The protostomes are arguably the most successful group of animals on Earth. They are the most diverse group, with more than one million species described for arthropods alone. Also, the protostomes contain the nematodes, which probably make up the most abundant and universally distributed of all animal groups.
The protostomes can be divided into two major clades—the lophotrochozoans (including bryozoans, annelids, and mollusks) and the ecdysozoans (including nematodes and arthropods)—largely on the basis of DNA sequence analysis.
Protostomes are a group of animals with several shared derived traits, but otherwise differ from each other radically in body form. The shared derived traits of protostomes include a blastopore that develops into a mouth, an anterior brain, and a ventral nervous system.
The protostomes are divided into two major clades, the lophotrochozoans and the ecdysozoans. Annelids and mollusks, among other groups, are members of the lophotrochozoans. Several lophotrochozoan groups are characterized by a filter-feeding structure known as a lophophore or by cilia-bearing larvae known as trochophores.
Arthropods and nematodes are members of the ecdysozoans. Ecdysozoans, which have a body covering known as a cuticle, must molt periodically in order to grow.
Textbook Reference: Concept 23.3 Protostomes Have an Anterior Brain and a Ventral Nervous System