Scientists have long observed snails "surfing" the oceans on such rafts, which can serve as flotation devices, egg-storage areas, and platforms for young snails. - http://news.nationalgeographic.com/news/2011/10/pictures/111...

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Note added at 51 mins (2017-07-30 21:24:48 GMT)
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In bubble-rafting snails, the eggs came first -
http://ns.umich.edu/new/releases/8604-in-bubble-rafting-snai...

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Note added at 53 mins (2017-07-30 21:27:39 GMT)
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"We thought of two possibilities," said Churchill, who did the work under the direction of Diarmaid Ó Foighil, a professor of ecology and evolutionary biology and a curator of mollusks at the U-M Museum of Zoology. "The first was that bubble rafting evolved from juvenile droguing." In many species of marine snails, the juveniles produce a mucus thread called a drogue that helps them drift from place to place like a kite on a string. Adding air-filled mucus bubbles to the drogue thread could result in something resembling a bubble raft.

The other possibility was that rafts represent modified egg masses. In wentletraps, which belong to the family Epitoniidae, females remain on their hosts, attached by stretchy mucus threads to tethered egg masses. These egg masses typically have egg capsules in various stages of development, from newly encased embryos to empty husks, and the researchers reasoned that in an intertidal species, the empty husks might trap air, making the egg mass and attached female temporarily buoyant. As in the drogue scenario, adding mucus-filled bubbles to this ephemeral raft could lead to development of permanent bubble rafts. Either way, getting to the surface would give the snails access to a completion-free food source: floating jellyfish. - https://www.sciencedaily.com/releases/2011/10/111011112925.h...