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, Pl. torva and Pl. 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,
Pl. gonocephala and Pl. 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, 11 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