THE ANATOMY OF AN AUSTRALIAN LAND PLANARIAN. 81 
example, in the genera Geoplana and Bipalium, and they always appear to be very 
numerous. In the forms with multicellular eyes, such as Rhynchodemus, there 
appears to be usually only a single pair. 
Von Kennel (3) states that in Rhynchodemus the eyes are two little pigment cups 
filled with small cells, whose nuclei stain pretty distinctly. Black pigment is 
irreoularly heaped around the contents. Similar eyes appear to occur in Geodesmus, 
according to Mecznikow and von Kennel; they are larger than those of Rhyncho- 
demus. 
The fresh-water Tvicladians have also multicellular eyes, whose structure is 
probably fundamentally the same as in Rhynchodemus. There is usually only a 
single pair, but they may be numerous. The latest account of these organs, so far as 
I am aware, is that given by Jijima. This author (7) states that the eye of Planaria 
folychroa consists of three parts—(1) a pigment cup, (2) the visual rods (“ Seh- 
kolben”), (8) the ganglion opticum. The pigment cup is formed of compact pigment 
granules, and has its opening directed outwards and upwards. In front of the opening 
is a collection of nervous substance, viz., granular substance and fibres surrounded by 
numerous nuclei, apparently belonging to ganglion cells. These form the ganglion 
opticum or retina, which lies only a little below the basal membrane. The boundary 
between the ganglion opticum and the cavity of the pigment cup is pretty sharp, but 
specially thick fibres pass over it into the cavity of the cup, where they increase in 
thickness, and end each in a swelling. Carriére considers the swellings as unaltered 
nuclei, but according to Hertwig nuclei are present within them, which, however, 
Jijima could not discern. The fibres in connection with the swellings can be traced 
into the optic nerves, which originate from the lower part of the ganglion opticum, 
and run obliquely backwards and downwards. How the optic nerves come off from 
the brain, Jijima was unable to determine. 
The aquatic Tvicladians then, and some of the terrestrial forms, have multicellular 
eyes, usually two in number, and probably innervated from the anterior ganglion. 
The remainder of the terrestrial forms have unicellular eyes, occurring im great 
numbers, with no special nerves, but lying in the nerve sheath, from which doubtless 
they are innervated. 
According to Moseley, Mecznikow considered that the great complexity of the eye 
in Geodesmus had been caused by terrestrial habits, but Moseley points out (and it 
will be readily seen from the above description) that the eye of the aquatic forms 
is aS complex as in Geodesmus. 
It appears to me, on the other hand, that the unicellular eyes of such forms as 
Geoplana ave of much later development than the multicellular type, and that they 
have arisen as a result of terrestrial habits by special modification of groups of 
