TUBIFEX. 365 



of the tube, but have a somewhat spiral arrangement. 

 The peritoneum, which completely invests the tube, is 

 composed of a single layer of very much flattened cells 

 with rather large, oval, nucleolated nuclei (PI. TV, 

 fig. 20, V t.). 



(b) The non-ciliated portion. The ciliated 

 part of the tube gives place gradually to the second part, 

 which is characterised by the absence of cilia and the 

 presence of a much thicker wall. Consequently, this 

 part of the canal has a diameter almost twice as great as 

 that of the first part (PI. I, fig. 2, v.d.2.). The lumen 

 of the tube is of exactly the same diameter throughout, 

 so its increase in size is due entirely to the greater 

 thickness of its wall. 



The epithelial cells which form the innermost layer 

 of the wall of this second part of the tube have the same 

 general form as those described above. That is, they are 

 ring-shaped, piled one on top of the other extremely 

 regularly, and each one is perforated in the centre by a 

 rounded lumen. Each cell has its greatest thickness 

 around the lumen of the tube, and tapers off considerably 

 at its outer edge (PL V, fig. 21). The nuclei of these 

 cells resemble very closely those of the first part of the 

 tube, and are situated near the inner edges of the cells 

 (PI. IV, fig. 19n). The greater thickness of the wall of 

 this part of the tube is due to a considerable increase in 

 the quantity of the matrix in which the cells are embedded, 

 and by which they are surrounded (PI. V, fig. 21, ma.). 

 This matrix still stains only with plasma stains, but it is 

 no longer structureless, for it is characterised by the 

 formation in it of a large number of exceedingly fine 

 fibrillae. These are not visible in the fresh material, but 

 are rendered very distinct by the action upon them of 

 fixing reagents ami by the use of appropriate stains. 



