These animals
are composed of a gelatinous material, and show scarcely any trace of
distinct organs. The Infusoria, Foraminifera, and sponges, with some other
forms, belong to this division.
PUPA (pl. PUPAE).--The second stage in the development of an insect, from
which it emerges in the perfect (winged) reproductive form. In most
insects the PUPAL STAGE is passed in perfect repose. The CHRYSALIS is the
pupal state of butterflies.
RADICLE.--The minute root of an embryo plant.
RAMUS.--One half of the lower jaw in the Mammalia. The portion which rises
to articulate with the skull is called the ASCENDING RAMUS.
RANGE.--The extent of country over which a plant or animal is naturally
spread. RANGE IN TIME expresses the distribution of a species or group
through the fossiliferous beds of the earth's crust.
RETINA.--The delicate inner coat of the eye, formed by nervous filaments
spreading from the optic nerve, and serving for the perception of the
impressions produced by light.
RETROGRESSION.--Backward development. When an animal, as it approaches
maturity, becomes less perfectly organised than might be expected from its
early stages and known relationships, it is said to undergo a RETROGRADE
DEVELOPMENT or METAMORPHOSIS.
RHIZOPODS.--A class of lowly organised animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the form of
root-like processes or filaments, which serve for locomotion and the
prehension of food.
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