These burrowing filter-feeders compose the earliest-branching chordate sub-phylum. A chordate () is an animal of the phylum Chordata. Vertebrates are the only chordate group to exhibit a proper brain. The invertebrate chordate classes are from "Fishes of the World". The chordate phylum has three subphyla, one of which is the vertebrates (see below). There are 19 chordate species assessed as extinct in the wild. There are two IL-17Rs (IL-17RA and IL-17RD) in the genome of the basal chordate "Amphioxus". On the basis of these findings, they constructed a new scenario for chordate evolution. "Pikaia" was a primitive chordate that lacked a well-defined head and averaged about in length. "Yunnanozoon" is similar to the form "Haikouella", which is almost certainly a chordate.