Acoels reproduce sexually, although some species also reproduce asexually by fission (asexual reproduction). Acoela have no distinct gonads; gametes are formed directly from the mesenchyme, or middle tissue layer. Acoel spermatozoa bear two flagella on each sperm cell, another primitive feature.
Acoels differ from other bilaterally symmetrical animals in the way in which their embryonic cells divide during development. A fertilized acoel egg divides once; then the two resulting cells subdivide further into many smaller cells. This pattern contrasts with the eggs of all other bilateral animals, each of which divides first into two and then into four cells that go on to divide into many smaller cells. According to Henry et al., the acoel pattern of embryonic development supports the hypothesis that the acoels branched off from the ancestral line of bilateral animals very early, and may thus represent an earlier evolutionary experiment in body structure.
Some adult acoela reproduce asexually by fission in one of three possible ways: architomy, in which smaller pieces separate from the maternal animal prior to organ differentiation; paratomy, or transverse fission, in which organ differentiation takes place within the fragments before separation; and budding, or local tissue reorganization in which a small bud or outgrowth detaches itself from the parent organism and lives independently.
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