when it does so, it is looked upon as the mark and characterisation of the allied genus Arachnoidiscus.
In a diagrammatic section of an AlôÙcyoôÙnarôÙian poôÙlype (Fig. 172), we have eight chamôÙbers set, symôÙmetôÙriôÙcalôÙly, about a ninth, which consôÙtiôÙtutes the ãstomach.ã In this arôÙrangeôÙment there is no difôÙfiôÙculty, for it is obvious that, throughout the system, three boundaries meet (in plane section) in a point. In many corals we have as {388} simple, or even simpler conditions, for the radiating calcified partitions either converge upon a central chamber, or fail to meet it and end freely. But in a few cases, the partitions or ãseptaã converge to meet one another, there being no central chamber on which they may impinge; and here the manner in which contact is effected becomes complicated, and involves problems identical with those which we are now studying.
In the great majority of corals we have as simple or even simpler