142 IVAN E. WALLIN 



of the eighth pouch is directly continuous into the oesophagus. 

 It is interesting to find that the band divides into two por- 

 tions within the oesophagus. These perhaps represent the 

 branches which were given off to the eighth pair of gill arches in 

 an ancestral form. 



Patches of ciliated epithelium are also present on the medial 

 aspect of each gill arch from the third to the seventh inclusive. 

 These ciliated patches undoubtedly were a part of the single 

 ciliated band on the gill arches in the younger larva. With the 

 growth of the non-ciliated epithelium of the gill arches, these 

 patches were cut off from the ciliated band. In the older larva 

 the ciliated band of the gill arch does not occupy the same rela- 

 tive position that it did in the young larva. The ventral end is 

 situated on the lateral side of the gill arch. When traced in a 

 dorsal direction it is found to take a slightly spiral course so 

 that the dorsal extremity which is continuous into the ciliated 

 band of the epipharyngeal ridge comes to occupy a cephalo- 

 medial position. This change in the course of the ciliated band 

 naay also be looked upon as the result of the growth of the non- 

 ciliated epithelium. The ciliated patches on the medial side 

 of the gill arch may acquire a sensory function as Schaffer 

 suggested. 



The arrangement of the ciliated epithelium on the epipharyn- 

 geal ridge in the older larvae is also the result of the growth of 

 the non-ciliated epithelium. In a larva between 8 and 9 mm. in 

 length the non-ciliated epithelium of the dorso-medial part of 

 the gill arches begins to invade the ciliated epithelium of the 

 epipharyngeal ridge. As a result of this invasion the continuity 

 of the ciliated band on the epipharyngeal ridge is lost. The two 

 cords of invading epithelium from the opposing gill arches fuse 

 in the median ventral line of the ridge. The invasion is continued 

 in a caudal direction dividing the ciliated band into two portions 

 which are pushed laterally. This fused portion {pL, text fig. 3) 

 becomes thicker and broader in a caudal direction and ends a 

 short distance cephalad to the attachment of the next pair of 

 gill arches. At the caudal end of this invading epithelium the 

 subsequent growth does not divide the cihated epithelium fur- 



