Annelids - Hormones in Growth and Reproduction

Studies on polychaetes have displayed that the endocrine glands play a key role in growth and reproduction. In addition to the brain neurosecretory cells, we observe that in annelids there is an infracerebral gland. This is situated in close association along with the brain immediately beneath or behind it. The infracerebral gland has its own secretory cells. In addition, the neurosecretory cells of the brain comprise their fibre terminations in the brain, close to the infracerebral gland, or even passing into the gland. Nereis and Nephthys are examples. The polychaetes are able of rapid growth when young, with proliferation of new segments. They are also able of extensive regeneration. However growth rate and regenerative ability decline along with age. An interesting phenomenon in polychaetes is epitoky. This occurs during sexual maturation in some Nereids. Sexual maturation in them is accompanied by changes in specific regions of the body. The changes involve enlargement of parapodia and development of modified chaeta. The animal is transformed into heteronereid phase, adapted for rapid swimming and sexual swarming.