The Echiura, or spoon worms, have a pear shaped, non-segmented body and a large and non-retractile proboscis. In the anterior ventral region there is a pair of setae or hooks. They live in burrows in sand, mud, corals and coral crevices. Some species are found inside the tests of dead sand dollars and between the shells of dead bivalves. There is strong evidence that echiurans are in fact modified annelids.
The subphylum Cephalochordata (called vernacularly as lancelets) is represented by a small (up to 6 cm long) animals characterized to have the notochord as an axial organ throughout the whole body length. The lancelet body plan is similar to that of fishes and other vertebrates: dorsally-situated neural tube, and ventrally-situated alimentary system sandwiched laterally with myotomes (or myomeres), arranged as a series of segmented muscle blocks on each side. They are exclusively marine, inhabiting intertidal to subtidal sandy bottoms in boreal to tropical waters around the world. Recently, however, the true deep-sea species was firstly described. The adults have at benthic life within the superficial layer of bottom sediment, and are filter-feeders. Adults can also swim, but only rarely and sporadically.
