Protostome genera

Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own phylum, Cycliophora, also called wheel wearers, shortly after being discovered in 1995. Cycliophora was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.

Parasagitta is a genus of arrow worms (phylum Chaetognatha) in the family Sagittidae. At one time these arrow worms were classified in the genus Sagitta.

Krohnittidae is a family of chaetognaths in the order Aphragmophora. It consists of a single genus, Krohnitta von Ritter-Záhony, 1910.

Krohnittellidae is a family of sagittoideans in the order Phragmophora. It consists of one genus, Krohnittella Germain & Joubin, 1912.