Hooge & Tyler, 2005 (citation): Acoels with a ventral mouth opening. Body-wall musculature with circular, longitudinal, and longitudinal crossover muscle fibers in both the dorsal and ventral body wall, and U-shaped fibers in the ventral body wall. Male copulatory organ with muscular, isodiametric, tubular penis, often very glandular. Penis musculature with inner circular and outer non-anastomosing longitudinal fibers. Penis is invaginated into a muscular seminal vesicle, if present. Male gonopore ventral, subterminal, supraterminal, or terminal at posterior end. Spermatozoa with 9+2 axonemes and cortical microtubules. Never with symbiotic algae. Ocelli, when present, do not contain platelets in the pigment cell.[From Hooge MD, Tyler S (2005)]
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