Hemichordata was earlier considered a sub-phylum under phylum Chordata, but it is now placed as a separate phylum under non-chordata. Hemichordates have a rudimentary structure in the collar region called the stomochord, which is similar to the notochord. The phylum consists of a small group of worm-like marine animals with an organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk. The circulatory system is of open type. Respiration takes place through gills. The excretory organ is the proboscis gland. Sexes are separate, fertilisation is external and development is indirect. Examples include Balanoglossus and Saccoglossus. The main exam focus is the change in classification: the stomochord resembles a notochord, which is why hemichordates were once grouped with chordates, but a true notochord is absent, so they are treated as non-chordates. They also differ from chordates in lacking the dorsal hollow nerve cord and pharyngeal gill slits as defined in Chordata. The three-part body of proboscis, collar and trunk is the easiest way to identify the phylum in a diagram-based question, and the proboscis gland is the excretory organ.