January 29, 2019


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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Nematoda roundworms Nematomorpha horsehair worms.

It is soft-bodied, slender marine burrowing animal. In the genus Cephalodiscusasexually produced individuals stay attached to the contractile stalk of the parent individual until completing their development.

Proceedings of the Royal Society B: A post-anal tail is present in juvenile member of the hemichirdata worm family Harrimaniidae. Asexual reproduction by budding is charxcteristics and often gives rise to colonies starting from a single individual. Sexes are separate, each sex uemichordata a single gonad with a gonopore on the right side.

Tree of Life Web Project. Together with the Echinodermsthe hemichordates form the Ambulacrariawhich are the closest extant phylogenetic relatives of chordates among the invertebrates. The first cleavage occurs from the animal to the vegetal cuaracteristics and usually is equal though very often can also be unequal.

The trunk is the longest part of the animal. Once this fourth division has occurred, the embryo has reached a 16 cell stage. Alimentary canal is straight and anus lies at the posterior end of the body. Both sexes have numerous gonads and fertilisation is external. Each subsequent individual zooid was housed within a tubular or cup-like structure called a theca.


Development mostly indirect through a free swimming tornaria larva. Synapticula are absent so that tongue bars hang freely in their gill-slits.

Anatomy of Uemichordata kowalevskii [6].

In the anterior part of the trunk is a single pair of gill-clefts. An original hemichorsata is produced sexually, on its trunk is a peduncle which bears buds, the buds do not become free and each forms a zooid. They live in burrows in the substrate mud or fine sand or under rocks, in both shallow and deeper waters.

Pterobranchs live in much deeper water than the Enteropneusts, this and their soft bodies means that they are difficult to study and much less is known about their ecology. At present there are about 90 known species Barnes et al. There are three classes of Hemichordates. The adult form is yet unknown.

Introduction to the Hemichordata

Habitat, Development and Affinities. The Enteropneusta are the acorn worms that remain extant. The mouth is located between the proboscis and the collar. The two classes share some attributes but are quite different in others. Their other organ systems are similar to those of the Enteropneusts see above.

Phylum Hemichordata

Circulatory system simple and well developed; closed type; usually with a contractile heart vesicle and two longitudinal vessels, one dorsal and one ventral, interconnected by lateral vessels and sinuses.

They turn out to be, nonetheless, very important in the study of the evolutionary origins of vertebrates. The proboscis is modified into into a shield which secretes the collagenous burrow in those species that live in cyaracteristics. Each ToL branch page provides a synopsis of the characteristics of a group of organisms representing a hemjchordata of the Tree of Life.

Introduction to the Hemichordata

The pterobranchs are sedentary, tube-dwelling animals having a short body measuring about mm in length. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles.


The Pterobranchia are also extant and form a small and obscure group that form colonies in which the individuals are interconnected by stems, hemichorxata stolons made from collagen protein. The fertilised 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.

As deuterostomes, the hemichordates are a sister group to both the true Chordates and the Echinoderms. Ptychodera is found in the sea of tropical and subtropical regions. Nematoida Nematoda roundworms Nematomorpha horsehair worms.

It is forced through the animal’s few vessels by the beating of a muscular heart that forces blood through the central sinus. Likewise most authors Meglitsch characteristlcs Schram, ; Ruppert and Barnes, have classified it into 2 classes— Enteropneusta and Pterobranchia.

Chxracteristics of Classification 4. They are all marine worm-like animals. The basic body plan is tripartite in three sections. The animal mesomeres of P. Each animal is housed in a zooid. Trunk cuaracteristics differentiated into anterior branchial region and posterior branchial region. The proboscis is small in both types and the collar is very small.

Proboscis and collar are short, while trunk is quite elongated.