"Instead of the sucker-feet of ordinary caterpillars they had rows of pincers like crabs"

Typical NZ velvet worm

Long long ago, a velvet worm of the Onychophora phylum and a primitive insect loved each other very much and wanted to make a baby together, so the daddy gave the mummy a spermatophore. This although the Onychophora and the Hexapoda are traditional enemies had been diverging since the last common ancestor some time in the Precambrian era. And the hybrid offspring was viable and the insects raised as if it were a full-blood.

I am not making this up. Nor is it the worst children's movie EVAH. This is a thing; it is the 'larval transfer' hypothesis. The idea is that the DNA from the velvet worm dominated in the first phase of the hybrid's life-span -- forming an entity that we might call a 'caterpillar', or 'grub', or more generally 'larva' -- before passing the baton to the other half of the genome, with the offspring reshaping itself into a phase more like a traditional insect, which we shall call an 'adult'. DNA time-sharing, as it were. The originator of the hypothesis can see no other way for insects to acquire the neat trick of metamorphosis that allows them to exploit more than one ecological niche. It has the imprimatur of Lynn Margulis, who arranged for space for it in PNAS.

Williamson further believes that the resemblance between the planktonic larvae of sea-urchins and the larvae of p3n1s-worms (enteropneusts to you) does not mean that sea-urchins are related to the Hemichordata phylum; and more indirectly, to chordates like us (Asimov, 1957*). Instead he proposes that echinoderms had no larval form until one of them was fertilised by enteropneust sperm, providing genes that are only expressed during the hybrid's infancy.** Unacountably, he omits the obvious extension that the difference between adult human beings and the larval form ("teenagers") is best explained by cross-breeding between Cro-Magnons and Neandertals.

Bonus sea-cucumber haiku from here.

All this becomes the subject of heated discussion at the Old Entomologist when the hour grows late and the empty pitchers of Durian Daiquari multiply upon the table. Head barmaid Evangeline Van Holsteren only puts up with the raised voices and the angry gestures because otherwise we'd be arguing about Nakobov's Blues and making even more noise. The agitation is understandable; if echinoderm eggs are that easily fertilised by alien sperm, there is the prospect of expensive paternity suits awaiting anyone who has ever masturbated into a rock-pool.

None of this should be confused with the Oval Transfer Hypothesis, which holds that the radical difference between the 'egg' phase of an animal's existence and the 'adult' phase is best explained if the animal's family tree contains an egg-shaped interloper as one of its ancestors.

Unexpected consequence of the OTH:
Finally there is the Visceral Symbiosis Hypothesis, which holds that various organs and organ systems evolved independently -- free-ranging hearts, eyeballs, intestines and so on -- until they discovered the advantage of submerging their individuality and autonomy within a cooperative Gestalt entity.

Margulis does not mention the VSH in any of her books, an omission which I incline to blame upon the machinations of our five-eyed sticky-fingered friends the Groaci (or else a priority battle since the VSH dates from 1971, right after the publication of Origin of Eukaryotic Cells).

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* "The Sea Urchin and We." Yes, I am an old fart. The lawn, its ownership. Its pristine state. To depart from it, vile fresh-hatched broodlings!

** Actually it is more complex than that, with a rich tapestry of interbreeding episodes among the various echinoderm genera, transferring the genes for the larval stage from sea-cucumbers to starfish to sea-urchins.