THYMUS-LIKE STRUCTURES IN AMMOCOETES 



141 



due to the nature of the dorsal attachment of what I have as- 

 sumed must be the eighth vestigial gill arch. This arch, like the 

 second, is not attached to the epipharyngeal ridge but to the 

 angle between the ridge and the dorsal wall of the respiratory 

 pharynx. Cilia, however, are present on the lateral side of the 

 eighth gill arch. They undoubtedly represent the vestigial 

 remains of a condition in which this arch had a dorsal attach- 

 ment to the ridge or its equivalent. The median ciliated band 



Text fig. 3 Diagram illustrating the arrangement of the ciliated bands on 

 the epipharyngeal ridge and the gill arches in a 31 mm. larva. Ventral view. 

 The gill arches which extend ventrally are here represented as extending laterally. 

 The ciliated epithelium is represented in heavy black, the non-ciliated by the 

 stippled part. Roman numbers indicate the gill pouches and Arabic numbers 

 the gill arches, pi., primitive thymus placodes. 



