Phylum Hemichordata and Phylum Chordata — General Features

Phylum Hemichordata is a small group of worm-like, exclusively marine animals — historically classified as a subphylum of Chordata because of certain superficial resemblances, but now recognised as a wholly separate, non-chordate phylum. The soft, cylindrical body is organised into three clear regions: an anterior proboscis, a collar, and a long trunk. Hemichordates are triploblastic, coelomate and bilaterally symmetrical, with organisation at the organ-system level, and the sexes are separate. Balanoglossus and Saccoglossus are the standard examples.

The phylum Chordata is defined, at some point in the life cycle (even if only in the embryo), by three fundamental features found together in no other phylum: a notochord; a dorsal, hollow (tubular) nerve cord, lying above the notochord — in sharp contrast to the solid, ventral nerve cord of annelids and arthropods; and paired pharyngeal gill slits, openings connecting the pharynx to the exterior, used for respiration in aquatic chordates and, in land vertebrates, retained only as an embryonic feature with no adult respiratory role. A fourth feature, a post-anal tail (a tail extending behind the anal opening), is present at some stage in nearly all chordates. …