[Pteropoda] Diacavolinia first develops a bowl-shaped embryonic shell (protoconch I), on top of which a conical section develops; soon afterwards the conical section broadens and a transverse slit starts to separate the dorsal from the ventral sides. All soft parts migrate into this bivalve-like shell and the embryonic shell is shed; the opening left at the base of the bivalve-like section is closed by bending the ventral and dorsal walls and secreting additional shell material, but no septum is formed; this fusion gives rise to the caudal joint (Diacavolinia in ventral view ).

Alternative form for Caudal joint : closing fold.