RHABDOPLEURA AND CEPHALODISCUS. 15 



connects the full-grown polypides of the colony. It is called by Lankester (13) the 

 " gymnocaulus," the hard stalk being termed the " pectocaulus." The gymnocaulus 

 gives off a series of buds in regular succession (text-fig. 7, g, f, e, 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 (_;'), which stretches across the tube, and 

 is traversed by the organic stalk of the colony, which shortly afterwards becomes 

 hardened (pectocaulus, h). 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 Rhdbdopleura 

 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 Rhabdopleura 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 Demiothecia (i.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 



