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Lester, S. M . 1988. Settlement and metamorphosis of Rhabdopleura ~~ormani (Hemichordata: Pterobranchia). (Department of Biological Sciences. Clemson University. Clemson. SC. U.S.A.) -Actu Zoo/. (Stockh.) 69. 11 1-120.Settling and metamorphosing larvae of the pterobranch hemichordate Rhabdopleura normani from Bermuda were examined with light microscopy, providing the first detailed description of the morphological changes during larval metamorphosis in pterobranchs. The swimming larvae settle within 24 h after being released from the parent tube. The settled larva immediately everts the ventral depression and surrounds itself with a translucent, sealed cocoon. During metamorphosis. which takes place over 7-10 days within the protective cocoon, structural changes occur in the following sequence: (1) differentiation of the oral shield from the anterior region; (2) budding of the two arms from the mid-dorsal surface; (3) development of the posterior stalk from the adhesive organ; (4) budding of the tentacles on the arms; (5) formation of the pharynx from invaginated ectoderm; ( 6 ) formation of the gut and pericardium from the remaining mesenchyme. The gut is the last structure to become functional. The larva relies on stored nutrients throughout metamorphosis. The metamorphosed juvenile builds a tube at an opening in the cocoon and begins to feed. Similarities with lecithotrophic bryozoan and enteropneust larvae are discussed.

Lester, S. M . 1988. Ultrastructure of adult gonads and development and structure of the larva of Rhabdopleuru normani (Hemichordata: Pterobranchia). (Department ol' Biological Sciences. Clemson University, Clemson, SC, U.S.A.) -Acra zool. (Stockh.) 69. 95-109.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 mitochondria1 filament in a midpiece appendix and a single flagellum with a 9+2 axoneme. Females brood 200 pm eggs and embryos in their distinctive. basally coiled tubes. The yolky eggs undergo radial cleavage and develop into ciliated. lecithotrophic, oblong larvae (400 p m in length) that are characterized by: ( I ) 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 epidermis. The ventral depression is not invaginating endoderm, but is instead a glandular epithelium that evidently secretes the larval cocoon and the adult tube. Internally. thc 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 myoepithclium. The discussion considers pterobranch affinities with other deuterostomes and with lophophorates.