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(nuclei belonging to the ciliated cells), and the other 

 basal (nuclei of the mucous cells). 



The ciliated cells are long and narrow, with, 

 however, an expansion at the periphery which forms a 

 kind of platform. The mucous cells are very large and 

 wide, their length varies according to the amount of 

 mucus present. Their peripheral ends are, however, often 

 quite attenuated. 



Bernard has made a very detailed study of the process 

 of mucus formation, examining pieces of living tissue 

 from the gland with the microscope. He concludes that 

 before the production of mucus the ciliated cells are all in 

 contact and form a continuous surface without grooves or 

 openings. The mucous cells do not reach at first the level 

 of the surface, but gradually they extend until they 

 appear between the ciliated cells. A small opening occurs 

 and from it the excretion pours out as a drop. The cell 

 continues to secrete mucus. 



The ciliated cells can detach their peripheral 

 portions, which go off as ciliated spherules minus nucleus 

 and with little protoplasm. Finally dead cells of both 

 kinds are expelled and may be seen in the excretion. 



THE ALIMENTAKY CANAL. 



The alimentary canal opens at the true mouth, at 

 the apex of a long retractile proboscis (fig. 11, Prob.), 

 probably not to be seen without dissection in the preserved 

 specimens. The opening which has been noted on the 

 surface of the head below the tentacles is not then the true 

 mouth. At this point the body wall is turned in to form a 

 permanent introvert of considerable length, part of 

 which, however, is again turned on itself to form 



The Proboscis. Text-fig. 4 explains this part of the 



