THE BRITISH FRESH-WATER PLANARIANS. 17
Planaria abscissa Ijima.
Planaria montana Chichkoff.
Description. Length 16 m.m.; width 5 m.m. Colour grey, greyish-
brown, brown, or dark green. Sometimes almost colourless specimens
are found, which show a reddish or greenish colour in the intestine. Ventral
surface lighter than dorsal. Tentacles well developed and pointed. Eyes
small, in the centre of small colourless oval areas close together, and some
distance behind the base of the tentacles. Mouth three-quarters of body
length from frontal margin. Genital pore just behind the mouth. Lobe-
formula of intestine 16-20, 2 (8-10), 16-20.
Habits. Found in cold springs and mountain streams.
For an account of distribution and temperature see p. 10.
The cocoons are spherical and about 1.6 m.m. in diameter, and
contain from 15 to 30 embryos. They are produced in winter
as a general rule, but in very cold situations they are deposited
throughout the year. Transverse fission is common (p. 9).
Distribution:—
Britain. Edinburgh Castle (Dalyell 6); Bavelaw, nr. Edin-
burgh (water temperature 11°C.) (H.W.). Isle of Man.
Ireland, France, Pyrenees, Belgium, Switzerland, Germany,
Austria, Sweden.
Polycelis nigra Ehrenb. Fig. 8.
Synonyms : Fasciola nigra Muller.
Planaria nigra Muller.
Polycelis tenuis Ijima.
Description. Length may attain 12 m.m.; width, about 1.5 m.m.
The numerous eyes, represented by black dots running along the frontal
margin and about one-third down the lateral margins of the body, together
with the absence of tentacles, distinguish this species from all others.
The frontal margin is slightly convex with a slight protrusion in the middle.
The colour is black or dark brown. Light brown specimens, with dark
stripes, have been regarded as a distinct species, but are now considered
to be a variety; var. brunnea. The mouth is a little less than two-thirds
the body length from the frontal margin. Intestinal lobe-formula 12-16,
2 (4-6), 12-16.
Habits. Found in ponds, ditches and streams and can with-
stand a wide range of temperature. The cocoons are deposited
on the shaded parts of water plants from spring to autumn.
They are brown, spherical or ellipsoid, and about 1 m.m. in
diameter. Dalyell observed, in August, 1812, that sexual union
took place and as he was not aware that planarians are hermaphro-
dite he regarded the smaller specimens as males. Each cocoon
contains several embryos and incubation takes place in from 21
B