Japanese Tricladida, Marioala. gc^ 



to immediately below the dorsal body-wall and undergoes laterally 

 very irregular and complicated outbulgings, but always showing a 

 bilateral symmetry, as is shown in figs. 7, 8 (PL 1). I regret I 

 am not able to demonstrate its form with adequate thoroughness. 



The Avail |)resents nothing peculiar. The inner wall is,. 

 except near the genital aperture, formed of a single laA^er, with 

 wide variations in thickness in different parts, and exhibiting no 

 nuclei at all. In certain portions the lining may be sufficiently- 

 distended to cause a virtual disappearance of the epithelium so 

 that the underlying basement membrane or muscles abut directly 

 on the cavi-ty. A similar condition has been demonstrated by 

 Lan(t (o7) in the penis of Ymvjia aurantiaca and by Luthee, (41) 

 in the atrium copulatorium of Castrada segne, in the bursa copula- 

 trix of the Mesostomidae, etc. Immediately external to the 

 epithelium there is a strongly developed muscular 'layer which also 

 varies in thickness in difïerent parts, and seems to be much 

 thicker on the hind wall than elsewhere. It is, although I am not 

 able strictly to say the kinds of the muscular fibres, for the most 

 part composed of three sets of fibres, viz., internal circular, 

 middle longitudinal and outermost circular, of Avhich the middle 

 circular layer is developed to a considerable degree. Externally 

 the muscular coat is surrounded by numerous pyriform cells which 

 perhaps represent partly the insunken parts of the lining epithe- 

 lium and partly unicellular glands. Moreover, around the vesti- 

 bulum muscles are developed to a considerable extent, which leave 

 the wall in various directions, passing through the body-paren- 

 chyma (PL I, fig. 8). Now, from the morphological point of view 

 I have come at length to the conclusion that the vestibulum of 

 Ect. Ihmdi is quite homologous with the vagina of St. trigonoce- 

 phala. 



In the Tiirbellaria we thus find an epithelial transformation 

 leading to the condition in the Trematodes and Cestodes, in 

 which the epithelium has been replaced by a thin refractive 

 membrane or a cuticula. This fact, I believe, points out beyond 

 doubt the probable homology between the Turbellarian epithelium 

 or epidermis and the Cestode cuticula. 



