112 GEORGE W. FIELD. 



development, and secondly on account of the briefness of the 

 time in which both pores are functional. However, a number 

 having two water-poreS were isolated, and upon subsequent 

 examination of these the right pore was found to have closed, 

 while in two instances the process was observed. 



But these facts, taken with the finding in the surface col- 

 lections of several undoubtedly normal individuals of a similar 

 age with two bilaterally symmetrical water-pores and water- 

 tubes (in one specimen the water-pore on the right side was 

 found to be obliterated, though the pore canal still persisted), 

 and the exceeding rarity of older larvse with two water-pores, 

 lead to the belief that this stage is a definite one in the onto- 

 geny, and not a pathological condition, as has hitherto been 

 assumed by the Continental students. 



Formation of the Ciliated Bands. 

 Circumoral Band. — From the time of the completion of 

 the segmentation until the formation of the larval digestive 

 tract, all the cells of the surface, of the oesophagus, and intes- 

 tine are ciliated. These cilia serve for locomotion, and for 

 propelling water through the digestive tract. But very early 

 this condition of general ciliation gives way to the restriction 

 of the cilia to definite band-like areas. Often even before the 

 completion of the oesophagus the ectodermal cells of the 

 ventral surface become flattened, and lose their cilia. There 

 are left, however, two narrow transversely extending ciliated 

 areas projecting slightly above the surface, one upon the 

 bulging portion of the body anterior to the mouth, the other 

 posterior, between the mouth and anus (see fig. 13, c. o. b.). 

 As the area where the cilia have disappeared extends dorsally 

 these two bands lengthen, the postoral one being of greater 

 length. The original body cilia disappear last at the apex of the 

 preoral lobe. Fig. 11, PI. XIII, shows a stage where the end 

 of the two parts of the ciliated bands pass into this remnant 

 of the original general ciliation. Later, by the further disap- 

 pearance of the original cilia the two ends of the preoral 

 portion unite with the corresponding ends of the postoral 



