12
THE ESSEX NATURALIST.
*Pl. gonocephala Duges.
Pl. alpina (Dana).
*Polycelis nigra Ehrenb.
Pol. cornuta (Johnson).
Those marked thus * indicate species that I have found in
Essex.
II. Species not yet recorded from Britain, but which
MAY OCCUR.
Dendrocoelum infernale (Steinmann).
Planaria vitta Duges.
Pl. cavatica Fries.
Pl. lugubris O. Schm.
Short descriptions of these species are given later, but they
are not included in the following key.
KEY TO BRITISH SPECIES.
This key is slightly modified from that of Steinmann and
Bresslau (36).
1. Eyes two (rarely three or four) .. .. .. (2)
Eyes numerous, along margin of head and anterior
of body .. .. .. .. .. .. (7)
2. With pointed tentacles, projecting like the ears of
a cat. Brow convex. .. .. .. Pl. alpina
Tentacles either absent or if present not pointed or
well defined .. .. .. .. .. . . (3)
3. Lateral lobes of head (p. 3), movable .. .. (4)
Lateral lobes of head not movable .. .. .. (6)
4. Head triangular, with one angle projecting forward
and the other two forming the lateral lobes ..
Pl. gonocephala
Head blunt, brow outline straight or concave. Sides
of body frilled ..........(5)
5. Colour white .. .. Dendrocoelum lacteum
Colour brown, often with dark spots
Bdellocephala punctata
6. Head margin straight or slightly convex with blunt
angles at side of head. (Fig. 6.) Colour brown to
black. Eyes squinting, behind widest part of
head .. .. .. .. .. .. Pl. torva