Cross references: Amphioxus Hormones Endostyle = Thyroid
Amphioxus Gut Hormones Lamellar Body = Pineal Gland
Amphioxus Pituitary Amphioxus Gonads Amphioxus Stress Hormones
Amphioxus Genomics Amphioxus Neuropeptides
Oxytocin Oxytocin Receptors Oxytocin Receptor Distribution
276<278 OxRcpDistCite Vasopressin
Quoting from: Oxytocin Receptors
"The earthworm Eisenia foetida is the most primitive species from which an OT-related peptide (annetocin) has been isolated ( 429 ). Injection of annetocin in the earthworm or in leechs results in induction of egg-laying behavior ( 430 ). "
"The above shows that oxytocin evolved before the split between the Protostomes and the Deuterostomes. What's more, it suggests that the original function of oxytocin-vasopressin was reproductive rather than water-balance. If so, then the water-balance function of human vasopression evolved after the reproductive function of human oxytocin. Further, it also suggests that the water-balance function only evolved when the lamprey swam inland to spawn and needed to adjust to the change in osmotic pressure from a salt water environment to a fresh water environment."
"Arginine-vasotocin stimulates smooth muscles from a wide variety of vertebrate species. It can stimulate contraction of oviducts from many jawed fishes and tetrapods."
It would seem from this that the affiliational function of oxytocin is, at least in the amphioxus and its immediate descendants, essentially sexual.