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.
D, Further advanced trochosphere (optical section).