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.
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