Mechanisms of rapid morphogenetic movements in the metamorphosis of the bryozoan Bugula neritina (cheilostomata, cellularioidea). I. Attachment to the substratum.

The larval morphology, settlement behavior, and the rapid morphogenetic movements that occur during the first 60 sec of metamorphosis of the cellularioid cheilostome bryozoan Bugula neritina have been examined and analyzed by light and electron microscopy. The larva attaches to the substratum at the onset of metamorphosis by the eversion of the internal sac. At the same time, the coronal cilia reverse their direction of beat, spreading an adhesive secreted by the neck region of the everting sac over the metamorphosing larva. During attachment, the larva goes through several configurations that coincide with the sequential contraction and relaxation of certain larval muscles. Histological and ultrastructural evidence indicates that the neck and wall regions of the internal sac are everted by the contraction of the muscles in the equatorial plane of the larva at the same time that the roof region in pulled toward the larval equator by the contraction of the axial muscles. The subsequent relaxation of the axial muscles allows the roof region to be everted by the antagonistic force generated by the sustained contraction of the equatorial musculature. After the roof region attaches to the substratum, the apical disc is temporarily retracted by a second contraction of the axial muscles. The apical disc subsequently reextends as the axial muscles relax just before coronal involution. A comparison of the ontogenetic sequence of rapid morphogenetic movements in the metamorphoses of cheilostome and ctenostome bryozoans indicates that cellularioid cheilostomes have undergone peramorphosis in the aspect of development.