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



mesenteries may be seen through it as clear stripes. In the external zone of its mesoglcea 

 lie the deposits above mentioned, consisting exclusively of Foraminiferal skeletons. 

 They are evenly distributed over the ccenenchyme ; but on the body- wall are ranged in 

 a most regular and elegant manner, the following facts being recognisable with the 

 aid of a lens. From the apex outwards run, in a well-grown individual, fifteen to twenty 

 looping rows of Foraminifera in clear elevated lines. Where the body-wall bends 

 downwards at right angles, each row bifurcates, and each branch so produced runs 

 downwards on the body-wall in a straight line ; a single row of Foraminifera is thus 

 situated over each mesentery, the insertion of the latter being externally clearly 

 recognisable, owing to the thinness of the wall. While therefore, from the apex of the 

 polyp outwards, the ridges agree in number with the pairs of mesenteries, in the lower 

 part of the body-wall there are present as many rows of shells as there are individual 

 mesenteries. Towards the base these become less plain, so that at the lowest part 

 of the polyps, as on the ccenenchyme, the Foraminiferal coating is evenly distributed 

 all over " (Erdmann). The rows of shells are continued on to that region of the body- 

 wall which has been drawn inwards ; and their arrangement can here be only understood 

 by referring to the point of transition from body-wall into oral disc. This occurs along 

 an undulating curve, since at one point the oral disc with its outer circlet of tentacles, 

 at another the body-wall with its rows of shells, projects the farthest. A horizontal 

 section therefore, through the region under discussion, meets alternately with rows of 

 Foraminifera and the origins of tentacles (PI. IY. fig. 8). Further, at the point of 

 junction, the body-wall forms a strongly projecting fold in which lies the greater part 

 of the sphincter (PL IV. fig. 7). The horizontal section represented in fig. 8 exhibits 

 this fold on the inner side, while on the outer lie the body-wall and oral disc, united 

 by mesenteries. 



The fold of the body-wall bears, on both sides, rows of Foraminiferal shells, 

 supported on ridge-like processes of the body-wall, and appearing therefore in trans- 

 verse section as coronets ; they are, as we learn from longitudinal sections, discontinuous 

 at the free edge of the fold, so that the outer and inner rows of shells do not pass into 

 each other. 



The sphincter embedded in the fold of the body-wall is mesoglceal and simple, and 

 forms here an evenly distributed complex mass of muscle-bundles, the latter being 

 variously shaped. It also overlaps a small strip of that region of the body-wall which 

 is not drawn inwards. 



The tentacles are, as in other cases, in two alternating circlets, and are in part 

 produced into long pointed filaments, in part contracted into short stumps. Their 

 muscles are ectodermal and slightly pleated ; the mesoglceal supporting lamina lying 

 at the base of the pleats sends processes into the epithelium. 



The stomatodseum is oval, and the siphonoglyphe only slightly expressed. 



