HISTOLOGY OF THE LAND-PLAN ARIANS OF CEYLON. 
145 
form ; they have a transparent cornea in front, which projects amongst the epithelium 
of the skin, and a posterior pigmented sac. From the pointed extremity of the sac a 
nerve-fibre can be traced a short distance. I had not sufficient material to allow of the 
examination of the internal structure of the eye. Leidy has given some account of the 
eyes of Planarici sylvcttica ( =Bhynchodemus sylvaticus, Diesing). He says the eyes consist 
of a vitreous humour, two thirds covered with pigment, and 5 ^- of an inch in diameter. 
Mecznikow (loc. cit.) describes the eyes of Geodesmus bilineatus as very complex. Their 
pigment-skin is composed of clearly definite hexagonal cells, and the eye contained a 
red-coloured crystalline body consisting of rod-like elements. The crystalline body 
evidently is homologous with the lens-like body in the eye of JBipalium, the similarly 
complex body in Leptoplana, and the vitreous body in Dendroccelum lacteum , which 
has usually a reddish tinge. The eye of Bhynchodemus Thwaitesii probably contains a 
corresponding structure. Mecznikow considers that the great complexity of eye of 
Geodesmus has been brought about by the animal’s terrestrial habits, it requiring to use 
its sight more on land than in water ; but the eye of Leptoplana is as complex as that 
of Geodesmus ; and it is possible that the aquatic ancestor of Geodesmus was already 
provided with highly developed ciliated sacs. 
In describing the habits of Bipalium , I described the manner in which that animal 
throws out tentacular-like projections from the anterior margin of its semilunar head when 
in motion, and evidently uses these temporary tentacles as sense-organs. In reading M. 
Humbert’s interesting account of Bipalium , I found that he had observed this habit of 
the animal as well as I, and had been led by his observation to seek for sense-organs or 
tentacular structures on the margin of the head. He was not successful in finding any ; 
but on very careful examination of well-hardened specimens I was more fortunate, and 
discovered a narrow band extending along the whole anterior margin of the head, 
entirely free from pigment, and occupied by a row of cylindrical rounded papillae placed 
vertically side by side, and with small oval openings between their superior extremities 
(Plate XIII. fig. 16). This row of papillae is in the upper part of the lower fifth of the 
margin of the head, so that it lies close to the ground when the animal’s head is lowered. 
The papillae are covered with short cilia ; but I could find no special structure in them, 
except that in their region, and that of the ciliated pits, there is a large quantity of 
tissue formed of small spindle-cells. The oval apertures between the papillae lead to 
ciliated pits, the appearance presented by which is shown in figs: 11, 12, & 13, Plate XV. 
In longitudinal and horizontal sections the appearance presented in fig. 13 is seen. The 
light bands, which appear to pass to the bottoms of the ciliated pits, are continuous 
with the vascular network of the head. Whether they represent tubes in communication 
here with the exterior I cannot say. They may convey nerves to the sacs. From the 
manner in which the animal uses the front of its head, there can be little doubt that 
the papillary line discharges some special sense-function ; but it is possible that this 
function is discharged by the papillae, whilst the ciliated pits with their communicating 
vascular stems act as excretory organs. The papillary line with its pits was found in 
JIDCCCLXXIV. u 
