97 



pericardium and the anterior body-cavity are the members of an anterior pair of coelomic spaces, 

 and that they have been segmented off respectively from the distal ends of the right and left 

 cavities of the proximal part of the bud. The view that the pericardium is the fellow to the 

 anterior body-cavity has been suggested for Balanoglossus by Bourne (89, p. 66) and Spengel 

 (93, pp. 682, 689) but it may be remarked that the strictly median position of the pericardium 

 in the later stages of Cephalodiscus and the fact that both proboscis-pores open into the atiterior 

 body-cavity are not in favour of this view. Some of the later stages {e.g. figs. 177 and 179 -f 180) 

 might suggest that the pericardium represents an independent somite. 



Whatever view is taken of figs. 161 — 163 there can be no reasonable doubt that they 

 indicate a segmentation of the coelom beginning at a very early stage in the budding, and that 

 Masterman (98, 2, p. 516) is mistaken in supposing that the coelom remains undivided until 

 after the alimentary canal has become obvious. 



Figs. 175 — 177 represent a later bud, cut nearly sagittally, the parent stalk being however 

 cut more or less transversely to its main axis. The coelomic cavities are now separated from one 

 another, although the third body-cavities are continuous with those of the parent stalk. This is 

 shewn in fig. 175, in which b.cr represents the collar-cavity of one side. It is so closely connected 

 with b. c? as to suggest that the two cavities have been developed as the result of the inter- 

 ruption of a single cavity by the formation of a transverse septum. The anterior body-cavity 

 [b. c?) appears separated from the collar-cavity by a considerable interval, but this is partly 

 because the proboscis is cut somewhat tangentially. Two sections further in the series, the walls 

 of the first and second cavities are continuous with one another, while in the nearly median 

 section shewn in fig. 176, both sets of paired cavities have disappeared, the proboscis being 

 occupied by the anterior coelomic sac [b. c?) in front, while at its base is a smaller vesicle {per.) 

 which can be recognised with certainty as the pericardium. In fig. 177, on the other side of the 

 middle line, four cavities are visible, the pericardium {per.) intervening between the anterior 

 body-cavity {b. c.'^) and the collar-cavity {b. c"), and already shewing the ventral ingrowth of its 

 wall which will give rise to the heart. The alimentary canal is distinguishable, in this bud, as 

 a thick-walled sac opening by the mouth (fig. 176, ;;/.), which is in its definitive position on 

 the ventral side of the proboscis. It is uncertain whether the whole of the internal mass in this 

 figure belongs to the alimentary canal, or whether the posterior portion of it may be part of 

 the wall of the third body-cavity, cut tangentially. A small part of the lumen of the alimentary 

 canal is cut in the neighbourhood of the mouth. As regards its external characters it may be 

 noted that the proboscis is spherical, shewing hardly any trace of the flattening which is .so 

 characteristic of the adult. The boundaries of the collar are slightly indicated externally. 



Fig. 164 is a sagittal section of the terminal sucker of an old stalk, on the anterior side 

 of which are two buds. The younger of these {b.^) has already been described. The older one ((5.-), 

 situated nearer the sucker, shews the continuity of its third body-cavity with that of the parent, 

 while the small ovoid vesicle {b. c.') is almost certainly one of the collar-cavities. In fig. 165, another 

 section of the same bud, the anterior body-cavity {b. c.'^) and the third body-cavity {b. c.^) are 

 unmistakeable, while it is highly probable that the vesicle marked per. is the pericardium and the 

 structure marked b. c.' is the collar-cavity of the opposite side to that which is shewn in fig. 1 64. 



SlBOGA-IiXPEDITIE XXVI W.f. 1 3 



