In Hydra, budding is stimulated by hormones
Animals have two major regulatory systems that release chemicals : The endocrine system and the nervous system.
The endocrine system secretes hormones, while the nervous system secretes neurotransmitters. Even though the two systems are separate, there is overlap between them, and together they work to regulate the body. Epinephrine (adrenaline), for example, functions as the fight–or–flight hormone secreted by the adrenal gland and as a neurotransmitter that sends a message from one neuron to another.
Invertebrate animals produce a variety of hormones in typical hormone–secreting endocrine cells and in neurosecretory cells. Some invertebrate hormones have homeostatic functions, such as regulation of water balance. However, we know the most about hormones that function in reproduction and development. In a hydra, for example, hormones stimulates growth and budding. In more complex invertebrates, the endocrine and nervous systems are generally integrated in the control of reproduction and development. In the mollusc Aplysia, for instance, specialized nerve cells secrete a neurohormone that stimulates the laying of thousands of eggs and also inhibits feeding and locomotion, activities that interfere with reproduction.
Ecdysone promotes molting and the development of adult characteristics, as in the change from a caterpillar to a butterfly.
Metamorphosis is a condition in which pupa stage replaces larval anatomy with the insect's adult form.Synthetic versions of juvenile hormone are now being used as insecticides to prevent insects from maturing into reproducing adults. The hormonal regulation of insect development has been studied extensively. Three hormones play major roles in molting and metamorphosis into the adult form. Brain hormone, produced by neurosecretory cells in the insect brain, stimulates the release of ecdysone from the prothoracic glands, a pair of endocrine glands just behind the head. Ecdysone promotes molting and the development of adult characteristics, as in the change from a caterpillar to a butterfly. Brain hormone and ecdysone are balanced by the third hormone in this system, juvenile hormone. Juvenile hormone is secreted by a pair of small endocrine glands just behind the brain, the corpora allata, which are somewhat analogous to the anterior pituitary gland in vertebrates. As its name suggests, juvenile hormone promotes the retention of larval characteristics. In the presence of a relatively high concentration of juvenile hormone, ecdysone can still stimulate molting, but the product is simply a larger larva. Only when the level of juvenile hormone wanes can ecdysone–induced molting produce a developmental stage called a pupa. Within the pupa, metamorphosis replaces larval anatomy with the insect's adult form.