allows the recognition of two 

 kinds of radial chambers; between the two of a pair is an intraseptal, 



FIG. 203. Transverse section of actinian (Adamsia diaphana) AB, plane of symme- 

 try, a second lies at right angles. I-IV, septa of four orders. 



between two pairs an interseptal chamber. New septa only appear in 

 the interseptal chambers. At one time all Hexactiuians have but six septa, 

 a pair of directives and, right and left, four lateral septa. With growth, 

 other septa of a secondary order may appear in the interseptal areas, giving 

 six of these. And so with septa of the tertiary order. Irregularities how- 

 ever occur, and forms are found which have abandoned this sexfold plan 



