18 PERCY SLADEN TRUST EXPEDITION 



of Actinians. The oral-disc muscle is continued as a much weaker layer into the outer 

 wall of the edge-zone. R. Hertwig (42) laid great stress on the systematic value of the 

 circular muscle for both the genera and species of Actinians. 



The mesenterial musculature consists of a longitudinal layer of muscle-j&bres on either 

 side of every mesentery attached to its mesoglasa (PI. 1, fig. 4, PL 2, fig. 13, PL 3, fig. 27). 

 As a rule, on all the mesenteries except the directives, the entocoelic layer is much better 

 developed than the exocoelic, the mesenteries bearing entocoelic mesogleeal pleats or 

 muscle-banners (Faurot's "feuillets") for the insertion of the fibres. These, usually known 

 as the "retractors" (Fowler, Bourne, etc.), are attached from the oral-disc to a varying 

 distance below the enterostome, and usually extend from the stomodaeum to the column- 

 wall; they are best developed in the stomodfeal region of the mesenteries. No specialised 

 parietal muscles are present as in Actinians ; the entocoelic muscular layer may, however, 

 be regarded as representing on that side both the parietal and the " faisceau unilateral " 

 of Faurot. By the contraction of this entocoelic layer, a downward pull is exerted on 

 the whole oral-disc, thus bringing about the shortening of the whole polyp. In the 

 non-pleatal region of the primary mesentery, a varying distance from the stomodaeal 

 attachment of the latter, the entocoelic muscle-fibres are obliquely directed inwards and 

 lie within the mesoglaea. They are obviously attached to the stomodeeum, and by their 

 contraction bring about the widening of the stomodeeum which is a characteristic 

 condition of the retracted polyps. The exocoelic layer has about the same vertical 

 extent, but the fibres are quite perpendicular, fewer and thinner ; by their contraction 

 they can only aid the entocoelic fibres in shortening the polyp, and hence the term 

 "protractors" as applied to them by Fowler (26, p. 252) and others is a misnomer*. 

 The exocoelic fibres are more distinct on the inner half of the primary mesentery in the 

 stomodseal region, presumably because the entocoelic fibres in the inner half function for 

 widening the stomodteum rather than for shortening the polyp. 



Fowler (44) believed that the exocoelic muscle-fibres of the mesenteries could be 

 continued into the tentacles to form their external longitudinal coat, but to this view 

 there are tw^o obvious objections : (a) the fibres have, on such a view, to pierce their way 

 through the mesoglaea of the oral-disc in their upward course to the tentacles ; (6) the 

 external tentacular muscle-fibres are said to be ectodermal, whereas the exocoelic fibres 

 are endodermal ; moreover Fowler's fig. 5 is only a diagram. Exocoelic pleats are present 

 only in a few species. 



The condition of musculature is reversed on the directive mesenteries. 



Hollard (71) and the Hertwigs described the muscular fibres on the faces of the 

 mesenteries of Actinians opposite the longitudinal retractor muscles as running trans- 

 versely, while Faurot could find neither fibres nor pleats on that side and suggested 

 that those authors might have mistaken the simple folds formed by the contraction 

 of the mesoglsea for permanent pleats. The parietal muscle of Actinians is unrepresented 

 on the exocoelic side of the mesenteries of coral polyps. 



* Gardiner observed a more or less similar condition in Caenopsammia. On p. 366 he says "On the faces 

 of the mesenteries, opposite to the great retractor muscles, there are a few isolated longitudinal muscles with a 

 similar course.... There do not appear to be any definite protractor or transverse muscles." 



