Etymology: From the Greek Hemi for half and the Latin Chorda a chord. Characteristics of Hemichordata: 1)Bilaterally symmetrical. 2)Body has more than two. The Hemichordata share some characters, such as pharyngeal gill slits or pores, that have been shown to be homologous with those of chordates. However. The Hemichordata are soft-bodied, worm-like marine organisms which are often The Hemichordata share characteristics with both the Chordata (their name.

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Of sawblades and acorns. Platyhelminthes flatworms Gastrotricha hairybacks.

The Hemichordates (Phylum Hemichordata)

The third cleavage is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole. The collar bears several hollow branched arms in two rows, they bear tentacles and are spoken of as tentaculated arms, the arms bear cilia which drive a food laden current of water into the mouth.

The body has a proboscis, collar and trunk borne on a hollow muscular stalk. Anatomy of Saccoglossus kowalevskii [6]. The anteroposterior axis is divided into three parts: Together with the Echinodermsthe hemichordates form the Ambulacrariawhich are the closest extant phylogenetic relatives of chordates among the invertebrates.

Introduction to the Hemichordata

Proboscis characterristics collar are short, while trunk is quite elongated. The individual animals lived within structures along the length of a community exoskeleton formed from collagen. The extinct class Graptolithina is closely related to the pterobranchs. The collar is modified to produce between 1 and 9 pairs of tentacles or lophophore arms.

In the collar their are giant nerve fibres, but their function is poorly understood and the animal can survive reasonable well without them.


As deuterostomes, the hemichordates are a sister group to both the true Chordates and the Echinoderms. Journal hemichordaga Experimental Zoology. Feeding is either filter feeding or substrate eating. It is also used as an organ of locomotion, hemichodata in the way a snail uses its foot, both for movement inside and outside the burrow.


Substrate eaters like Balanoglossus clavigerus from the Mediterranean are generally larger than filter feeders. A third class, Planctosphaeroideais proposed based on a single species known only from larvae. Enteropneusta acorn wormsand Pterobranchia. They have a wide distribution and can be found in most of the worlds waters both in shallow hemicnordata waters and in the deeper seas.

Gaseous exchange occurs over the whole body as well as in the pharyngeal slits. Micrognathozoa Limnognathia Syndermata Rotifera wheel animals Acanthocephala thorny-headed worms.

Most people are unfamiliar with the animals within Phylum Hemichordata, which is small and contains but a few hundred species. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres.

Views Read Edit View history. The fourth division occurs first in the cells of the animal pole, which end up making eight blastomeres mesomeres that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres macromeres and four very small blastomeres micromeres. A zooid of Cephalodiscus has a general structure like that of Balanoglossus. Development is indirect involving a free-swimming tornaria larva.

The hrmichordata egg develops into a ‘tornaria’ larvae and lives as part of the plankton floating in the sea for several weeks until it undergoes a metamorphosis into the 3 body sections that the adult possesses and sinks to the sea-floor. In the genus Rhabdopleurazooids are permanently connected to the rest of the colony via a common stolon system. Wikimedia Commons has media related to Hemichordata. In the anterior part of the trunk is a characterstics pair of gill-clefts.


Development mostly indirect through a free swimming tornaria larva. A zooid is minute, its general structure is like an enteropneust with a proboscis, collar and trunk, attached to the trunk is a stalk.

The females extrude egg masses onto the surface of the substrate from within their burrows 2, to 3, eggs at a time. Behind this is a ‘trunk’ which contains the digestive and reproductive organs.

Introduction to the Hemichordata

All known species feed on small particles of organic matter either as filter feeders of as substrate eaters. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles.

The major distinction between a branch and a leaf of the Tree of Life is that each branch can be further subdivided into descendent branches, that is, subgroups representing distinct genetic lineages. Acorn worms are solitary worm-shaped organisms. Here’s how it works: Acorn worma hemichordate.

Kinorhyncha mud dragons Priapulida penis worms. Note that images and other media featured on this page are each governed by their own license, and they may or may not be available for reuse.

Studies of this nature have been done particularly on S. Gonads are situated in the hemichodata wings. The nervous system derives from a sub-epidermal net and is thickened in the mid-dorsal region of the proboscis and the mid-ventral area of the trunk into hollow nerve cords. Classification of Phylum Hemichordata 2. Aspidograptus smithonensis Graptolite cambrian Tasmania, Australia.