468 fx. HERBERT FOWLER. 



but of a peripheral plate (a) at the side of which is fastened a 

 lateral plate (6) ; the latter unites below with a central plate (c) 

 which projects radially outwards from the oral disc and stomo- 

 dseum, and is of course merely that part of the mesentery 

 which is on the central side of the stoma or perforation repre- 

 sented in fig. 23. The peripheral and central parts are alone 

 shown in fig. 11, a section taken above the level at which the 

 lateral plates grow out. In the thickened and slightly 

 muscular free edge of the peripheral plates it is easy to recognise 

 the homologue of the muscular thickenings of the mesenteries 

 numbered. 



Continuous centrally with {c), laterally with (a) and {b), and 

 inferiorly and peripherally by a sort of tendon {d) with the 

 pedal disc, is the huge retractor muscle, the general relations 

 of which certainly suggest that it is shifting off from the 

 mesentery. This interpretation is even more strongly suggested 

 by figs. 19 to 22, which are taken from a " vertical'^ series, — 

 that is, parallel to the plane of section in fig. 23, and read from 

 without inwards. They are, therefore, approximately trans- 

 verse to the muscle itself, and exhibit its relations to the pedal 

 disc. The whole structure is of considerable interest, and, if 

 the Anthozoan origin of coelomate animals be accepted, may 

 even be taken to indicate the method of evolution of primitive 

 muscles (aggregates of muscle-fibres) at the sides and from the 

 walls of the archenteric coelomic pouches. 



The mesenteries which exhibit this curious shifting of the 

 muscles are eight in number, and, as already mentioned, they 

 are grouped in the same manner as in the Edwardsise (fig. 12) ; 

 a pair of " dorsal " directives is followed on each side by two (not 

 a ''pair'' of) mesenteries, with their muscles on the "ventral" 

 sides ; and these face towards a pair of '' ventral " directives. 

 They carry what appears to be a trilobed filament ; the ova lie 

 in the usual position, embedded in the mesogloea lamina 

 (fig. 22). 



Phylogenetic Considerations. 

 The anatomy of Octineon having been described, its syste- 

 matic position demands consideration. That it is a.Hexac- 



