-15- 



openinp Into the thyroid they are continued on the floor of the 

 pharynx as a single, shallow, median groove (Figs, 5 j-k), 

 which finally disappears posterior to the thyroid as a groove, 

 hvit continues to t^e oesophagus as a ridge of thickened epithe- 

 lium. In an embryo out sagitally so that the pharynx may be 

 laid open and exposed from above, the ciliated grooves m?.y be 

 seen easily with the naked eye, and still more plainly with a 

 lens, but n£ indication of grooves or bands running from the dor- 

 sal ciliated ridge to the ventral groove is to be made out. 

 A transverse section through one of the gill arches showed the 

 presence of very fine, short cilia, but their arrangement could 

 not be made out. '^his arrangement of ciliated grooves is quite 

 different from that described by Willey in his interesting book 

 "Amphioxus and the Ancestry of the "ertebrates. " On page 168 

 he saye:-"These grooves curve forwards and upwards in front of 

 the gill clefts (after the obliteration of the first gill-pouches), 

 and then proceed backwards on either side of the dorsal middle 

 line of the pharynx as far as the commencement of the oesoph- 

 agus. Fere they appear to curve downwards again, and uniting 

 together, extend forwards as a median ventral groove to the pos- 

 terior lip of the hypobranchial aperture." 



The arrangement of the ciliated bands and grooves 

 then is briefly as follows:- On the floor of the pharynx, begin- 

 ning at or near the opening of the oesophagus. Is a ridge of 

 epithelium on "vhich no cilia could be made out. This ridge. 



