284 ME. J. E. DUEEDEJf ON 



histological alteration. Its floor presses upon the blind end of the 

 archenteron, and the result is a mutual flattening, leading to a 

 convexity in the former and a considerable lateral extension in 

 each. The two ends ultimately break down in the middle, and a 

 communication between the interior and exterior is thereby 

 established. The lateral walls of the invaginated ectoderm and 

 of the archenteric endoderm form the lining of the adult 

 oesophagus. 



Once the oesophagus is fully established, the lumen at first is 

 extremely small, and circular in transverse section. Even in the 

 older larvse the outline may be circular in some places, but oval 

 in others, the longer axis always corresponding with the sulco- 

 sulcular axis of the mesenteries. No histological change sug- 

 gestive of a gonidial groove ever takes place at either end, a 

 condition not to be expected, as gonidial grooves are not present 

 in the adult. 



The lining is more strongly ciliated than that of the columnar 

 ectoderm ; indeed, it is the most strongly ciliated layer of the 

 whole larva. As in the column a small, deeply-staining enlarge- 

 ment occurs at the base of each eilium. 



The nuclear zone is very pronounced ; the nuclei are oval, 

 closely arranged, and stain a brilliant red in carmine. Clear 

 gland-cells appear to be absent, but granular gland-cells occur 

 here and there, and long, narrow nematocysts similar to those of 

 the column. The mesogloea is extremely thin. Only in some of 

 the most favourable sections of the older larvae can suggestions 

 of a nervous layer be observed, though this is very pronounced 

 in the adult. 



Aborally the oesophagus opens directly into the gastro-coelomic 

 cavity. In perfectly horizontal sections the sulcar and sulcular 

 ends terminate at about the same level. At the inner extremity 

 a slight reflection of the strongly-nucleated lining takes place all 

 round, so that in transverse sections at this point the latter 

 appears cut through twice (PI. 19. fig. 18). The reflected portion 

 is more or less connected with the mesenterial filaments at their 

 origin ; it is further discussed under the section Mesenterial 

 ^Filaments. 



In the adult polyp the oesophagus is greatly elongated trans- 

 versely, so that it stretches nearly across the ccelenteron, the 

 directive mesenteries being much shorter than the four lateral 



