VESICULATED AXONS IN HAEMAL VESSELS OF AN HOLOTHURIAN, CUCUMARIA FRONDOSA

¹ Department of Anatomy, University of Chicago, Chicago, Ill. 60637, and Mount Desert Island Biological Laboratory, Salisbury Cove, Maine

Segments of the primitive haemal rete of the holothurian, Cucumaria, were fixed in glutaraldehyde and in osmic acid, embedded in epoxy resin, sectioned for electron microscopy and stained with uranyl acetate and lead citrate. Multifibered nerve strands were found among the epithelial cell processes of the wall of the haemal vessels. Individual axons containing large (0.2 to 0.3 micron) membrane-bounded dense-cored vesicles are distributed to the non-striated muscle cells. The vesicles arise in association with the Golgi region of the neurone and large numbers are found in proximal cell processes. The vesicles containing dense aggregates are distributed along the axons, with a few present at the tapered terminal portions at the muscle cell.