Just as equal cleavage
is characteristic for architypical mollusks, it appears to be so in Annelida and Platyhelminthes.
For example, van den Biggelaar (1996) found a trend in cleavage type and D-quadrant specification from equal cleavage and late specification in "archaeogastropods" to unequal cleavage and early specification in opisthobranchs and pulmonates.
Equal cleavage with 4d cell formation at the 64-cell stage as in "archaeogastropods" is assumed to be the ancestral condition.
Nemertean embryos typically display equal cleavage
; but the identity of the four cell quadrants differs from those described above.
rosaceus follows the typical pattern for echinoids with planktotrophic larvae, beginning with three equal cleavages
followed by the formation of mesomeres, macromeres, and micromeres at the 16-cell stage (Fig.