140 IVAN E. WALLIN 



ated bands. The third and remaining arches have a broad 

 ciHated band covering the medial and cephalic aspect. These 

 bands are directly continuous with the ciliated band on the epi- 

 pharyngeal ridge. They have no connection with the endostyle 

 in this stage of development and I have been unable to deter- 

 mine whether such a connection exists or not in younger larvae. 



The moving apart of the second pair of gill arches is very 

 suggestive of an approach to the condition in fishes where the 

 arches are attached in the lateral part of the roof of the pharynx. 

 Accompanying this lateral migration there is a loss of the ciliated 

 band on the arch. 



The arrangement of the ciliated bands as described above 

 does not persist in older larvae. This undoubtedly accounts 

 for the contradictory descriptions given by Anton Schneider 

 ('79) and Schaffer ('95, 1 and 2) and others. In a larva 9.5 mm. 

 in length growth and differentiation of the epithelium of the gill 

 arches has resulted in a new arrangement of the ciliated bands. 

 This new arrangement has gained its permanent larval condition 

 in a 15 mm. larva. 



In the older larvae the median dorsal ciliated band which 

 represents the fused continuation of the dorsal ciliated grooves 

 of the first arch ends in the median dorsal line betw^een the sec- 

 ond pair of gill arches. Immediately caudad of the dorsal at- 

 tachment of the second pair of gill arches two ciliated bands 

 appear on the ventro-lateral part of the epipharyngeal ridge. 

 Tracing these bands in a caudal direction, they are seen to come 

 together and fuse in the median ventral part of the ridge at the 

 caudal end of the dorsal equivalent of the third gill pouches. 

 From this single band a branch is given to each of the third pair 

 of gill arches. In the median line the ciliated band ends as a 

 pointed process in the angle between the dorsal attachments of 

 the third pair of gill arches. This arrangement of the ciliated 

 bands is repeated in the remaining arches and dorsal equiva- 

 lents of the gill pouches (text fig. 3, and c.e. in text fig. 1). In 

 the eighth pouch, however, the median A'entral fused part does 

 not give off any lateral branches corresponding to the ones given 

 off to each gill arch in the third to the seventh arches. This is 



