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 systematist. 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, Pl. polychroa
(fig. 5), convex with a small median projection as in Polycelis
nigra (fig. 8), or triangular as in Pl. gonocephala (fig. 4). Planaria
alpina (fig. 3), and Polycelis cornuta (fig. 7), have a rounded
frontal margin with two lateral projections (tentacles1) like the
ears of a cat. The two remaining British species, Bdellocephala
1 In Pl. gonocephala and Pl. polychroa the head widens out behind the eyes into a pair
of lateral lobes or "auricles" which are readily distinguishable from tentacles.