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



is removed from the inside. The colour is caused by numerous small granules, which 

 completely fill the endodermal epithelial cells. 



The lower end of the sac-shaped body, the aboral pole of the longitudinal axis, is 

 denoted by a small depression, which is equally visible on either side, and is caused, it 

 seems to me, by a microscopically small opening found at this spot (PI. XL fig. 11). 

 In order to settle this question by transverse sections, I cut out the portion of the wall 

 containing the opening, and laid it to stain in carmine, but unfortunately it got lost. 



On the exterior in the periphery of the aboral depression there are six sliallow, radial 

 furrows, which soon become less distinct as they run upwards. Instead of them, thii-ty- 

 six longitudinal streaks begin, the external signs of the origins of the septa, which run 

 at an equal distance from one another to the margin of the oral disk. The inside of 

 the wall is covered with a smooth layer of circular muscular fibres, which are never 

 compacted into a distinct sphincter, which explains why the upper end of the wall is not 

 contracted at all. 



The structure of the oral disk, which projected like a proboscis in the animal examined, 

 requires a more detailed description. Thii'ty-six stomidia lie close to its periphei'al 

 margin ; these are fine, longitudinal fissures enclosed by thickened hps, their greatest 

 diameter extending in a radial direction. Through the stomidia we see the inside of the 

 body, looking alternately into an intraseptal and an interseptal space ; they therefore 

 alternate with the septa which are inserted into the narrow portions of the oral disk, lying 

 between two stomidia. Sections perpendicular to the surface of the disk and parallel 

 to its margin thxough the region of the stomidia are, therefore, divided into as many 

 pieces as there are septa, and each piece consists of a septum and the section of the oral 

 disk belonging to it (PL XL fig. 9). 



Distinctly marked radial thickenings (PL II. fig. 11 ; PL XL fig. 6), extending to close 

 upon the oral opening, proceed inwards from the stomidia. They are broad and shallow 

 in the middle of the oral disk, but rise towards the outside and towards the inside into 

 narrow comb-like ridges, the outer ridges being divided into two small folds. These 

 folds end near the stomidia and twist repeatedly during their course so as to produce 

 S-shaped figures. Each two contiguous folds enclose a fissure along which we can pass 

 a needle a little way into the interior of the corresponding radial thickening, which shows 

 that the inner part of the thickening is hollowed out by a radial invagination. 



From the varying relations of the diS"erent parts of the oral disk the transverse 

 sections also present very different figures, according as they are made nearer to or 

 further from the oral margin. Fig. 7 gives a section corresponding to the line S in fig. 6. 

 The supporting lamella is thickened in the middle between each two septa and covered by a 

 repeatedly folded muscular layer. Inwards from this point, in the region of the line e, above 

 each radial chamber, the supporting lamella rises like a ridge which also bears a thickly 

 pleated muscular lamella, as shown in fig. 5. The section given in fig. 3, whose position 



