EPITHELIUM OF TURBELLARIA 261 



at least, due to shrinkage, as suggested above for Planocera. 

 Occasional rhabdoids are scattered through the epithelium and 

 ciliary layer. The main point of interest, however, in the epi- 

 thelium of Polychaerus is the occasional occurrence of nuclei. 

 These are found mainly dorsally, but I have observed at least 

 one or two clear cases of their presence ventrally, one of which 

 is shown in the figure. There can, I think, be no question as to 

 the nuclear character of this and similar bodies in the epithelium. 

 Their structure is identical with that of the parenchyma nuclei. 

 They are round or oval in outline, average 4.7 by 3.5^* in diam- 

 eter^ and contain a dense chromatic network surrounded by a 

 fairly definite membrane, which is however obviously incom- 

 plete in some cases. It is not impossible, however, that they 

 may owe their presence here to distortion of the tissue produced 

 by contraction of the worm during fixation. This would explain 

 their greater number in the dorsal epithelium where the tissues 

 (in my preparations) are much more distorted than they are 

 ventrally. The fact of their occurrence in the ventral epithelium, 

 however, where but little distortion has occurred and the further 

 fact of their occurrence in the epithelium of other Acoela as recorded 

 by von Graff ('91) renders it probable that their occasional pres- 

 ence here is normal. 



In Bdelloura propinqua (fig. 3) there appears to be an entire 

 absence of nuclei in the epithelium examined. The surface of 

 this worm is covered by a layer of cilia similar in appearance to 

 that of Planocera already described. Below the cilia is the epi- 

 thelium which also, except for the absence of nuclei, has a struc- 

 ture similar to that of the last named species. Here is found 

 the same layer of basal swellings of the cilia at the points of their 

 insertion in the epithelium, the same fibrillar network with its 

 meshes approximately perpendicular to the surface, and con- 

 taining numerous vacuoles which are probably artefacts pro- 

 duced by shrinkage, as they are sometimes absent or but poorly 

 developed. Here too exists the same difficulty in determining 

 the precise relation between the epithelial fibrillae and the cilia 



* Average of ten measurements. 



JOURNAL OF MORPHOLOGY, VOL. 23, NO. 2 



