Most of the investigations into annelids have been carried out on polychaetes, particularly several species of nereids. It has been shown that growth, reproduction, regeneration and metamorphosis are controlled by neurosecretions from the brain.
Normal growth is regulated by a growth hormone, and when the mature form is achieved, either the amount of secretion wanes, or the tissues become less sensitive to it. Maturation of the sex cells is inhibited by a “juvenile” hormone from the brain, but when the level of this is sufficiently reduced, maturation begins. Later, for full development of the oocytes, a very low level of the hormone is necessary. There is some justification for regarding this “juvenile” hormone as being identical with them, may secrete a hormone which stimulates the final development of ripe eggs.
Regeneration of segments cut off, or lost naturally in annelids, takes place first by the formation of a growing point or blastema at the site of loss. This blastema is particularly sensitive to growth hormones and new segments are formed, at first slowly and then much more rapidly. It has been shown that both the blastma and the hormone are necessary for regeneration to take place.
Metamorphosis from the Nereid to the heteronereid condition is also hormonally controlled. Again, this may depend on the concentration of the growth hormone, or, if it exists, of a low enough, metamorphosis will proceed. There is some possibility that the developing oocytes may produce a hormone influencing metamorphosis.
The facts of hormone control of certain processes are clearly established, but as yet there is no certainty as to how many hormones are involved. Some workers have suggested that the growth hormone controls all the above processes by its varying concentration. Others suggest that there may be at least three different hormones, two produced by neurosecretion from the brain and one by secretion from the oocytes.