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Why do earthworms and leeches have no free-swimming larval stage?

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annelid *The structure of polychaetes.* (Left) Free-moving polychaetes. (A) *Neanthes,* (B) *Nereis.* (Right) Tube-dwelling (sedentary) polychaetes. (C) *Amphitrite,* (D) *Sabella.*

Earthworms and leeches do not have a free-swimming larval stage because their development occurs entirely within a protective cocoon.1

Unlike many polychaete annelids, which utilize a free-swimming trochophore larva for dispersal and feeding in marine environments, earthworms and leeches have evolved to bypass this pelagic phase. In these groups, the cocoon provides a stable, nutrient-rich environment—typically containing albumin—that supports the embryo until it is sufficiently developed to emerge as a juvenile that resembles the adult.123

This direct development is an adaptive strategy that eliminates the need for a vulnerable, free-living larval stage, allowing these worms to thrive in diverse habitats, including terrestrial and freshwater environments where a pelagic larval life would be impractical or hazardous.14