The crural gland produces a venom secretion containing at least nineteen peptides; superadded by non-nitrogenous components. 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 OvDLPs are related to, though distinct from, those involved in reptilian venom production. This appears to be an example of convergent evolution of venom genes from existing immune system genes (defensins). A unique feature of the venom is the presence of a D-amino acid. This is the only known such example in mammalian systems. This venom appears to be connected to several species that are not part of the platypus's evolutionary lineage, such as certain fish, reptiles, insectivores, and spiders, sea anemones, and starfish.