256 R. T. YOUNG 



worm is covered by a layer of cilia averasing- 5.7// in tiiickness 

 dorsally and 5.4 ventrally.^ In fixation, they become matted 

 together to form a tangled mass, in which it is difficult to observe 

 individual cilia. Where they are inserted in the epithelium, 

 the characteristic basal swellings give the appearance of a thin 

 dense layer at the surface, which has been interpreted by various 

 writers as a cuticula. The epithelial cells have a fibrillar struc- 

 ture presenting numerous small spaces^ which are very likely the 

 result of shrinkage. 



The course of the fibrillae is more or less irregular, though in a 

 general way perpendicular to the surface, producing the striations 

 mentioned by various authors in the turbellarian epithelium. 

 They form a close network, varying in density from point to 

 point, in the meshes of which lie the spherical or ovoid nuclei with 

 a distinct chromatic network and a definite membrane which 

 stains similarly to the latter, and in most, but not all cases, ap- 

 pears to be complete. Frequently, but not always, the network 

 is condensed at one or more points to form false nucleoli. This 

 appearance may be the result of shrinkage. These nuclei aver- 

 age 4.4 by 5.5fi in diameter.^ Besides nuclei, numerous rhabdoids 

 occur in the epithelium, an account of which does not concern 

 us here. The fibrillae appear to be continuous with basal exten- 

 sions of the cilia, but on this point I cannot speak positively. 



Cellular outlines, faintly evident in surface views, are indis- 

 tinguishable in cross sections.'* I have not observed a differen- 

 tiation of epithelial and interstitial cells, as described by Lang ('84) 

 for polyclads. Nor can I distinguish the difference in size of 

 nuclei which he describes and figures. 



1 Average of seven measurements. Variations in thickness of the ciliary layer 

 of from 3 to 8^ occur. These are probably not altogether normal, but where indi- 

 cations to the contrary are lacking I have included them in my averages. 



2 Except in the denser surface layers. 



* Average of twenty measurements. 



* Occasionally I find an apparent fibrilla passing through the epithelium from 

 outer to inner surface of the latter. These may represent cell boundaries but 

 they are distinguishable from other fibrillae only by their extent from surface 

 to surface of the epithelium and by their more nearly vertical direction. In 

 some places where shrinkage has occurred apparent cell outlines may be seen. 



