Sense-organs of tJie Turhellaria. 311 



is certainly possible that they really act as refractive media, or 

 at any rate are homologous in their origin with the lenses of 

 other eyes of Khabdocoela, 



The eyes of Plagiosfoma ochroleucum, maculatum, reticu- 

 latum, and sulphureum agree essentially in their structure 

 with the eyes of Enterostoma striatum. Smaller diiferences, 

 of course, exist, and more will probably be found on further 

 investigation. Thus, for example, the contents of the pig- 

 ment-capsule in Plagiostoma ochroleucum do not consist of 

 two globular structures, as in Enterostoma striatum, but only 

 of one. The tendency to break up into several pieces in the 

 eyes of Plagiostoma sulphureum is also known. 



I must, however, specially notice the eyes of Plagiostoma 

 Girardi. In this animal the contents of the pigment-capsule 

 consist of two clearly distinguishable substances. The larger 

 posterior portion of the capsule is filled with a perfectly homo- 

 geneous substance which only becomes faintly coloured by 

 reagents. In front of this there is a narrow band which does 

 not stain at all, but shows a distinct horizontal striation. The 

 limit of this band is very sharp and distinct both inwardly 

 and outwardly. Before the pigment-capsule there is an 

 aggregation of cells, of which the central ones are larger than 

 the peripheral. They also show a difference in their behaviour 

 towards colouring materials, the smaller cells stain more 

 strongly than the large central ones. The figure given by 

 von Graff, in his monograph of the Turbellaria, of the eyes 

 of Plagiostoma Girardi does not agree with my representa- 

 tion. In my opinion von Graff had before him indifferently 

 preserved specimens, and crushed preparations in this case 

 only too readily give rise to illusions. What von Graff de- 

 scribes as the lens is undoubtedly the contents of the pigment- 

 capsule shrivelled during preparation, and which I regard as 

 the terminal nervous apparatus, I believe, with some justice. 



A. Lang * and I. lijima f mention in the Playiarice ex- 

 amined by them a nervous plexus, which is readily demon- 

 strable, especially at the back of the animal. In Planaria 

 gonocephalaj also, there is both at the dorsal and at the ven- 

 tral surface a subcutaneous nervous plexus, which may be 

 particularly demonstrated in the cephalic part, and here again 

 very distinctly in the auricular processes. In connexion with 

 this subcutaneous nervous plexus I have observed in the 

 auricular processes an apparatus which is probably to be in- 

 terpreted as a terminal nervous apparatus. 



* Das Nervensystem der Tricladon. 



t Untersuchungen iiber deii Bau und die Entwicklungsgeschichte der 

 Siisswasser-Deudroccelen. 



