Phylum : Hemichordata / Stomachordata

PHYLUM: HEMICHORDATA / STOMOCHORDATA

Hemichordata was earlier considered as a sub-phylum under chordata. But now it is placed as a separate phylum under non-chordata.

The phylum consists of a small group of worms like marine animals with organ system level of organisation.

Its characteristics are :

(i) In hemichordata or stomochordata, true notochord is absent. gill slits are present but they are dorso lateral in position. They are worm like. bilaterally symmetrical. triploblastic and entero-coelomate animals.

(ii) Stomochord is a hollow outgrowth arising from the roof of the buccal cavity, also called 'buccal diverticulum'. It is present in the proboscis.

(iii) Dorsal heart, ventral hollow nerve cord, no respiratory pigments. Circulatory system is open. Respiration by gills.

(iv) Development is mostly indirect through a free swimming tornaria larva. e.g. Balanoglossus (acorn, tongue worm), Glossobalanus. Sexes are separate. Fertilization is external.

(v) Believed to be connecting link between non-chordates and chordates.

(vi) Due to the absence of true notochord in hemichordata, many taxonomists do not consider these animals as chordates.

(vii) Excretory organ is proboscis gland.

(viii) The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk.

Examples : Balanoglossus, Saccoglossus

Phylum Hemichordata

This phylum is made up of marine deuterostome creatures. They are sometimes referred to as the Echinodermata's sister group. Phylum Hemichordata is a small phylum with only 100 recognized species and animals that resemble worms. Some species live alone, while others live in colonies. Hemichordata was previously classified as a sub-phylum of the Chordata phylum. However, because these species lack a post-anal tail or even the notochord, which is a distinguishing trait of chordates, they are now classified as a different phylum under non-Chordata.

Hemichordates, on the other hand, have a primitive structure comparable to the notochord in the collar region. This phylum is made up of a tiny group of worm-like marine organisms that are organized at the organ-system level.This phylum is made up of marine deuterostome creatures. They are sometimes referred to as the Echinodermata's sister group. Hemichordatesare bilaterally symmetrical and triploblastic animals.The body is divisible into proboscis, collar, and trunk.Simple, open, and well-developed circulatory system is present. The alimentary canal is a straight, complete, or U-shaped tube.Excretion by a single glomerulus situated in the proboscis.Reproduction is mainly sexual. Sexes are usually separate.External fertilization takes place in seawater.Development can be direct or indirect.An important example of a hemichordate is Balanoglossus or acorn worm. Other species are Saccoglossus, Rhabdopleura, Atubaria, and Dendrograptus.