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REP: The Sea tubes — Description
Sea tubes are very primitive animals that are spread across the whole world's oceans and are primarily free-swimming but adapted to a sessile life in multiple cases. Most species live in the photic zone. They swim by peristaltic contractions of their bodies, filtering out food particles with their mucus-covered tentacles that line the inner walls, where they are digested in special cell cavities; sea tubes don't possess a real mouth. They are usually very small animals with the biggest species reaching 1 cm. An array of calcite concretions in the front body is used for orientation, both as statoliths as well as part of a primitive eye system where the concretion casts a shadow on a lining of photoreceptors. Sea tubes reproduce through spores.
The more primitive, very similarly looking water tubes don't possess muscles and are using flagellae for movement.

Mucous u-tube: U-tubes in general are a branch that adapted to a sessile life. The more primitive examples use mucus for attaching with come species having a symbiotic relationship with silicate-mineralizing bacteria that stabilize the mucous coat in exchange for a part of the nutrients the tube collects.
Eightail: The members of this group use long hind tentacles to collect food particles instead of an inner lining of short tentacles. They are more prevalent in greater depths.
Mason tube: A member of the u-tubes that mineralizes a calcite shell. Higher species have tentacles and feeding cavities on their outer wall for collecting food particles as well.