ON THE DIPLOCHORDA. 719 



3. The Pericardial Sac and Heart, Pre-oral Cavity, 

 Pre-oral Canals, Glomerulus, Collar Cavities, and 

 Blood-vascular System. — The pericardial sac lies ante- 

 riorly to the distal extremity of the subneural gland. 

 In most specimens it is nearly square in cross-section, 

 but may be compressed at its base as in fig, 6. Roughly 

 its cross-section is about "05 mm., and its length about 

 •08 mm. It appears to be a closed sac formed of very 

 delicate endothelium; its posterior wall is invaginated to 

 form the heart. This inner wall is thickened, and has 

 numerous muscular fibres stretching across the cavity of the 

 sac to its outer wall (figs. 6 and 2). It is doubtless con- 

 tractile, and the shape of the pericardial sac varies greatly 

 according to its state of contraction. On its ventral wall 

 there is a fairly constant transverse groove (fig. 1). The sac 

 lies in the blood-space or cavity between the walls of the 

 pre-oral and collar cavities, and its walls do not differ except 

 in their extreme delicacy from those of these cavities. In 

 transverse sections it is seen that the pericardial sac is bent 

 over the apex of the subneural gland dorsal ly and ventrally 

 (figs. 7 and 8). Laterally it is bounded by the wall of the 

 pre-oral cavity, which is thickened into an epithelial lining of 

 the pre-oral canal. In fig. 6 both pre-oral canals are clearly 

 seen, and the right pre-oral canal is cut thi'oughout its length 

 from the pore at the base of the ectodermal pit to the inner 

 opening on the wall of the pericardial sac. The canal is 

 lined by a delicate columnar epithelium, apparently ciliated. 

 In this connection we may note the statement of Ehlers (2) 

 that the "proboscis canals" of Cephalodiscus end in blind 

 sacs. There can be no doubt whatever that Mcintosh and 

 Harmer were perfectly correct in stating that they open 

 freely into the pre-oral cavity, though in a specimen examined 

 as a transparent object the pre-oral canal might appear to 

 terminate in the pericardial sac. 



I have elsewhere (7) described the blood-vascular system 

 of Cephalodiscus, and wo have here to notice that, as 

 indicated by Harmer (4), the organ I first took to be the 



