Extant hemichordates consist of two groups with different This idea goes back to the early nineteenth in relation to de novo formation of chordate-specific, dorsal structures of the embryo. Chordate: Chordate, any member of the phylum Chordata, which includes the Some classifications also include the phylum Hemichordata with the chordates. Chordates enter into a wide variety of symbiotic relationships and are especially . vertebrates): Notochord extends to the back of a well-developed head;. PDF | Chordates and vertebrates are two groups of higher animals. The main difference between chordates and vertebrates is that . Chordates: Vertebrates, urochordates, and cephalochordates are . Welcome back!.
The phylum Chordata typically includes three subphyla, Cephalochordata, Vertebrata, and Tunicata, the last showing a chordate body plan only as a larva. Hemichordates, in contrast, have pharyngeal gill slits, an endostyle, and a postanal tail but appear to lack a notochord and dorsal neural tube.
Because hemichordates are the sister group of echinoderms, the morphological features shared with the chordates must have been present in the deuterostome ancestor. No extant echinoderms share any of the chordate features, so presumably they have lost these structures evolutionarily. We review the development of chordate characters in hemichordates and present new data characterizing the pharyngeal gill slits and their cartilaginous gill bars.
We show that hemichordate gill bars contain collagen and proteoglycans but are acellular. Hemichordates and cephalochordates, or lancelets, show strong similarities in their gill bars, suggesting that an acellular cartilage may have preceded cellular cartilage in deuterostomes. Our evidence suggests that the deuterostome ancestor was a benthic worm with gill slits and acellular gill cartilages.
The Global Diversity of Hemichordata
Chordates traditionally include vertebrates, lancelets cephalochordatesand tunicates, but tunicates do not exhibit a chordate body plan as adults Zeng and Swalla fig. Hemichordates are sister group to echinoderms and both phyla are an outgroup to the rest of the chordates Cameron, Garey, and Swalla ; Peterson ; Smith et al.
Xenoturbella has been recently included in the deuterostomes as molecular evidence unites them with hemichordates and echinoderms, but their exact position within the deuterostomes is not yet clear Bourlat et al. All five chordate characteristics postanal tail, dorsal nerve cord, notochord, endostyle, and pharyngeal gill slits have at one time or another been suggested to have homologous structures present in hemichordates, but all these features are lacking in echinoderms and Xenoturbella, the closest relatives to hemichordates, suggesting that they were lost during their evolution Smith et al.
Echinoderms and hemichordates are sister groups, while tunicate relationships are still not well resolved.
The placement of Xenoturbella is still not exactly certain. Chordate characters indicated on the phylogeny that unite tunicates with cephalochordates and vertebrates are present only in the tunicate tadpole larva. Redrawn from Swalla and Zeng and Swalla Recent evidence suggests that hemichordates lack a notochord and a dorsal central nervous system CNSwhich are found in chordates Lowe et al.
These two structures are linked in chordates as the notochord induces the CNS during development Von Straaten et al. The notochord is a stiff rod-like structure present in chordates that provides support against the hydrostatic skeleton for propelling the animal forward when it swims Kardong In hemichordates, an anterior diverticulum of the gut has been described as histologically similar to the chordate notochord, so has been called a stomochord Willey ; Balser and Ruppert Brachyury is a T-box transcription factor that specifies notochord and is expressed in tunicate, cephalochordate, and vertebrate notochords mesoderm during development Wilkinson, Bhatt, and Herrmann ; Yasuo and Satoh; Holland et al.
However, it has been shown that this gene is not expressed in the hemichordate stomochord, though it is expressed in other tissues Peterson et al.
In sea urchins, brachyury is expressed in the secondary mesenchyme cells mesoderm Harada, Yasuo, and Satohand in Drosophila, it is expressed in the posterior gut endoderm Singer et al. Thus, developmental gene expression data do not confirm the homology between the notochord and the stomochord, but it does not completely rule it out either see Gerhart, Lowe, and Kirschner The dorsal neural tube is a chordate characteristic at one time thought to be present in hemichordates, albeit in a more limited form, namely the collar nerve cord Bateson Hemichordates have a diffuse nervous system composed of a middorsal nerve cord, a midventral nerve cord in the collar and trunk, as well as an epidermal nerve net throughout all body regions Knight-Jones The dorsal nerve cord in the collar region has been hypothesized to have homology with the chordate neural tube because it is hollow and forms by rolling up into a tube during development Morgan However, Lowe et al.
These results suggest that the deuterostome ancestor may have had an epidermal nerve net rather than a CNS, yet the anterior-posterior patterning of the ectoderm was already present Holland In contrast, three of the chordate characteristics are present in hemichordates, a ventral postanal tail, an endostyle, and pharyngeal gill slits.
A ventral postanal tail is present in juvenile harrimaniid hemichordate worms Bateson ; Burdon-Jones ; Cameronand it has been proposed to be homologous to the chordate postanal tail due to posterior Hox gene expression data Lowe et al. Furthermore, they share specific amino acid motifs with the echinoderms Petersonstrongly supporting their sister-group relationship Zeng and Swalla It is interesting that a postanal tail has only been reported in species of the direct-developing Harrimaniidae Bateson ; Burdon-Jones ; Cameronnot in the Ptychoderidae Urata and Yamaguchi These two families are paraphyletic with 18S rDNA analyses, with harrimaniids a sister group to the colonial pterobranchs Halanych ; Cameron, Garey, and Swalla ; however, theses two families are monophyletic when a 28S rDNA Winchell et al.
A postanal tail not being present in the indirect-developing ptychoderid acorn worms means that either it was lost in ptychoderids or the direct-developing harrimaniid worms more closely resemble the chordate ancestor.
The endostyle is an iodine-binding organ present in the tunicate and cephalochordate pharynx and is considered by many researchers to be a homologous organ to the vertebrate thyroid Sasaki et al.
The Global Diversity of Hemichordata
The hemichordate epibranchial ridge, like the chordate endostyle, is composed of specialized secretory cells Ruppert, Cameron, and Frickand these cells bind iodine. However, iodine binding is not restricted to this region but instead occurs all throughout the pharynx Ruppert Likewise, the hemichordate homolog of the gene NK2. Based on these data, it appears that the endostyle function in chordates is accomplished broadly by the pharynx in hemichordates.
The most convincing homology between hemichordates and chordates is the pharyngeal gills. Hollandlamprey Ogasawara et al. The expression of the most anterior Hox gene, Hox1, is first seen in vertebrates in the second pharyngeal slit and the expression of Hox1 in hemichordates is seen at the level between the first and second gill slits, suggesting that the location of the gill slits along the anterior-posterior axis is also homologous Lowe et al.
Acorn worms are generally slow burrowers. To obtain food, many acorn worms swallow sand or mud that contains organic matter and microorganisms in the manner of earthworms this is known as deposit feeding. At low tide, they stick out their rear ends at the surface and excrete coils of processed sediments casts. They rarely leave their burrows, which may have several openings.
Another method that some acorn worms use to obtain food is to collect suspended particles of organic matter and microbes from the water. This is known as suspension feeding. Organic material adheres to mucus on the proboscis and is moved by cilia to the mouth.
The mouth can be covered by the collar to avoid eating inorganic or other undesirable items. Some acorn worms live in other environments, such as in vegetation seaweed or plant roots or sand in a shell, and specimens in deep water have been observed moving freely across the ocean floor. Acorn worms have separate genders that release eggs and sperm into the water for external fertilization.
Hemichordata - New World Encyclopedia
In some, eggs develop into free-swimming larvae that look very similar to echinoderm larvae. After a number of weeks, the larvae change into tiny acorn worms and settle on the surface and take on the burrowing lifestyle.
Others do not have a larval stage, but develop directly into small juveniles. Pterobranchia Pterobranchia is a class of Hemichordata that live in secreted tubes on the ocean floor, and feed by filtering plankton out of the water with the help of cilia attached to tentacles.
Unlike the enterpneusts, the pterobranchs possess only one, or even no, pharylgeal slits and each animal has only a single gonad, while enterpneuts have numerous gonads. The collar has between one and nine pairs of tentacles, each of which has a double row of smaller ciliated tentacles.
There are about 30 known living species in the group. These are small, and range from one millimeter to 12 millimeters. Pterobranchia was established by Ray Lankester in It contained, at that time, the single genus Rhabdopleura. Rhabdopleura was at first regarded as an aberrant Polyzoon, but with the publication of the Challenger Report Cephalodiscus init became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta.
Recent advances in electron microscopy have suggested that pterobranchs belong to the same clade as the extinct graptolites. New insights from phylogenetic analyses of deuterostome phyla. Acorn worms and pterobranchs. Version 01, January