12 
THE ESSEX NATURALIST. 
*Pl. gonocephala Duges. 
PI. 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. 
PI. cavatica Fries. 
PI. 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. .. .. . . PI. 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 .. 
PI. 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 .. .. .. .. .. .. PI. torva 
