REPORT ON THE ACTINIARIA. 19 



(PL II. fig. 4). This circular ridge appears in transverse section as a pushing out of 

 the body-waU, the circular muscle exhibiting a very different structure in the different 

 regions. At the base of the organ the pleating of the muscle-lamella is insignificant, 

 indeed weaker than at other points of the body-wall, but at both edges of the 

 evagination it is exceptionally strong, and more especially so at the boundary of the 

 oral disc. When the section comes to the actual spot on which one of the stomidia 

 is set, the inner sphincter — as we call the nearest muscular pleating — is beautifully 

 recognisable as a ridge projecting inwards, into the axis of which protrudes a mesogloeal 

 ingrowth. From this axial ingrowth are given off on both sides richly branching 

 mesogloeal lamellae, clothed by powerful muscle-fibres in transverse section. At the 

 remaining points, where the oral disc presents no stomidia, the sphincter is less clearly 

 bounded, and resembles more the outer sphincter, which is essentially nothing but an 

 approximation of muscular folds at two closely- adjacent points. 



Relatively to the size of the animal, both sphincters are weak ; a consequence of 

 this is the fact that they have not drawn up the body-wall over the mouth disc, but 

 that stomatodseum and oral disc have rather been pressed outwards. 



The oral disc recalls in appearance a toadstool, having a faintly flesh-coloured 

 surface, covered by whitish, slightly elevated spots. These spots are the stomidia or 

 tentacles, which are distributed nearly up to the mouth, leaving but a narrow strip 

 free. Between the stomidia the radial furrows run in undulating lines. Their number 

 is difiicult to determine, but may amount to about 400. 



The stomidia are openings in the oral disc, surrounded by a slightly developed 

 ridge, and projecting a little above the surface ; roughly speaking, they are distributed 

 uniformly over the oral disc, or allow only of a vague distinction into several zones. 

 Of these zones one is peripheral, set close to the edge of the oral disc ; one is 

 central, not far from the oral opening ; and two intermediate zones are placed between 

 them. The openings increase in size from without towards the centre, and at the same 

 time undergo an alteration of shape ; in the peripheral zone they are like radially-set 

 slits, with a long axis of 0-7 -I'd mm. ; in the intermediate zones they are circular, 

 with a diameter of 1-2 mm. ; and in the central they again form slits of 2-0-2-5 mm. 

 in the longer diameter, but are here placed at right angles to the radii. 



The structure of the stomidia can best be exhibited by figures of transverse 

 sections. Each stomidium completely occupies the intermediate space between two 

 neighbouring mesenteries, and forms a tube, opening peripherally by a wide mouth. 

 The walls of the tube appear in section to be direct continuations of the adjacent septa ; 

 morphologically their lower part is to be regarded as oral disc, their upper part as 

 rudimentary tentacle; accordingly, they exhibit below the numerous muscular pleatiugs 

 which at other points cause tlie radial ridges on the oral disc, while above these pleat- 

 ings are absent. A remarkable structure is a small circular fold projecting below into 



