660-630 million years ago


Cnidarians possess primitive gonads and a homolog of the hormone oxytocin.

In cnidarians, reproductive cells can divide to produce a small groups of cells, forming the most primitive ovaries and testes (Beklemishev, vol. 2). These "gonads" cannot be considered as organs, they are simply small aggregations of reproductive cells (Hyman). It is interesting that in both these cnidarian "gonads" and the first true gonads found in higher flatworms, the sex cells must migrate to the gonads rather than originate there, since this is also a characteristic of higher animals as well (Hyman, p. 431; Hickman). In craniates from hagfish to humans, spermatogonia and oogonia are produced in yolk sac and migrate to the gonads (Romer, p. 421).

Oxytocin and vasopressin family members are known from 4 invertebrate phyla and all groups of vertebrates (Youson, 1999). Cnidarians produce oxytocin/vasopressin (Thorndyke, ).