The toxins are excreted through their skin, and adhere with the oils that keep their skin moist. It is a defense that keeps other animal from eating/touching them. They are not really facilitated to bite as a defense. They pull prey in, and their mouth mostly crushes, and is used to swallow.
From the Natural History Museum UK website -
The hallmark of venom is that it’s introduced via a wound. It can be injected through a number of means, including teeth, a sting, spines or claws. ‘Poison is different as there is no wound involved. It can be absorbed into the bloodstream through the skin, inhaled or ingested,’