Noting the earlier mention of
ajolotes, I hasten to bring you an even stranger clade of burrowing beasties, the
Caecilians! (Sadly the Apodan Amphibia are one of the groups
Tree of Life haven't got around to doing yet)
Caecilians are wormlike amphibians up to a metre-and-a-half long, with adorable sensory tentacles. They've been around since at least the Jurassic, OM NOM NOMing on earthworms, termites, and anything else they stumble across. They also enjoy penetrative sex, being the only amphibians duly equipped to do so. In at least one species, the babies flay their mother alive for food.
Here's a video.
Click to view
Happily, the mother survives this, since she's grown a specially thick fatty skin layer just for this purpose (see, cellulite DOES have a purpose!)
In other species the babies gnaw at the inner lining of her oviducts for 12 months instead.