Pharyngeal slits are filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the pharyngeal slits. It is postulated that this is how pharyngeal slits first assisted in filter-feeding, and later, with the addition of gills along their walls, aided in respiration of aquatic chordates. [1] These repeated segments are controlled by similar developmental mechanisms. Some hemichordate species can have as many as 200 gill slits. [2] Pharyngeal clefts resembling gill slits are transiently present during the embryonic stages of tetrapod development. The presence of pharyngeal arches and clefts in the neck of the developing human embryo famously led Ernst Haeckel to postulate that "ontogeny recapitulates phylogeny"; this hypothesis, while false, contains elements of truth, as explored by Stephen Jay Gould in Ontogeny and Phylogeny . [3] However, it is now accepted[ who? ] that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates.[ citation needed ] Pharyngeal arches, pouches, and clefts are, at some stage of life, found in all chordates. One theory of their origin is the fusion of nephridia which opened both on the outside and the gut, creating openings between the gut and the environment. [4]
In vertebrates, the pharyngeal arches are derived from all three germ layers. [5] Neural crest cells enter these arches where they contribute to craniofacial features such as bone and cartilage. [5] However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest-independent mechanisms of pharyngeal arch development. [6] The first, most anterior pharyngeal arch gives rise to the oral jaw. The second arch becomes the hyoid and jaw support. [5] In fish, the other posterior arches contribute to the brachial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus. [7] The genetic and developmental basis of pharyngeal arch development is well characterized. It has been shown that Hox genes and other developmental genes such as dlx are important for patterning the anterior/posterior and dorsal/ventral axes of the branchial arches. [8] Some fish species have jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation. [9]
The presence of pharyngeal slits in hemichordates led to debates of whether this structure was homologous to the slits found in chordates or a result of convergent evolution. [10] With the placement of hemichordates and echinoderms as a sister group to chordates, a new hypothesis has emerged-suggesting that pharyngeal gill slits were present in the deuterostome ancestor . [11] Intriguingly, extant echinoderms lack pharyngeal structures, but fossil records reveal that ancestral forms of echinoderms had gill-like structures. [12] Comparative developmental and genetic studies of these pharyngeal structures between hemichordates and urochordates have brought about important insights regarding the evolution of the deuterostome body plan. [13] Comparative molecular biology has revealed that the Pax1/9 genes (which encode for transcription factors) are expressed in similar patterns between hemichordates and urochordates; In vertebrates, Pax 1 and Pax 9 are expressed in the pharyngeal pouches and are important for thymus development. [14] Applying excess retinoic acid (excess retinoic acid in vertebrates results in pharyngeal abnormalities) leads to the absence of gill slits in developing amphioxi, suggesting that retinoic acid may act through the same mechanism in vertebrates and amphioxi. [15] These studies indicate that the pharyngeal slits found in hemichordates and chordates are indeed homologous in a molecular sense.
A chordate is a deuterostomic animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation.
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is an historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (ontogeny), goes through stages resembling or representing successive adult stages in the evolution of the animal's remote ancestors (phylogeny). It was formulated in the 1820s by Étienne Serres based on the work of Johann Friedrich Meckel, after whom it is also known as Meckel–Serres law.
Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The class Graptolithina, formerly considered extinct, is now placed within the pterobranchs, represented by a single living genus Rhabdopleura.
Vetulicolia is a phylum of bilaterian animals encompassing several extinct species belonging to the Cambrian period. The phylum was created by Degan Shu and his research team in 2001, and named after Vetulicola cuneata, the first species of the phylum described in 1987.
The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of humans and most animals.
A craniate is a member of the Craniata, a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia, and the much more numerous Gnathostomata. Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish as vertebrates, making living craniates synonymous with living vertebrates.
Ostracoderms are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. The closest non-hemichordate relatives of the Enteropneusta are the echinoderms. There are 111 known species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii. Two families—Harrimaniidae and Ptychoderidae—separated at least 370 million years ago.
The pharyngeal arches, also known as visceral arches, are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arches.
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In contrast, bony fishes have a single outer bony gill covering called an operculum.
Shark anatomy differs from that of bony fish in a variety of ways. Variation observed within shark anatomy is a potential result of speciation and habitat variation.
Walter Garstang FLS FZS, a Fellow of Lincoln College, Oxford and Professor of Zoology at the University of Leeds, was one of the first to study the functional biology of marine invertebrate larvae. His best known works on marine larvae were his poems published as Larval Forms and Other Zoological Verses, especially The Ballad of the Veliger. They describe the form and function of several marine larvae as well as illustrating some controversies in evolutionary biology of the time.
Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates. Along with the cornutes, they form one half of the Stylophora.
Branchial arches, or gill arches, are a series of paired bony "loops" that support the gills in fish. As gills are the primitive feature of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arches varies between taxa. In jawed fish, the first arch pair develops into the jaw. The second gill arches develop into the hyomandibular complex, which supports the back of the jaw and the front of the gill series. The remaining posterior arches support the gills. In amphibians and reptiles, many pharyngeal arch elements are lost, including the gill arches, resulting in only the oral jaws and a hyoid apparatus remaining. In mammals and birds, the hyoid is simplified further.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.
In evolutionary developmental biology, inversion refers to the hypothesis that during the course of animal evolution, the structures along the dorsoventral (DV) axis have taken on an orientation opposite that of the ancestral form.
The calcichordate hypothesis holds that each separate lineage of chordate evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group.
Most bony fishes have two sets of jaws made mainly of bone. The primary oral jaws open and close the mouth, and a second set of pharyngeal jaws are positioned at the back of the throat. The oral jaws are used to capture and manipulate prey by biting and crushing. The pharyngeal jaws, so-called because they are positioned within the pharynx, are used to further process the food and move it from the mouth to the stomach.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Billie J. Swalla is a professor of biology at the University of Washington. She was the first female director of Friday Harbor Laboratories, where she worked from 2012 to 2019. Her lab investigates the evolution of chordates by comparative genetic and phylogenetic analysis of animal taxa.
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