LINEUS. 249 



suggested that this arrangement may subserve the purpose 

 of respiration, and the vascular network which surrounds 

 this region of the alimentary canal (PL II., fig. 2, oesL), 

 lends some support to such a view. 



Histologically the oesophagus, like the rest of the 

 alimentary tract, is lined by ciliated epithelium. 

 Squeezed in among the ciliated cells (PL IV., 

 fig. 3) is a number of large unicellular gland 

 cells, among which two, or possibly three, types 

 may be distinguished. After staining with borax carmine, 

 followed hj picro-nigrosin, many of these gland cells (PL 

 IT., fig. 3, fi) take a yellow stain. They appear to be full 

 of large coarse granules. Others similarly granulated, 

 though much less numerous, shew an intense crimson 

 colouration (PL IT., fig. 3, y). Except for the difference 

 in staining reaction, these two types are indistinguishable. 

 The greater number of the gland cells, however, present 

 a different appearance. Their contents consist of a coarse 

 spongy network, which presents a slaty-purple hue (PL 

 IV., fig. 3, a). Whether these various types are in reality 

 distinct, or whether they represent different stages in a 

 single type, is a question which must be left for future his- 

 tological investigation to decide. 



Besides being found between the ciliated cells, 

 these unicellular gland cells also occur massed 

 beneath the ridges of the oesophageal epithelium. 

 It seems feasible that these large glandular oesopha- 

 geal cells supply the active juices of digestion, whilst the 

 intestinal region is more concerned with absorption. The 

 more granular of them bear a close resemblance to the 

 large unicellular glands of the integument which, as has 

 already been seen, are probably concerned with the 

 markedly acid skin reaction. Such a fact lends some 

 support to the view that the oesophagus is derived from 



