In echinoderm development, conversion of the oral-aboral axis takes place during metamorphosis, and the left and right sides of larvae become the oral and aboral sides of juveniles, respectively. Since the rudiments of the adult organs are formed before metamorphosis, late brachiolaria larvae of Asterina pectinifera show distinct bilateral asymmetry. On the other hand, early bipinnaria larvae look almost symmetric bilaterally, except for hydropore and posterior coelomic pouch formation which take place only on the left side. The relationship between the axis of the adult rudiment and the bilateral asymmetry in the coeloms of early stage larvae was examined by (1) bisecting larvae, (2) discarding left or right anterior coeloms (lac, rac) and/or left posterior coelom (1pc) and (3) replacing lac or rac with the counterpart.
In normal brachiolaria larvae, adult organs such as hydrolobes and adult spicules are formed in the posterior part of the larval body. In the left half of the adult rudiment, a row of five adult arm-type spicules are aligned along the median plane. Hydrolobes, which form the hydrovascular system in the juvenile, were observed on the left of the arm-type spicules. The right half of the rudiment was covered with adult disc-type spicules (Fig. 1). The left half derived from larvae older than 40 h (mouth formation stage) lacked adult disctype spicules, and two rows of arm-type spicules existed on both side of the median plane (L-type; Fig. 2a).