114 GEOEGE W. FIELD. 



at first supposed by Semon in his earlier work (36), but cor- 

 rected in his recent paper (38). 



The cilia of the oesophagus are found to disappear except 

 over a certain area, forming a loop-shaped ciliated band 

 extending posteriorly and ventrally into the oesophagus, with 

 its anterior end in connection with the above-described cili- 

 ated band bounding the rira of the oral depression, and with 

 this circular band constituting the adoral ciliated band. 



Now to return to a further consideration of the circuraoral 

 band. How does this band, at first single and continuous 

 (fig. 14), reach the condition shown in figs. 24, 30, and 29, 

 the condition characteristic of the Bipinnaria? The change 

 occurs in the six days old larva. The original single bilater- 

 ally symmetrical circumoral ciliated baud (fig. 14, c. o. b.) by 

 a transverse division at the apex of the preoral lobe of the 

 ) ( shaped portion, and by fusion of the divided ends takes a 

 ^ form ; so that a plane passing between the links of the 

 baud, after the division and subsequent fusion of the broken 

 ends, lies at right angles to its first position. At first per- 

 pendicular to the dorsal and ventral surfaces of the larva, it is 

 after the division parallel with them (figs. 25 and 26). There 

 are thus formed from this single band two complete bands : 

 the upper (see fig. 26) bounds the dorsal and postoral area ; 

 while the lower {p. v. a.) comes to lie entirely upon the 

 ventral surface, bounding the preoral ventral area (fig. 30, p. 

 V. a.). The whole history of the ciliated bands can be followed 

 in figs. 13, 11, 12, 14, 23, 25 and 26, 24, 30, and 29. 



By this division of the original single circumoral baud into 

 the two ciliated bands characteristic of the Bipinnaria onto- 

 genetic proof is given, as pointed out by Semon (38), of the 

 correctness of Gegenbaur's hypothesis that the two ciliated 

 bands of the Bipinnaria are equivalent to the single band of 

 the Auricularia (see Balfour's ' Embryology,^ vol. i, pp. 554 

 and 557). Semon calls the stage with the single bilaterally 

 symmetrical ciliated band the Auricularia stage of the 

 Bipinnaria. 



The ciliated bauds in cross -section (fig. 34, c.o.h.) show a 



