Fine morphology and ultrastructure of the collar ring muscle and tentacle elevators of Phoronopsis harmeri larvae. A – C. Fine morphology of the collar ring muscle and tentacle elevators (SEM). A. Overview of the collar ring muscle (white arrowheads). B. Part of the collar ring muscle (cr) and two tentacle elevators (te) in a competent larva. C. Disorderly net of muscle cells of the collar ring muscle in an advanced larva. D – F. Ultrastructure of the collar ring muscle (TEM). D. Overview of muscle cells with nueclei (n). E. Desmosomes (arrows) between muscle cells. F. Organization of one of the muscle cells (light blue), which has a cilium, basal body (bb), accessory centriole (ac), several Golgy complexes (G), and long processes that spread on the basal lamina (bl) and form bulbs (black arrowheads) opposite the nerve fibers (ne). G. Semi-thin cross section of the tentacle in a competent larva shows tentacular blood vessel (bv), coelomic lining (cl), frontal side epidermis (fs), the base of the tentacle elevator (teb), and the terminal bulge of the tentacle elevator (tet). H. Part of longitudinal section of the tentacle; a muscle cell with sarcomeres (s) is evident. I. muscle cells of the tentacle elevator (te), which is squeezed between the basal lamina (bl) of epidermis (ep) and the cells of the coelomic lining (cl) of the tentacular coelom.