286 GELBERT C. BOURNE. 



I will conclude witli a few remarks on the histology of the 

 labial grooves and alimentary tract. 



The right and left labial grooves pass without any distinct 

 line of demarcation into the mouth. The right groove as 

 stated on p. 260 is shallow nnd smooth for a large part of its 

 course posterior to the mouth ; the left groove, on the con- 

 trary, soon becomes deep, and is thrown into numerous 

 vertical ridges. In both grooves the vertical ridges are 

 covered by a very high, ciliated, columnar epithelium, in 

 which no gland cells can be distinguished. But the smooth 

 portions of both grooves are lined by a characteristic epithe- 

 lium shown in fig. 23. The ciliated columnar cells are very 

 distinct, and have a doubly refractive border. Between them 

 are two kinds of gland cells, elongated granular, and ovoid 

 clear cells. The former are elongated and occupy the spaces 

 between the ciliated cells, their free ends reaching to the 

 surface. They are filled with fine yellow granules, and their 

 nuclei are to be found in the inner third of their length. As 

 these nuclei are identical with those of the ciliated cells it is 

 probable that the granular gland cells are modifications of 

 ciliated cells. The ovoid clear cells are ver}^ large, with clear 

 contents staining pink in picro-indigo carmine ; their nuclei 

 stain uniformly dark red in borax carmine. From their 

 staining properties these ovoid cells appear to be mucous 

 cells, and they are similar in size and appearance to the 

 mucous cells of the foot, but, unlike the latter, are not 

 rendered polygonal by mutual pressure. 



The epithelium of the labial grooves passes into the 

 oesophagus, but the finely -granular cells soon disappear, and 

 their place is taken by large coarsely-granular gland cells. 

 The mucous cells at the same time disappear. The oesophagus 

 is surrounded by a very distinct layer of subepithelial 

 muscular fibres. The epithelial lining of the oesophagus 

 passes gradually into that of the stomach. In this cavity the 

 glandular cells, as has been already described on p. 267, are 

 restricted to the side walls and floor; the roof is thin and 

 lined by moderately long ciliated cells only. It is noticeable 



