2
THE ESSEX NATURALIST.
LITERATURE. O. F. Muller (26), the Danish naturalist, in
the year 1773, first recognized and described species of these
worms. Later he gave them the general name of Planaria,
a name which should now be restricted to a genus. Paludicola
is another name for the fresh-water Tricladida. The name
Planarian is now in common use by field naturalists and is used
throughout this paper.
In 1814 Sir John Graham Dalyell (6), of Edinburgh, noticed
that many of the specimens which he kept under observation were
more or less mutilated. He tried the experiment of cutting off
parts of a healthy specimen, and found that the missing parts
grew again. This led him to suspect that the animals might
reproduce themselves by spontaneous transverse fission, and his
suspicions proved correct. He says: "It" (Polycelis nigra)
"is privileged to multiply its species in proportion to the violence
offered to its otherwise delicate frame. It may be almost called
immortal under the edge of the knife." Dalyell also noted that
sexual union took place; he observed the deposition of the
cocoons and the periods of incubation of the various species.
He also described how some species sometimes use the surface
film when gliding, and how the animals frequently lower them-
selves from the surface by means of mucus threads.
Within the decade a Bristol physician, Dr. J. R. Johnson
(19, 20), made similar observations. Michael Faraday (11)
was fascinated to watch the regeneration of mutilated planarians.
Two important discoveries were published about 1826:
the Dane, O. Fabricius (10) found that a large proportion of the
surface of the creature's body was covered with cilia, and C. E.
v. Baer (1) observed that planarians were hermaphrodite.
Five years afterwards Ehrenberg (9) applied the name
"Turbellaria" (turbellae = disturbances) to all ciliated, unseg-
mented worms.
The order Turbellaria is now divided into four sub-orders
according to the character of the gut. (1) Sub-order Acoela
(without gut), marine; (2) Sub-order Rhabdocoelida (with a
straight, unbranched gut), small forms, fresh-water and marine;
(3) Sub-order Tricladida (gut with three main branches), fairly
large, found in the sea, in fresh-water or on land. (This paper
deals exclusively with the fresh-water Tricladida); (4) Sub-order
Polycladida (gut with many branches), marine.