The aptly named Ping-pong tree sponge (Chondrocladia lampadiglobus) is a carnivorous sponge. Those ping-pong ball looking things are covered in tiny spicules which the sponge uses to catch tiny crustaceans.
Sea Butterfly photographed by Ingo Arndt. Tiny marine shelless snails known as sea butterflies take many forms, including heart-shaped, such as this species in Antarctica's Weddell Sea. via National Geographic
Clione limacina, common name Naked and Common Clione, is a sea angel found from the surface to greater than 500 m in depth. Distribution of Clione limacina is mostly in cold waters, from the Arctic Ocean to North Carolina, Alaska
Nudibranch by Dermal Denticles via ferrebeekeeper: Gastropod mollusks - Range: shallow intertidal surf to Most are benthic seafloor dwellers, but some float upside down at ocean's surface or swim in water column.
*SAGAMINOPTERON PSYCHEDELICUM NUDIBRANCH ~ Originally described from Guam, this animal is now known from many parts of the tropical western Pacific Maximal size : 20 mm. Frequently on the reef flat zone and the fore reef zone. (psychedelicum, indeed.