Early development of the sabellid polychaete bispira volutacornis

Early development of Bispira volutacornis is described up to the early trochophore stage using light and scanning electron microscopy. Gametes were obtained through both natural and induced spawning of the polychaetes. The spermatozoon is an ect aquasperm, primitive type, and c. 58.5 μm in total length. Mature oocytes, 118 μm in diameter, are encased in a vitelline envelope, 275 μm in diameter, negatively buoyant, opaque and olive green in colour under reflective light. Division of the oocytes is by equal spiral cleavage and a transitional three-cell stage is observed. The blastula is ciliated and rotates clockwise while still within the vitelline envelope. Gastrulation is by invagination, producing a gastrula that is oblong in shape. Prior to the gastrula assuming a basic trochophore configuration, elliptical in profile, the vitelline envelope is discarded. An equatorial ciliary band, the prototroch, an elliptical tuft of cilia and a central red eyespot are all prominent features of the trochophore.