6 THE VOYAGE OF H.M.S. CHALLENGER. 



I have as yet only given a general survey of the anatomical and histological parts 

 composing the organisation of the Actiniaria ; it now remains for me to discuss the 

 differentiations shown by the histological elements in their nature, distribution, and 

 arrangements in the various parts of the body, and to show how we may thereby acquire 

 a knowledge of the more accurate characteristics of these parts. 



The pedal disk does not present much worthy of notice ; it has a slightly developed 

 endodermal musciilar layer, always running circularly, which is often even wanting ; in the 

 centre there are sometimes, but rarely, one or more small openings, through which the 

 water can find entrance and exit ; as yet, however, such openings have only been 

 observed where the pedal disk and wall pass continuously the one into the other, which 

 condition is usually described as absence of the pedal disk. Radial furrows may also 

 run on the outside of the pedal disk, and usually correspond to the insertions of the 

 septa on the inside (PI. IV. fig. 2 ; PI. IX. fig. 5). The wall is much more complicated 

 both on its endodermal and its ectodermal sides ; on the former there often lies a layer 

 of circular muscular fibres, which appears everywhere as a flat or slightly folded lamella, 

 but is also often more strongly developed in certain places, and forms a special muscular 

 cord acting as a sphincter. The sphincter or circular muscle usually lies immediately 

 below the upper margin of the wall, which it draws together like a bag over the oral 

 disk and the tentacles if the latter require shelter from any threatened danger. A 

 second sphincter, lying further down, may also be added to the upper sphincter. 



The nature of the sphinet-ers varies greatly. We talk of a " difiuse " s^jhincter 

 when it merely arises from repeated pleatings of the musculax lamella ; because in that 

 case it is not sharply defined at the upper and lower margins (PI. V. fig. 8), it does not 

 strike the eye in looking at the surface, and is shown in transverse section only by the 

 local thickening of the wall in whose substance it is completely embedded. A " circum- 

 scribed " sphincter is formed when the pleated muscular mass projects above the inner 

 surface of the wall, with which it is connected only by a narrow band, so that an annular 

 swelling arises which is easily observed both in looking at the surface and in transverse 

 section (PI. VII. figs. 2, 4). Finally, in the " mesodermal " sphincter, the muscles 

 have left their original position in the epithelium, and are completely hidden in the 

 supporting substance, which consequently increases doubly or trebly in thickness (PI. 

 VII. fig. 7 ; PI. VI. figs. 1-3). 



The complete absence of the sphincter is comparatively rare. I have only observed it 

 in a few species {e.g., in the representatives of the genus CoraUimorphus), almost invari- 

 ably animals which are not caj)able of contracting the upper margin of the wall over the 

 oral disk. This is, however, also the case in animals with a weak sjahincter, such as the 

 Antheadse. On the other hand, the existence of a strong circular muscle can often be 

 inferred with tolerable certainty from a high degree of contraction. The capacity for 

 concealing the oral disk plays an important part in the systematic division of the 



