to 
7he Irish NaiurMtst. 
January, 
position. But on this occasion only half of the worms 
were found to have crossed the dish after the lapse of 
•fifteen minutes, and a further exposure up to a total of 
half an hour produced no effect : the laggards remained 
stationary. Apparently the young nemertines were fatigued 
by their forced marches or their appetite for light had 
been satiated. 
On the following day, the 23rd of May, the final experi- 
ment was made. The worms, now grown to a length of 
nearly 2.5 mm., were transferred to a rectangular glass 
dish, inches long and filled with sea water. The worms 
having been focussed at one end of the dish by the method 
used in previous experiments, the dish was reversed, 
so that the group of worms was separated by an interval 
of yh; inches from the opening, J inch square, by which 
light was admitted. After half an hour's exposure all 
save six of the worms were found to have travelled the 
whole length of the dish and congregated round the light 
opening. 
The uniformity of the results obtained in these experi- 
ments appears to me to fully justify the inference that the 
young Lineus gesserensis almost as soon as it is hatched is 
highly sensitive to light, and this notwithstanding the 
rudimentary character of the eye -spots as organs of sight, 
or rather of light perception. The action of heat rather 
than light is, I think, out of the question. Mcintosh, in 
Part I. of his Monograph of the British Annelids," pub- 
lished in 1874, speaking of the eye -specks of the mature 
worm, in the Anopla section of the Nemertines, to which 
this species belongs, says they " are simply masses of black 
pigment arrayed on the sides of the snout with greater or 
less regularity and without any special optical structure. 
The textures of the head and nerve -fibres themselves are 
so unfavourable for observation that I have had difficulty in 
making out nerve branches thereto." Subsequent observers 
liave detected such nerve branches, but nevertheless the 
development of the eye -spots as organs of light perception 
must be regarded as elementary. 
In the mature Lineus gesserensis the eye-spots differ both 
in number and position from those of the young worm. 
