THYMUS-LIKE STRUCTURES IN AMMOCOETES 



139 



iiig pairs of gill arches is the ventro-lateral angle of the epi- 

 pharj^ngeal ridge. Ventrally, they are attached near the ven- 

 tral median line, a Uttle to the side of the endostyle. The 

 second pair of arches are pecuHar in that they contain no cili- 



t h i 



Text fig. 2 Camera lucida outline drawings to illustrate the course of the 

 dorsal ciliated grooves (the ventral grooves are not shown) from the diverticula 

 in the first pouch to the point where the grooves meet in the median dorsal line 

 fo form a single band of ciliated epithelium, e.g., dorsal ciliated groove; d.a., 

 dorsal aorta; e.p.. epithelial placode; «., notochord. 



