Phylum Hemichordata

Hemichordata (Greek hemi, half; chorde, string) were originally treated as a subphylum of Chordata (Prochordata), but are now recognised as an independent phylum of invertebrates positioned close to Echinodermata on the classification tree. Members of this group carry a genuine mix of invertebrate and chordate characters, which is why their placement has historically been contested.

The phylum is a small group of soft-bodied, worm-like marine animals, mostly tube-dwelling, commonly called acorn worms or tongue worms. They are bilaterally symmetrical, triploblastic and coelomate, with organ-system organisation. The cylindrical body is divided into three distinct regions: an anterior proboscis, a short collar, and a long trunk. Most hemichordates feed by ciliary currents. Their circulatory system is simple, open or lacunar, with a dorsal heart. Respiration takes place through paired gill slits opening into the pharynx — the chordate-like feature that once placed this group inside Chordata. Excretion is carried out by a single proboscis gland (glomerulus) located in the proboscis, and the nervous system remains primitive. …