This venom appears to be related to that of several species that are not part of the platypus's evolutionary lineage, such as certain fish, reptiles, insectivores, and spiders, sea anemones, and starfish. This is the only known such example in mammalian systems. A unique feature of the venom is the presence of a D-amino acid. This appears to be an example of convergent evolution of venom genes from existing immune system genes ( defensins). The OvDLPs are related to, though distinct from, those involved in reptilian venom production. Those peptides that have been sequenced and identified fall into three categories: defensin-like peptides (OvDLPs), C-type natriuretic peptides (OvCNPs), and nerve growth factor (OvNGF). The crural gland produces a venom secretion containing at least nineteen peptides and some non-nitrogenous components.
The spur normally lies flat against the limb but is raised when required. The spur is attached to a small bone that allows articulation the spur can move at a right angle to the limb allowing a greater range of attack than a fixed spur would allow. Female platypuses, in common with echidnas, have rudimentary spur buds that do not develop (dropping off before the end of their first year) and lack functional crural glands. The venom is produced in the crural glands of the male, which are kidney-shaped alveolar glands located in the upper thigh, and delivered through a spur, or calcar, on each hind limb.
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