Ultrastructure of Adult Gonads and Development and Structure of the Larva of Rhabdopleura normani (Hemichordata: Pterobranchia)

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Abstract
Sexually mature adults, embryos and larvae of the pterobranch Rhabdopleura normani from Bermuda were studied with light and electron microscopy. The sexes are separate among the zooids of a colony, but a given colony may contain females and males. In zooids of either sex the single gonad is associated with a large haemal sinus in the trunk sac and is displaced laterally (to the right or to the left). The wall of the gonad is composed of three layers: an outer metasomal peritoneum, an internal lining of germinal epithelium and an intervening genital haemal sinus. The mature gametes lie in the lumen within the gonad. The spermatozoon is characterized by an elongate nucleus, no obvious acrosome, a long mitochondrial filament in a midpiece appendix and a single flagellum with a 9+2 axoneme. Females brood 200 μm eggs and embryos in their distinctive, basally coiled tubes. The yolky eggs undergo radial cleavage and develop into ciliated, lecithotrophic, oblong larvae (400 μm in length) that are characterized by: (1) yellow coloration peppered with black pigment spots; (2) a deep ventral depression; (3) a posterior adhesive organ; (4) an anterior apical sensory organ; (5) an evenly ciliated epitdermis. The ventral depression is not invaginating endoderm, but is instead a glandular epithelium that evidently secretes the larval cocoon and the adult tube. Internally, the peritoneum of the coelomic cavities begins to split from the periphery of a large, central mass of yolky mesenchyme cells. The larva swims using cilia, but also undergoes contractions, evidently powered by the peritoneal cells, which constitute a myoepithelium. The discussion considers pterobranch affinities with other deuterostomes and with lophophorates.