Fig. 6.

Drawings of invertebrate receptor terminal complexes. a The basal tunnel of nematocytes from the cnidarian, Coryne tubulosa (Holtmann and Thurm 2001a, b), contains pre- and postsynaptic processes, like the invaginations of vertebrate photoreceptor rods and cones. Apparent presynaptic terminals (pre) in the tunnel have densecored vesicles. The nematocyte terminal area has a single magnovesicle (v). b The retinula (photoreceptor) axon terminals of crayfish can have invaginating complexes (Hafner 1974) that look very like those of vertebrate photoreceptor rods and cone synapses. c The photoreceptor terminals in the octopus form large bags (b) or carrots that contain three kinds of invaginating processes, including postsynaptic spines, tunnel fibers (t), which are nerve trunks of small neurons, and vesicle-filled finger twigs (asterisks) that extend from adjacent bags; the spines are blue and have a presynaptic cluster of vesicles (Dilly et al. 1963; Case et al. 1972) (c1 is a low magnification of a whole bag and parts of 2 adjacent bags; c2 is an enlargement of upper left part of central bag) (Color figure online)