TUBAKIUM OF CEPHALODISCUS NIGRESCENS. 21 



of the tubarium in which the tubes are embedded. The two substances are 

 doubtless of a similar nature, but in consequence of the above differences it 

 will be convenient in the following pages to refer to them as .the "tube" and 

 the " test " respectively. 



The edge of the mouth of each tube is produced into a blunt lip, and the 

 roughness of the surface of the colony is due mainly to these projecting lips. 

 In some portions of the colony the tubes, not merely their lips, project beyond 

 the general surface, even to the extent of 4 mm., as though there had been a 

 deficiency in the production of the common test between the tubes. The lower 

 part of each tube, i.e. the part farthest from the external aperture, is unoccupied, 

 and extends obliquely towards the axis of the branch (see fig. 4, plate 3, and 

 fig. 10, plate 4). 



A transverse section of one of the branches of the colony (fig. G) shows 

 around its edge the lips and distal portions of the tubes, with some of the 

 animals within them ; the central part shows the empty deeper parts of tubes 

 which open nearer to the extremity of the branch than the level of section. 



In the middle of the length of a branch of a colony of average size the tubes 

 have a length of 12 to 17 mm. Their width is nearly uniform throughout (1 -2 or 

 1 • 3 mm), but near the external opening the cavity becomes very slightly wider, and 

 the end of the tube remote from the external opening is usually enlarged into a 

 kind of bulb, with a slightly narrowed neck. This end is closed and blind (fig. 11). 

 There is nowhere in the colony any sign of branching of the tubes, nor of the 

 communication of one tube with another. 



The blind end of the tube has a variable number of thin septa, mostly 

 hemispherical, but occasionally irregular. These one may reasonably suppose to 

 have been secreted successively, the last-formed one of the series being that which 

 is nearest to the polypide, or rather its buds, for these are found towards the blind 

 end of the tube (fig. 11). 



If a tube of the colony be dissected from its neighbours by cutting away the 

 common test that surrounds it, and a longitudinal section be taken, the mode of 

 lengthening of the tube by additions to its margin, and the mode of increase of 

 the common test by the deposition of successive strata between the tubes at once 

 becomes apparent (see figs. 11 and 12). The layers added to the rim of the 

 tube by the polypide are continued down the interior of the tube for some distance 

 as very thin sheets, of darker colour than usual. The terminal portion of the tube 

 is slightly wider than lower down, and these thin sheets have the effect of 

 diminishing the width, so that when the part under consideration is no longer 

 terminal it is no longer wider than the average of the tube. 



The softer common test between the tubes lias a stratification which is more 

 intimately related to the general surface of the liranch than to the tubes. How 

 these layers of the test are deposited by the polypides it is difficult to understand. 



V 



