Sense-organs of the Turhellaria. 309 



many Gasteropoda, e. g. Helix j)omatta, in which also the 

 part of the dotted substance from which the sense-nerves 

 and, indeed, especiallj the nervus opticus are given off 

 is distinguished from the rest bj the above-mentioned 

 properties. 



The cells of the ganglion opticum possess a large nucleus, 

 which is surrounded by only a narrow plasmatic border. 

 They are unipolar, but this process divides immediately into 

 a number of smaller ones, which, so far as I could ascertain, 

 all but one enter into the dotted substance, probably to unite 

 here with each other and with the fibres of the nervus opticus. 

 One of the fibres produced by the division of a cell-process 

 turns, however, towards the aperture of the pigment-capsule, 

 and before entering it undergoes a more or less strong geni- 

 culation. In the cavity of the pigment-capsule it swells into 

 the so-called terminal club. These terminal clubs completely 

 fill up the pigment-capsule. Hitherto they have been 

 described as hyaline structureless formations ; in Planaria 

 gonocephala they present a more complex structure. The 

 fibres in question become thickened first of all into a small 

 pestle-like formation, which sometimes shows a fine longitu- 

 dinal striation. Upon this, like a hood, is seated a crescenti- 

 form, finely granulated, terminal piece, and between the two 

 there is intercalated a thin, hyaline, intermediate plate. In 

 Planaria Iheringii * I do not find the intermediate plate ; in 

 this the terminal piece enveloped the club for a certain 

 distance. 



I have been unable to detect any lenses or lentiform struc- 

 tures. I suppose that the function of the lens is performed 

 by the parenchymatous tissue situated between the retina and 

 the epithelium, which during life is viscous and transparent. 

 I regard as the retina the ganglion opticum and the terminal 

 clubs, as has already been done by others. 



Among the Rhabdocoelous Turbellaria I have hitherto 

 particularly devoted my attention to the Alloiocoela. Among 

 these the Plagiostomidaj, when compared with the Monotidse, 

 possess the more complex eyes, and of these two or four. 



Vorticeros auriculatum possesses two eyes which are placed 

 in direct contact with the brain, as indeed is the case in all 

 other forms. The pigment of the pigment-capsule is, in the 

 Plagiostomidse, very frequently connected by pigment-cords 

 with the pigment of the body, so also in Vorticeros auricu- 

 latum. The aperture of the pigment-capsule is turned 



* Planaria Iheringii, a new Tricladous Turbellarian from Brazil, de- 

 scribed bv the author in the same number of the * Zooloofischer Anzei^er.' 



