4 
THE ESSEX NATURALIST. 
punctata (fig. 2), and Dendrocoelum lacteum (fig. 1), have tentacles, 
and also a sucker in a median position on the ventral surface. 
The eyes (18) are numerous in the two species of Polycelis 
and appear as a row of black dots extending across the head 
and some distance along the sides of the body. Other British 
species of planarians have generally two eyes. In Dendrocoelum 
lacteum and Bdellocephala.punctata they appear as small black 
dots, one near the base of each tentacle. In Planaria alpina 
the black dot is in the centre of a whitish oval. In the species, 
Planaria gonocephala, PI. torva and PI. polychroa, each eye is 
in a circular or oval white area and is not in the centre but on 
the circumference near the middle line ; this disposition gives 
the animal a curious squint. Occasionally three or four eyes 
may be seen on one individual. The eyes are connected with 
the brain by means of short ganglia. To study the nervous 
system it is necessary to cut and stain sections for microscopical 
examination. The brain consists of two lobes connected by 
a transverse commissure. Two lateral nerves pass backwards 
from the brain and extend the full length of the body. 
In addition to the organs described above, Planaria torva y 
PI. gonocephala and PI. polychroa each have on the dorsal surface, 
between the margin and the eyes, a pair of grooves, termed the 
auricular organs (au.o. figs. 4, 5 and 6); doubt still exists as to the 
function of these. 
The pharynx (ph., figs. 1, n and 12), or proboscis, lies on the 
ventral surface near the middle of the body, and consists of a 
straight muscular tube which lies in a sheath ; (ph.s., fig. 12). 
This tube, with the mouth (mo.) directed backward, is protruded 
when the animal feeds. The intestine consists of three portions 
(hence, Tricladida) ; a single tube, bearing pairs of branches,, 
is directed forward, and a pair of tubes, with branches on the outer 
sides, passes backward alongside the proboscis towards the 
posterior extremity. (See fig. 1.) The whole system may be 
compared to a tuning fork in which the handle is pointed forward 
and the two prongs backward. 
In the section dealing with classification, formulae are given 
to denote the number of lobes to each branch of the intestine,, 
commencing at the left fork, then the handle where there are 
paired lobes, and finally the right fork. For example, the lobe- 
formula for D. lacteum is 16-19, 2 (10-15), 16-19. This means 
