Invertebrates

Among most invertebrates (animals without backbones), endocrine glands
are not in evidence. Specialized cells known as neurosecretory cells serve as
endocrine tissue. The cells, which resemble neurons (the functional cells of
the nervous system) are hormone producers. In invertebrate animals such
as the hydra and planaria, the secretions (hormones) of the neurosecretory
cells seem to influence growth and may be the underlying cause of the tremendous
powers of regeneration possessed by the animals. There are indications
that the development of sexuality, the laying of eggs, and the release
of sperm may be under hormonal control in these animals. Attempts to establish
the link between hormones and invertebrate behavior when the hormones
are produced by neurosecretory cells have inherent problems. A
common method of studying hormone influence involves removal of the secreting
organ, which causes a hormone deficit. Changes in physiology or behavior
are observed. A hormone is then provided to the animal to see if the
original condition can be restored. Use of this method is complicated by the
difficulty in removing all the functioning neurosecretory cells. In addition, the cells regenerate rapidly. This prevents an accurate assessment of the effects
of hormone deficit.
Hormone effects are observable and measurable in the more developed
invertebrates such as the Arthropoda. Studies carried out on insects and crustaceans
indicate the presence of both neurosecretory cells and endocrine
glands. Among the behaviors and activities controlled by the hormones released
from either the cells or the glands are molting, sexual differentiation,
sexual behavior, water balance, and diapause. Because arthropods are encased
in an outer skeletal structure, it is necessary for the animals to shed
their outer structure in order to grow. During the growth years, the animals
go through cycles of shedding the outer skeleton—or molting—growing,
and reforming an outer coat. There is evidence that insects are under hormonal
control when they enter a state of diapause, or arrested behavior in
adverse times. 332