THE MARINE TURBELLARIA. 295 



'plug-like' mass of tissue derived in part from the parenchyma and in part from the epidermis, 

 which differs from the tissue surrounding it in being denser and in staining more deeply. It 

 occurs in every papilla through which my sections pass, but I cannot 

 offer any suggestion as to its significance. 



Behind the level of the pharynx, which resembles that of other 

 species of the genus, the main gut gives off numerous branches. These 

 do not all arise from it in the same plane, but at the same time no 

 branches are given off from the ventral side. Von Plehn [10] in de- ^^^~^ T t 1 

 scribing two new species, which she calls Thysanoplana indica and eyeso{ Thysamzoonplehni. 

 Thysanoplana viarginata, pointed out that a similar peculiarity occurs 



in those species, but to a much greater extent, for in them apparently the branches may in 

 addition originate from the ventral side of the main gut. I have also found — from a careful 

 examination of a series of sections in the Laboratory Cabinet of the Owens College Zoological 

 Department — that in the neighbourhood of the male aperture of Thysanozoon brocchii, and 

 for some little distance behind it, the gut branches do not all rise in exactly the same plane, 

 though only from the lateral walls of the main gut. It is in the same neighbourhood, viz. in 

 that part of the body where the main gut is largest, that this feature is most marked in von 

 Plehn's species of Thysanoj)lana as well as in Thysanozoon plehni. 



Genital organs. Excepting in detail these resemble very closely those of other species 

 of the genus, and especially those of Th. auropunctatus (Coll.) as figured by von Stummer- 

 Traunfels [14]. The terminal parts of the female ducts are in no way peculiar. The eggs, 

 like those of other species of the genus, exhibit large centrosoma. 



Male apparatus. The vasa deferentia unite to enter the vesicula seminalis by a short 

 common duct (" gemeinschaftlicher Einmiindungsgang " of von Stummer-Traunfels), which runs 

 dorsalwards to open into the lower side of the vesicula near its hinder end. The vesicula 

 extends forwards and a little downwards for the greater part of its length, but close to its 

 anterior end it curves suddenly downwards and narrows into the duct to the penis. This duct 

 is long, more than twice the length of the vesicula, and after a tortuous course enters the 

 base of the penis, where it is joined by a very short duct running from the small prostate 

 gland, which lies close above and behind the penis. It is lined with secretory epithelium, and 

 has a compact muscular wall composed of diagonal fibres. The penis is armed with a short 

 tubular chitinous stylet and projects into a cavity, whose walls form the penis-sheath. This 

 cavity opens immediately below the penis into the antrum masculinum, which in turn ojjens 

 to the exterior by the male gonopore. 



It is of some interest to find that, whereas the gonopore is situated on the middle line, 

 the penis with the penis-sheath lies very decidedly to one side of it, the right side. But the 

 cavity of the antrum is extended fairly equally both to the right and to the left of the middle 

 line. This peculiarity perhaps indicates that this species is descended from a form in which, 

 as in many other species of Thysanozoon, the penis is paired, and that one of the pair, the 

 left in this instance, has disappeared leaving the right penis to open unsymmetrically into the 

 symmetrical antrum. In Lang's Monograph, T. 30, fig. 17, a diagrammatic figure is given of 

 the terminal male apparatus of Pseudoceros maximus — a species belonging to a closely-allied 

 genus — which possesses a pair of penes. If we picture one of these penes to be atrophied, the 

 diagram would represent very closely the appearance seen in a section of the present species 

 passing through the penis and antrum masculinum. 



G. 38 



