8
THE ESSEX NATURALIST.
nigra appear indifferent to the direction of running water. Poly-
celis cornuta reacts only feebly.
Nutrition. The food of planarians consists of small
crustaceans, insect larva, small worms, and sometimes dead
animals or even plants. The planarians discharge quantities
of rhabdites which, when in the water, produce mucus, wherein
the prey becomes entangled and is then devoured. A planarian
is able to penetrate the joints of fresh-water amphipods and
isopods by means of its muscular proboscis, although no signs of
teeth can be detected. It is probable that a special secretion
assists in this difficult process. When once the skin has been
penetrated, the proboscis is inserted and ingestion commences.
A recently-fed planarian usually shows the ramification of the
intestinal lobes, owing to the presence of partly digested food.
The undigested portion is returned through the mouth. (See
p. 5)
Planarians have a marvellous power of withstanding hunger,
and they are able to live for months, and even for a year, without
food. Stoppenbrink (37) has studied this subject carefully and
he found that the size of the animal diminished steadily during
starvation. Specimens of Planaria alpina, kept for nine months
without food, had diminished almost to one-fourth of their original
length. The sexual organs degenerated in the inverse order of
their development, while the nervous, alimentary and excretory
systems underwent little change. This is a somewhat unusual
occurrence in nature—the immediate sacrifice of the race for the
welfare of the individual.
Reproduction and Regeneration. All fresh-water
Planarians are hermaphrodite, and under favourable conditions
reproduce sexually by means of eggs, several of which are
deposited in a cocoon.
The time for cocoon production depends upon the species,
and upon the temperature rather than the season. Generally
speaking, however, Bdellocephala punctata, Planaria alpina and
Polycelis cornuta are sexually mature during the winter and early
spring; Planaria gonocephala and Pl. torva in summer and
autumn. Bohmig (3) states that Dendrocoelum lacteum is a
winter, and Polycelis nigra a summer, layer. I have found