THE LARVAL DEVELOPMENT AND ECOLOGY OF THORACOPHELIA MUCRONATA (TREADWELL)

Abstract

1. The larval development of Thoracophelia mucronata follows the same pattern as that of O phelia bicornis, but is much more rapid, the eggs being less than a quarter of the volume of those of O. bicornis when mature. 2. The larval development shows differences in the formation of the anterior segments and the anterior regions of the gut. 3. Larvae of both species show the development of the adhesive papillae at the time of settling, and this is regarded as an adaption to settling on an unstable substratum. 4. The great development of hemoglobin in Thoracophelia mucronata may be interpreted as an adaptation against surf action, the hemoglobin functioning as an oxygen-store when the stirring of the surface layers of sand causes the worms to burrow more deeply. 5. Attention is drawn to the development of sensory papillae, and the elongated chaetae of the most posterior segments in relation to rectal respiration which is carried on near the surface when the sand is not being disturbed. 6. The optimal tidal level for adult worms is above that of the younger stages; the relation between zonation and the size of the worms is discussed.