fig. 9) in the rinys which constitute the creeping portion of the tubes. Lankester (84, p. 626) 

 also emphasises this supposed difference between the peristomial rings and those of the attached 

 parts of the tubes. The sutures of the peristomial rings are, however, clearly described and 

 figured b)- ScuiiroTiEFF (04, p. 6, PI. I, fig. 3). 



There can be little doubt that the tube-rings of Rhabdoplejira are successively constructed 

 by the proboscis or buccal shield, as stated by L.\nkester (pp. 624, 626). It may fairly be 

 assumed, with M'Ixtösh (87, p. 6) that the coenoecium of Cephalodiscus is formed by the same 

 organ, the greatly developed glandular epidermis of wliose anterior side apj^ears well fitted 

 for this task. 



C. Icvinsetii. (PI. II, figs. 11 — 13. PI. IX, figs. 108 — iio). 



The coenoecium of this species is characterised b)- the following features : — (I) by its 

 definite la\v of growth; (II) by the existence of a distinct and separate tubular cavity for each 

 of the zooids; and (III) by the prolongation of this tube into a well marked free portion or 

 peristome, the orifice of which has a characteristic oblique form. These features are illustrated 

 by PI. I, fig. 10 and PI. II, figs. 11 — 13. 



Fig. I 2 represents the details of the peristome A of Fig. 1 1 . The tube has an accurately 

 cylindrical cavity and is lined by a certain number of longitudinally disposed lamellae, which 

 are not shewn in this figure, although some of them are represented in fig. 13. The wall of 

 the tube is specially thickened on the side which passes into the projecting lip of the orifice. 

 This thickened side forms a sort of mid-rib, which clearly serves as a supporting structure. The 

 lamellae of which the peristome is composed closely resemble the tube-rings of Rhaódopletira, 

 as is well .seen when they are examined in optical section ie.g., left hand side of fig. 13). The 

 ring then appears evenly convex distall)', and concave proximally where it overlaps the next 

 lamella. In the greater part of the peristome, the lamellae are very oblique, sloping proximally 

 from the abaxial side, or side of the mid-rib. In the more proximal part of the peristome, they 

 are broad, indicating a rapid period of growth ; while more distally they become more crowded 

 and irregular (fig. 12). They probably never form complete rings, but the majority of those 

 which are not merely concerned in the construction of the mid-rib form half rings, starting from 

 the mid-rib, passing across the whole of one of the lateral surfaces of the tube, and ending in 

 a point on the axial side. This structure is illustrated by the lamella a in fig. 12. The deposition 

 of these lamellae may be alternate, so that if a forms jjart of what may be termed, for descriptive 

 purposes, the "right" wall of the peristome, ó will form part of the "left" wall, the two lamellae 

 overlapping at the mid-rib. Distally this arrangement is by no means constant ; but in the proximal 

 region of rapid growth the lamellae may alternate quite regularly for a considerable distance. 

 When the peristome is examined from the axial side, the interdigitating pointed ends of the 

 half rings are often conspicuously seen. 



A orlance at fisf. 1 2 will shew that the lamellae which constitute the mid-rib are far 

 more numerous than those which form the rest of the tube ; and it will further be seen that 

 this disparit)- is due to the fact that many of the lamellae are concerned exclusively in building 



SIBOGA-EXPEDITIE XXVI iÜM. 2 



