RHABDOPLEURA AND CEPHALODISCUS. 15 
connects the full-grown polypides of the colony. It is called by Lankester (18) the 
“ gymnocaulus,” the hard stalk being termed the “ pectocaulus.” The gymnocaulus 
gives off a series of buds in regular succession (text-fig. 7, 9, /, ¢, d, c) at the basal end 
of the terminal “ proliferous polypide,” which remains incompletely developed so long 
as it continues to produce buds from its stalk. Each bud remains in that part of the 
axial or growing part of the colony in which it was formed, and becomes partitioned 
off by the formation of a transverse septum (j), which stretches across the tube, and 
is traversed by the organic stalk of the colony, which shortly afterwards becomes 
hardened (pectocaulus, 2). The bud, which has already developed a shield and a pair 
of plumes, now breaks through the side of the portion of the tube within which it is 
enclosed, and forms a lateral tube, commencing at the point of eruption, by the secre- 
tion of successive rings. It now enlarges and differentiates further, and becomes an 
adult polypide. 
This succession of buds along the stalk of a terminal polypide of Ehabdupleura 
appears to have no direct equivalent in Cephalodiscus, and in Cephalodiscus there is 
no differentiation into ordinary polypides and proliferous polypides. All the polypides 
of Cephalodiscus seem to be capable of budding, the buds being formed around that 
area at the free end of the stolon where the severance of the polypide in question 
took place, when, as a full-grown bud, it separated from its parent polypide. Search 
was made for some such regularity of bud-succession as occurs in Rhabdopleura, but 
without success (see plate 7, figs. 69-84). 
Returning to the consideration of the tubarium, it is to be noted that the tubes 
of Cephalodiscus levinseni and C. nigrescens do not arise as outgrowths from a main 
tube as do those of Rhabdopleura, but, so far as one can judge from a study of non- 
living material, each new cavity is from the first independent of other cavities 
(see plate 4, fig. 10). The newest tubes are the short ones at the apex. In 
other words, the bud of C. nigrescens separates off from its parent and forms 
an entirely new tube of its own, whereas the bud of Mhabdopleura breaks out 
laterally from the main or axial tube and remains in organic continuity with the 
parental stolon. 
In the species of Cephalodiscus of the sub-genus Demuothecia (1.e., C. dodecalophus, 
C. hodgsoni, C. sibogae, and C. gracilis) the polypides are more social than those 
of Idiothecia, and co-operate in the secretion of a common envelope, which may 
in a general way be described as an irregular branching tube with numerous ostia. 
The part played by the younger and the older polypides respectively in the building 
of the tubarium is not known, but most probably all polypides act in concert, and 
additions to the existing envelope are made by young and old indifferently. 
As regards the general external appearance of the polypides, Cephalodiscus and 
Rhabdopleura have much in common—the division of the body into a buccal shield, 
a collar region with plumes, and a trunk region, larger than the others, and with a 
ventrally placed stalk (cf. text-fig. 8 and plate 6, fig. 49). 
VOL, II. R 
