TUBAKIUM OF CEPHALODISCUS NIGRESCENS. 23 



branches. These arise mainly from this upper surface, and where a young branch is 

 being developed the apertures of those tubes on the relatively main branch that lie 

 close around it are symmetrically disposed around its base, and have the lips on the 

 edges of the tubes most remote from the axis of the new branch. 



The above generalisation holds good in the main, but in places one meets 

 with a most irregular disposition of the apertures of the tubes, and a pair of tubes 

 opening to the exterior within 10 mm. of one another may have their apertures 

 facing one another, and yet have no trace of a developing branch between them. 



A secondary branch behaves in its development as a foreign organism. The 

 polypides of the new branch, having begun to secrete their tubes at some small area 

 of the surface, incommode the polypides of the main branch in their immediate 

 vicinity, and cause them to distort their tubes so that the openings are well outside 

 the area settled upon. If a young branch be roughly handled, it breaks off and 

 leaves an irregular flat or concave scar composed of soft test only, none of the tubes of 

 the secondary branch running into the main branch (fig. 5, a and b, plate 3). Should 

 any deserted tubes of the main branch open within the area upon which a new branch 

 is growing, it is covered over with common test, and, unlike the inhabited tubes, is 

 found in the scar (fig. 5, c). 



In a young branch there is a marked contrast between the new tubes of its base 

 and the old tubes of the main branch opening around its base. The old tubes have 

 longer, thicker, and browner lips. The common test of a young branch resembles 

 that of the apex of an old branch in being softer than usual, paler, and more 

 transparent, and in being composed of thicker strata. 



Until this species of Cephalodiscus shall have been studied in the living state it 

 will be impossible to make any definite statement with regard to the mode of growth 

 of the colony, but so far as one can judge from the material available, the deductions 

 are as follows : the fully-grown buds, after severing their connection from the parent 

 stolon, emerge from the tube within which they have been growing, and wander over 

 the surface of the branch. They are gregarious, and eight or ten of them collect 

 either at the apex of the branch, or, in the case of an old branch, settle in a patch 

 at some convenient spot on the surface and start producing a new branch. They 

 secrete profusely a soft investment common to them all, within with each lives in a 

 sharply bent or curved tubular cavity with a bulbous blind end. By the addition of 

 numerous thin layers to the interior of the neck of the bulb the " tube " becomes 

 differentiated from the " test." These lining layers are continuous with the margin 

 of the tube, where they are comparatively thick layers and resemble so many super- 

 imposed rings. The mouth of the tube projects slightly above the general test, and 

 has no lip. 



As younger polypides than these migrate to the apex of the new branch and 

 secrete profusely, the polypides under consideration lengthen their tubes in an 

 obliquely radial direction in order to avoid being covered in incidentally developing 



