252 GILBERT C. BOURNE. 



peribrancliial cavities are puslied further and further towards 

 the dorsal surface, so that the stigmata, which in the posterior 

 part of the branchial sac were confiued to its ventral and 

 ventro-lateral aspects, are in the anterior part confined to its 

 dorso-lateral aspects ; and the branchial sac itself becomes 

 progressively wider iu its dorsal and narrower in its ventral 

 moiety as one proceeds forwards. A little below the level of 

 the branchial sphincter the ventral wall of the branchial sac 

 is continued into the spout-shaped ventral prolongation of the 

 branchial siphon, but the relations are somewhat complicated 

 by the fact that in the mid-ventral line the branchial sac is 

 produced into a short, forwardly directed diverticulum, lying 

 ventrad of the branchial siphon. The walls of this diver- 

 ticulum are thrown into longitudinal folds and are sur- 

 rounded by tlie strong muscular fibres of the sphincter and 

 the radial muscles taking their origin from the ring of 

 thickened tissue round the mouth. 



The result of these arrangements is that the obliquely 

 directed mouth-opening faces the opening of the oesophagus 

 into the branchial sac, and the stigmata are confined to a 

 narrow zone running obliquely across from the ventral to the 

 dorsal side of the branchial sac, nearly parallel to the mouth. 

 These relations are clearly shown in the diagram, fig. 34. 

 The space between the stigmata and the mouth is the pre- 

 branchial zone. 



The branchial sac is too short and the stigmata are too 

 closely crowded near its ventral surface to leave much room 

 for an endostyle. Reference has already been made to the 

 fact that in the entire animal, notwithstanding the trans- 

 parency of its tissues, no endostyle could be seen ; never- 

 theless one is present, though in a very reduced and rudi- 

 mentary condition. 



I had great difiiculty in making out the relations of the 

 endostyle, peribrancliial grooves, etc., because the epithelial 

 liuing of the branchial sac wis largely peeled off and lay in 

 strips and patches in its cavity. In places, however, the 

 epithelium remaiued adherent to the walls, and even in the 



