Advances in the study of invertebrate endocrine systems have lagged behind those in vertebrate endocrinology, largely due to the problems associated with adapting investigative techniques that are appropriate for large vertebrate animals to small invertebrates. It also is difficult to maintain and study appropriately some invertebrates under laboratory conditions. Nevertheless, knowledge about these systems is accumulating rapidly. All phyla in the animal kingdom that have a nervous system also possess neurosecretory neurons. The results of studies on the distribution of neurosecretory neurons and ordinary epithelial endocrine cells imply that the neurohormones were the first hormonal regulators in animals. Neurohemal organs ...(100 of 5184 words)
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