What is "heteractinida"

The Heteractinids are a grade of sponges that are paraphyletic with respect to Hexactinellida. Their distinctive trait is their six-pronged (snowflake-like) spicules, whose symmetry historically suggested a relationship with the triradial Calcarean sponges. Nevertheless, they actually represent a polyphyletic grade, from which the hexactinellids arose.