ANOPLURA : ALIMENTARY CANAL 557 



and like it are constantly changing their shape with the contractions 

 of the muscle fibres in the wall. 



The wall of the mid-gut (fig. 3) consists of a layer of digestive cells 

 and circular and longitudinal muscle fibres. The cells are cylindrical 

 and somewhat irregular in shape, no two cells being exactly alike, 

 so that the lumen presents an irregular outline in sections. The 

 inner half of each cell is rounded and free from its fellows on each side, 

 and may be slightly swollen. There is a well marked striated border 

 extending all round the free portion of the cell, and a slightly denser 

 and more deeply staining layer immediately internal to this. The pro- 

 jecting portion of each cell contains a collection of small vacuoles set 

 closely together, which probably represent the digestive secretion of the 

 cell collected at this point previous to discharge into the lumen of 

 the gut. Each cell contains from one to three nuclei, differing markedly 

 from those of any of the digestive epithelia so far considered. They 

 are small, irregularly oval in shape, and in sections stained with haema- 

 toxylin show a clear area surrounding a central mass of deeply stained 

 chromatin. The remaining portion of the cell contains many large and 

 small clear vacuoles, scattered irregularly throughout the protoplasm. 

 The whole epithelium differs markedly from that of the Diptera or the 

 fleas. 



Both the circular and longitudinal muscle bundles are well developed 

 and are regularly arranged. The inner (circular) fibres are closely press- 

 ed against the bases of the digestive cells ; no basement membrane can 

 be distinguished. 



On the right side of the ventral surface of the mid-gut there is a 

 small collection of specialised cells, forming an oval patch, distinctly 

 raised above the level of the rest of the wall, and sharply distinguished 

 in the fresh condition on account of the yellow pigment which the cells 

 contain. Each cell is cylindrical in shape, and is directed inwards to- 

 wards the lumen, the bases being arranged in the rows of about eleven 

 cells each. The function of these peculiar cells is not known. 



The hind-gut commences at the point of insertion of the four Mal- 

 pighian tubes, and at once turns forwards ventral to the mid-gut ; after a 

 short course it again turns backwards to reach the anal aperture. It 

 is much narrower than the mid-gut, and is of fairly uniform diameter 

 throughout, though somewhat swollen about the middle of its length. 

 There is no rectal ampulla, the rectum being only distinguished by the 

 presence of certain chitinous plates, probably representing rectal 

 papillae, in the wall. . . . 



