258 K. T. YOUNG 



the parenchyma, and by its replacement in Polychaerus caudatus 

 by a fibrous network directly continuous, through the sub-epi- 

 thelial muscle layers, with the parenchyma, as is the case in 

 Taenia serrata and its larva (Young, '08) . Woodworth ('91 , p. 20) 

 on the contrary believes the basement membrane of Phagocata 

 gracilis to be a hypodermal product. In the fibrillar groundwork, 

 some deposit is probably formed, either by the epithelium or the 

 parenchyma, or by both, which intimately unites the fibrillae 

 into a homogenous mass, and is the cause of the differential 

 staining capacity of this membrane as described above. Nuclei 

 in the basement membrane, as described by Lang ('84) are not 

 present here.'^ 



Conditions in general similar to the above exist in several other 

 turbellarians studied by me (viz: Planaria maculata, Dendrocoe- 

 lum lacteum, Phagocata gracilis, Bothromesostoma personatum, 

 Mesostoma tetragonum, Mesostoma sp. and Phaenocora(?). 

 In not all, however, are the nuclei as numerous as in Planocera 

 inquilina. While the abundance of nuclei depends in a large 

 measure on the condition of expansion or contraction of the worm, 

 still by comparison of several specimens of each species similarly 

 fixed, it is possible to construct a series with reference to nuclear 

 abundance leading from the last named form to those in which 

 nuclei arc seldom or never found in the epithelium. 



Such a form is Polychaerus caudatus (fig. 2) which presents 

 an advanced stage in the development of an insunken epithelium. 

 ]^eneath the cilia, which in ])reserved material appear to form 

 an almost continuous layer, is a loosely fibi-illar, vacuolated 

 layer representing the epithelium, while a basement mem,brane 

 is not differentiated. The fibrillae form an ij-regular network, 

 showing no definite arrangement with reference to the surface, 

 and apparently in direct continuity with that of the under-lying 

 j)arenchyma on the one hand and with the bases of th(^ cilia on 

 the other. I cannot speak positively i-egarding this however. 

 The vacuoles in the epithelium of this worm I ]:)olieve are, largely 



" Kosarditifi 1 his iiiciiil)i-aiic, Lang says however (1. (!. J). 04) .... sieauf 

 vielen I'l ajjaraten ganz homogen aussicht, weil veilc fi'ir die iibrigen (icwcbc des 

 Kfirpcis ti(>fHiclH' Titictioiisinil f (^1 di('S(>lbo d ffiis farlxMi." 



