On the Early Development of the Vestimentiferan Tube WormRidgeiasp. and Observations on the Nervous System and Trophosome ofRidgeiasp. andRiftia pachyptila
Stages in the development of the vestimentiferan Ridgeia sp., based on lengths of preserved specimens processed for scanning electron microscopy, were examined. Features of the nervous system and trophosome of Riftia pachyptila were studied by light, scanning-, and transmission-electron microscopy. Development proceeds from a trochophore-type larva with an anterior prototrochal ciliary ring and a posterior assemblage of transient larval setae, through intermediate stages, some of which lack endosymbiotic bacteria but all of which display additional transient features such as larval branchial filaments, a ventral medial process, and digestive tract, to a young juvenile stage that possesses endosymbiotic bacteria and exhibits the morphology characteristic of adult vestimentiferans. Larval branchial filaments are resorbed in later developmental stages and replaced by paired rows of ciliated branchial filaments with pinnules. The larval gut is divisible into foregut, midgut, and hingut regions based on cytological features of the epithelium. The establishment of the symbiotic association in the midgut region is confirmed. The development of the gut and the establishment of the endosymbiotic association appear to be correlated with the timing of settlement by the young juveniles. Aspects of the development of the nervous system include the appearance of the brain near the base of the ventral medial process followed by the development of a nerve cord in the epithelium of the body wall throughout the length of the juvenile. The nerve cord includes one or two giant axons, except in its most posterior region. The trochophore larva likely serves as a dispersal stage in the life history of vestimentiferans. The trochophore larva in the early development of vestimentiferans strengthens the assertion that Vestimentifera and Annelida are closely related.