This larva is termed the Trochosphere larva, and typically (as it is held) is an egg-shaped larva with two bands of cilia, one preoral and one postoral, with an apical nervous plate surmounted by a tuft of longer cilia, and with a simple bent alimentary canal, with lateral mouth and posterior anus, between which and the ectoderm is a spacious cavity (blastocoel) traversed by muscular strands and often containing a larval kidney.
A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger.
But in these epibolic forms, just as in the embolic Paludina, the embryo proceeds to develop its ciliated band and shellgland, passing through the earlier condition of a trochosphere to that of the veliger.
B, The diblastula has become a trochosphere by the development of the ciliated ring y r (optical section).
C, Side view of the trochosphere with commencing formation of the foot.
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