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70 Submitted manuscript<br />

Chapter IV<br />

Same stage as in J with animal view onto the future dorso-anterior part of the larva. The two<br />

fields of TtrNot expressing cells which are interconnected by a narrow band of TtrNot expressing<br />

cells (arrowhead) are visible. (L) Elongated gastrula with small blastopore (asterisk). The TtrNot<br />

expressing cells are distributed equally along the posterior part of the future larval apical lobe.<br />

(M) Ventral view of an early three-lobed larva with apical tuft (at), anterior lobe (al), mantle lobe<br />

(ml), and pedicle lobe (pl). The blastopore (asterisk) is closed. The border between ectoderm<br />

(ec) and mesoderm (ms) is visible in the mantle and pedicle lobe. TtrNot expressing cells are<br />

distributed in the posterior part of the apical lobe (indicated by the dashed line). (N) Ventroanterior<br />

view of a larva with all larval lobes fully established: apical lobe (al), mantle lobe (ml),<br />

pedicle lobe (pl). The blastopore (asterisk) is closed and TtrNot expressing cells are distributed<br />

in a ring along the posterior part of the apical lobe. (O) Dorsal view of a fully differentiated<br />

larva with apical lobe (al), mantle lobe (ml), pedicle lobe (pl), and setae (s). TtrNot is no longer<br />

expressed. (P) Posterior view of a juvenile at 360 hours after settlement (hps) with pedicle (p),<br />

anlage of both valves (v) ,and larval setae (s), which extend beyond the valves. TtrNot is not<br />

expressed.<br />

Fig. 4 Cdx expression in<br />

Terebratalia transversa. Scale bars<br />

equal 50 µm, age is given in hours<br />

after fertilization (hpf). (A) Gastrula<br />

stage, TtrCdx (purple) is expressed in<br />

the ectoderm of the posterior pole of<br />

the blastopore (asterisk). (B) Gastrula<br />

stage slightly older than the one in A,<br />

TtrCdx expression on the posterior<br />

side of the blastopore (asterisk) is<br />

more intense than in A. (C) Maximum<br />

projection image of a confocal<br />

reflection scan of a specimen with<br />

similar expression pattern as in B,<br />

showing that TtrCdx is still limited to<br />

the ectoderm. (D) Early larva with<br />

modest expression of TtrCdx (arrow)<br />

in the ventral part of the future mantle<br />

lobe (ml), immediately posterior to<br />

the blastopore (asterisk). The future<br />

apical lobe (al) and posterior lobe (pl)<br />

can already be distinguished. (E) Early<br />

three-lobed larva with apical lobe (al),<br />

mantle lobe (ml), and pedicle lobe<br />

(pl). TtrCdx (arrow) is expressed in<br />

the posterior part of the mantle lobe,<br />

further posterior from the almost closed blastopore (asterisk) than in previous stages. The<br />

larval apical lobe (al), mantle lobe (ml), and pedicle lobe (pl) are further developed. (F) Fully<br />

established larva with expression of TtrCdx (arrow) in the center of the ventral part of the mantle<br />

lobe (ml). Apical lobe (al), mantle lobe, and pedicle lobe (pl) are fully developed and four sets<br />

of larval setae (se) extend from the mantle lobe.

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