EPITHELIUM OF TURBELLARIA 263 



different layers. The following measurements^ (in n) indicate 

 the variations referred to: 



At the edge of the body, where numerous glands open, the 

 basement membrane is reduced to a very narrow band. 



While nuclei are typically present in the surface epithelium of 

 the Turbellaria, they are frequently either very rare in or entirely 

 absent from the pharynx and pharyngeal sack, and cell boundaries 

 can only be demonstrated by special methods, the epithelium 

 thus assuming the appearance of a cuticula (fig. 4). The dis- 

 tribution of the cilia in this region is also very variable according 

 to accounts of various authors, these being in some cases present, 

 in others absent throughout the pharnyx; in some present on 

 its outer but not on its inner wall, while in others the reverse 

 holds true. In general, the epithelium of the gut forms a definite 

 one-celled layer, but in some cases (Bohmig, I.e. in Plagiostoma 

 bimaculatum, maculatum and sulphureum; Fuhrmann, '98 in 

 Plagiostoma violaceum), etc., it appears to be intimately con- 

 nected with the surrounding parenchyma, while in the Acoela, 

 as is well known, the gut is replaced by parenchyma, the pharynx 

 opening directly into the latter. The oesophagus, the transition 

 from pharynx to gut, may have an epithelium of the ordinary 

 type, or an insunken one (Luther, '04, etc.). 



The statements regarding the epithelium of the excretory ducts 

 are again conflicting. In general an epitheliim appears to be 

 present, at least in the main ducts; but in some cases (Luther, I.e.), 

 the wall is not sharplj^ differentiated from the surrounding paren- 

 chyma, while a non-nucleated terminal duct in Monoophorum 

 striatum is described by Bohmig (I.e.), so that the existence of a 

 typical epithelium in the excretory system of all Turbellaria 



' These measurements are arranged in order from five worms, the first of each 

 set referring to one specimen, the second to another, etc. 



