THE BRITISH FRESH-WATER PLANARIANS.
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in absolute alcohol, which should be occasionally changed. At
the end of an hour the animal is fixed. To get rid of the mer-
curic chloride it is necessary to place the planarian in a weak
solution of iodine in 95% spirit, and afterwards in clean spirit
to free the specimen from the iodine. Specimens prepared by
this method may then be mounted in Canada balsam after
dehydrating with absolute alcohol and clarifying in xylol or ben-
zol. If a stained preparation is desired, either borax carmine or
haematoxylin differentiated by acid alcohol give good results.
Sections may be cut from material killed and preserved in
this way. Double staining with Delafield's haematoxylin and
eosin or Orange-G. gives satisfactory results.
HABITS AND LIFE-HISTORY.
Locomotion. The gliding motion of planarians, which
resembles that of snails, is due not (as in the case of the mollusc)
to muscular contraction, but to the movement of the cilia cover-
ing the under surface of the body. It has been suggested that
the slime glands render the movements of the cilia easier and that
the secretion from the ventral surface prevents the coagulation
of mucus from the lateral gland. Planarians sometimes creep
along on the under side of the surface film of water. Dalyell
(6) noted this in the case of Poly cells cornuta.
When planarians are irritated the gliding movement passes
into one in which the body, with the head fixed to the substratum,
is stretched fully and then contracted violently. This operation
is repeated until the animal is out of reach of the irritating object.
Dendrocoelum lacteum usually adopts these tactics when attempts
are made to capture it.
Reactions to Light and to Running Water. All fresh-
water planarians avoid light, seeking the shade of water
plants, stones and submerged leaves. They recognize the
direction from which light comes and move rapidly away into
shade. Further interesting information on this branch of the sub-
ject is to be found in a paper by Prof. Raymond Pearl (30).
Some species show a tendency to arrange the. long axis of
their bodies parallel to the direction of the current, with the head
facing up-stream. Planaria alpina, Pl. gonocephala and Dendro-
coelum lacteum are examples of this, whilst Pl. torva and Polyceli