Cilia and other surface structures of the trochophore of Harmothoë imbricata (Polychaeta).

Abstract

Ciliary aggregations occur as the prototroch, neurotroch, apical system and as tufts associated with the eyes and superficial glands. The major collection of cilia is the locomotory organ or prototroch that runs around the equatorial plane of the larva. This band is composed of four contiguous rows of cells, the two medial rows bearing the long locomotory cilia. The cilia occur in clumps, with several clumps arising from each prototroch cell while both the main cells contribute to each clump. The central filaments of these cilia are orientated at right angles to the long axis of the clump, the direction of ciliary beat being at right angles to the progression of the metachronal wave along the prototroch. The neurotroch, extending from the mouth to the posterior pole of the larva, beats away from the mouth. The rate of beating is rapid, and the cilia are short. The apical area of the larva is bordered by five single lines of compound cilia that surround a few stiff cilia. All the cilia beat occasionally. A further line of cilia, the akrotroch, exists at a position halfway between the apical area and the prototroch on the same side as the mouth. These cilia beat towards the prototroch. Some of these cilia are associated with sets of glandular openings. The fine structure of the glands and cuticle is described. The glands are small mucous glands that open via a projecting pore which is encircled by rings of microvilli. They often occur in groups of four or in pairs. The cuticle is similar to that described previously for adult polychaetes.