Nemertea analyses by molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. Most protostome phyla outside the Lophotrochozoa are members of Ecdysozoa ("animals that molt"), which include Arthropoda, Nematoda and Priapulida. Most other bilaterian phyla are in the Deuterostomia, which include Echinodermata and Chordata. The Acoelomorpha, which are neither protostomes nor deuterostomes, are regarded as basal bilaterians. Nemertea Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Although
Klein four-group representation on four points: In this representation, V is a normal subgroup of the alternating group A (and also the symmetric group S) on four letters. In fact, it is the kernel of a surjective group homomorphism from S to S. Other representations within S are: They are not normal subgroups of S According to Galois theory, the existence of the Klein four-group (and in particular, the permutation representation of it) explains the existence of the formula for calculating the roots of quartic equations in terms of radicals, as established by Lodovico Ferrari: the map corresponds to the resolvent cubic,