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Cubomedusae (box jellyfish) are well known for strong directional swimming, rapid responses to visual stimuli, and complex lensed eyes comparable to those of more advanced multicellular animals. They possess a total of 24 eyes that are of four morphologically different types, yet little is known about the neural organization of their eyes. The eyes are located on ganglion-like structures called rhopalia. Each of the four rhopalia contains an upper and a lower lensed eye (with a cornea, lens, and retina), two pit ocelli, and two slit ocelli. Transmission electron microscopy was used to examine the synaptic morphology of the eyes and pacemaker region of four species of cubozoans (Tamoya haplonema, Carybdea marsupialis, Tripedalia cystophora, and Chiropsalmus quadrumanus). Invaginated synapses were found in all four species, but only in the upper and lower lensed eyes. Density measurements indicated that the invaginated synapses were located close to the basal region of photoreceptor cells, and size differences of invaginated synapses were observed between the upper and lower lensed eyes, as well as between species. Four additional types of chemical synapses-clear unidirectional, dense-core unidirectional, clear bidirectional, and clear and dense-core bidirectional-were also observed in the rhopalia. The invaginated synapses of the lensed eyes may be useful as markers to help sort out the neural circuitry in the retinal region of these complex cubomedusan eyes.

The musculature of the cubomedusa Tripedalia cystophora was investigated using immunohistochemical staining with an anti-actin antibody and histochemical staining with fluorescent phalloidin. The subumbrella is lined with a sheet of circular, striated muscle that is interrupted at the perradii, and by the nerve ring. The sheet is continuous with circular, striated muscle of the velarium, which turns radially on each face of the four velarial frenula. Perradial strips of smooth muscle run radially from just above the level of the rhopalia into the manubrium and lips. The strips give off perpendicular offshoots that run a short distance in parallel with the circular swim muscle. Musculature of the tentacles and pedalia is longitudinal and limited to the oral side of the pedalia. The pedalial muscle connects with bundles of smooth muscle that runs circularly from the tentacle base well into the subumbrella. The arrangement of striated muscle in the frenula suggests that these structures may function in directional nozzle formation of the velarium during turning. In addition, the perpendicular branching of the radial strips and the circular extensions of pedalial muscle may function in hinge formation to aid bending of the pedalia and tentacles into the subumbrella during feeding and protective responses.