CONTRIBUTION TO KNOWLEDGE OP RHABDOPLEURA. 627 



attached or recumbent portions of the tubarium are not true 

 rings, but are formed by two obliquely-set pieces, which appear 

 to overlap one another. Gradations between the extreme form 

 of two oblique half-segments and a single complete seamless 

 ring are found (PI. XXXIX, fig. 4). This difference in the 

 structure of the proximal and distal portions of the tubarium 

 is simply due to the form of the buccal-disc by which 

 they are secreted. The complete rings of the upstanding 

 tubes are secreted by the complete circular buccal-disc of a fully- 

 grown polypide (PI. XXXVIII, figs. 1 and 5). The curious ob- 

 liquely-divided segments of the recumbent portion of the tuba- 

 rium are always secreted by an immature polypide, 

 such as that seen at a, fig. 1, PI. XXXIX. In the immature 

 polypide the buccal-disc has, as pointed out by Allman, a very 

 curious bilobed form. The axial portions of the tubarium are 

 necessarily always formed by what may be termed the 

 '^ leading'^ bud of each branch, and, similarly, the proximal 

 portion of each polyp-tube is formed by the polypide when in a 

 young condition. The assumption of the adult form, either 

 by the leading bud of a branch, or by any one of the nume- 

 rous lateral buds of a branch, coincides with the change of 

 direction of growth of the corresponding part of the tubarium, 

 viz. it proceeds to grow at right angles to the surface of at- 

 tachment; it coincides also with the production of complete 

 ring-segments, in .the place of the obliquely-notched pieces. 



A further examination of the tubarium of Rhabdopleura 

 shows that it is divided internally by transverse septa 

 (PI. XXXIX, fig. 1, p) into a number of separate chambers. 

 These septa occur in the various branches of the axial portion 

 of the tubarium, and are so placed that a portion of the axial 

 tube giving off a single polyp-tube from its side is shut off from 

 similar portions in front and behind, each of which also gives 

 origin to one polyp-tube. 



The significance of this arrangement is seen when the ter- 

 minal portion of a budding branch is examined (fig. 1, a, and 

 fig. 7, PI. XXXIX). It is then seen that the buds lie one 

 behind the other in this portion of the axis, and that they are 



