EXTRA-ECTODERMAL STRANDS IN THE CILIATED BANDS OF THE ECHINOPLUTEUS.

An ectodermal nerve net and some extra-ectodermal, axon-like elements have been demonstrated in the ciliated bands of Psammechinus miliaris larvae. It is proposed that these extraectodermal structures, together with the microvillous epithelial cells and the ectodermal nerves, participate in the co-ordination of responses on sensory stimulation from the outside of the larvae.

SOME ASPECTS ON PIGMENT CELL DISTRIBUTION AND FUNCTION IN THE DEVELOPING ECHINOPLUTEUS OF PSAMMECHINUS MILIARIS.

Pigment cells in Psammechinus miliaris larvae are frequently found in regions which are morphogenetically active. They can be seen in living larvae from the the midgastrula stage up to metamorphosis. Their early development and typical characters as seen by the electron microscope are described. Histochemical methods have revealed carotenoids which are preferentially located in the pigment granules. The findings are discussed in relation to published data on photopigments. It is suggested that caroteno-proteins of the pigment cells are involved in photo-activated processes which support larval migration and/or morphogenesis.

Secretory cells in the foregut of the echinopluteus.

Metachromatically-staining substances have been found in the secretion of the stomodeum and esophagus of the echinopluteus (Psammechanus miliaris). This secretion seemed to derive from specialized cells in the wall of the foregut. Ultrastructural examination revealed that these cells are rich in large cisternae containing an electron-translucent material. They are also rich in ribosomes and possess large Golgi complexes. The number of secretory cells increases with age. It is tempting to speculate that the function of these cells is analogous to that of the glandular cells of the endostyle in some chordate phyla.