THE BRITISH FRESH-WATER PLANARIANS. 
3 
. The literature published during the last half century on 
European Tricladida is chiefly in French and German. E. Ray 
Lankester (23) in an article, “ On the Planaria of our Ponds and 
Streams,” written in 1867, laments the sad neglect of this branch 
of zoology in this country. He complains that even the British 
Museum list of non-parasitical worms (21), published in 1865, 
is practically worthless so far as the Turbellarians are concerned. 
Houghton (16), in 1868, gives very little information. The 
most helpful recent contribution is that by Gamble, in the 
Cambridge Natural History (12), where figures are given which 
help to identify the commoner species, though there is little 
information as to habits. 
GENERAL APPEARANCE AND STRUCTURE. The 
more obvious characters are observable by means of a pocket 
lens ; allusion to microscopic details is made only when necessary 
to elucidate some special point. 
The bodies of planarians are very soft and flexible. Being 
capable of rapid extension and contraction they present an ever- 
varying form. The shapes here described are those assumed 
when the animal is fully extended during gliding. 
British planarians range in length from 12 mm. to 36 mm. ; 
the body is usually from four to six times as long as broad, in 
Polycelis nigra about eight times. They are all flat; the thick¬ 
ness rarely exceeding half a millimetre. This film-like, flexible 
thinness permits the ventral surface of the animal to be closely 
applied to the stones under which it lies and to the water plants 
among which it crawls. The outline of the anterior region 
or “ head ” of the planarian, the number of the eyes and their 
position claim first the attention of the systematise In many 
species the “ head ” is divided from the “ body ” by a slight 
constriction ; its frontal margin may be simply rounded, as in 
Planaria torva (fig. 6), shaped like the head of an axe, PI. polychroa 
(fig. 5), convex with a small median projection as in Polycelis 
nigra (fig. 8), or triangular as in PI. gonocephala (fig. 4). Planaria 
alpina (fig. 3), and Polycelis cornuta (fig. 7), have a rounded 
frontal margin with two lateral projections (tentacles 1 ) like the 
ears of a cat. The two remaining British species, Bdellocephala 
1 In PI. Qonocephala and PI. Polychroa the head widens out behind the eyes into a pair 
•of la eral lobes or “ auricles” which are readily distinguishable from tentacles. 
