Protostomia is a clade of animals. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria comprising animals with bilateral symmetry and three germ layers; the major distinctions between deuterostomes and protostomes are found in embryonic development and is based on the embryological origins of the mouth and anus. In most, but not all protostomes, the mouth forms first the anus, whereas the reverse is true in deuterostomes. In animals at least as complex as earthworms, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut. In deuterostomes, the original dent becomes the anus while the gut tunnels through to make another opening, which forms the mouth; the protostomes were so named because it was once believed that in all cases the embryological dent formed the mouth while the anus was formed at the opening made by the other end of the gut. It is now known that the fate of the blastopore in protostomes is variable.
While the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name'protostome' is disputable. Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, the anus forms secondarily. In deuterostomes, the anus forms first. Most protostomes have schizocoelous development, where cells fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms through invagination of the endoderm; the common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to about 60,000 deuterostome species. Protostomes are divided into e.g. arthropods, nematodes.
A modern consensus phylogenetic tree for the protostomes is shown below. The timing of clades radiating into newer clades is given in mya; the Taxonomicon for Karl Grobben Media related to Protostomia at Wikimedia Commons
The bilateria, bilaterians, or triploblasts, are animals with bilateral symmetry, i.e. they have a head and a tail as well as a back and a belly. The bilateria are a major group of animals, including the majority of phyla but not sponges, ctenophores and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm and ectoderm. Nearly all are bilaterally symmetrical, or so. Except for a few phyla, bilaterians anus; some bilaterians lack body cavities, while others display primary body cavities or secondary cavities. Some of the earliest bilaterians were wormlike, a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan have a head end and a tail end as well as a back and a belly. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.
The body stretches back from the head, many bilaterians have a combination of circular muscles that constrict the body, making it longer, an opposing set of longitudinal muscles, that shorten the body. They have a gut that extends through the cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; the hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian". The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups, while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla have lost body cavities secondarily; the first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago. Earlier fossils are controversial.
Fossil embryos are known from around the time of Vernanimalcula, but none of these have bilaterian affinities. Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, are believed to be at least 585 million years old. There are superphyla, of Bilateria; the deuterostomes include the echinoderms, chordates, a few smaller phyla. The protostomes include most of the rest, such as arthropods, mollusks, so forth. There are a number of differences, most notably in. In particular, the first opening of the embryo becomes the mouth in protostomes, the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes and Spiralia; the arrow worms have proven difficult to classify. A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial and the tree has changed between 2000 and 2010, it is indicated when clades radiated into newer clades in millions of years ago.
The original bilaterian is hypothesized to have been a bottom dwelling worm with a single body opening. It may have resembled the planula larvae of some cnidaria. Embryological origins of the mouth and anus Tree of Life web project — Bilateria University of California Museum of Paleontology — Systematics of the Metazoa
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are pronounced; some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as chordates produce a third layer, between these two layers. Making them triploblastic. Germ layers give rise to all of an animal’s tissues and organs through the process of organogenesis. Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. In 1817, Heinz Christian Pander discovered three primordial germ layers while studying chick embryos. Between 1850 and 1855, Robert Remak had further refined the germ cell layer concept, stating that the external and middle layers form the epidermis, the gut, the intervening musculature and vasculature; the term "mesoderm" was introduced into English by Huxley in 1871, "ectoderm" and "endoderm" by Lankester in 1873. Among animals, sponges show the simplest organization. Although they have differentiated cells, they lack true tissue coordination.
Diploblastic animals and Ctenophora, show an increase in complexity, having two germ layers, the endoderm and ectoderm. Diploblastic animals are organized into recognisable tissues. All higher animals are triploblastic, possessing a mesoderm in addition to the germ layers found in Diploblasts. Triploblastic animals develop recognizable organs. Fertilization leads to the formation of a zygote. During the next stage, mitotic cell divisions transform the zygote into a hollow ball of cells, a blastula; this early embryonic form undergoes gastrulation, forming a gastrula with either two or three layers. In all vertebrates, these progenitor cells differentiate into all adult organs. In the human embryo, after about three days, the zygote forms a solid mass of cells by mitotic division, called a morula; this changes to a blastocyst, consisting of an outer layer called a trophoblast, an inner cell mass called the embryoblast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida and undergoes implantation.
The inner cell mass has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears; the epiblast in this region moves towards the primitive streak, dives down into it, forms a new layer, called the endoderm, pushing the hypoblast out of the way The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now called the ectoderm; the endoderm is one of the germ layers formed during animal embryonic development. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm; the endoderm consists at first of flattened cells. It forms the epithelial lining of the whole of the digestive tract except part of the mouth and pharynx and the terminal part of the rectum, it forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas. The endoderm forms: the pharynx, the esophagus, the stomach, the small intestine, the colon, the liver, the pancreas, the bladder, the epithelial parts of the trachea and bronchi, the lungs, the thyroid, the parathyroid.
The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm; the formation of a mesoderm leads to the development of a coelom. Organs formed inside a coelom can move and develop independently of the body wall while fluid cushions and protects them from shocks; the mesoderm has several components which develop into tissues: intermediate mesoderm, paraxial mesoderm, lateral plate mesoderm, chorda-mesoderm. The chorda-mesoderm develops into the notochord; the intermediate mesoderm develops into gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, dermis; the lateral plate mesoderm develops into the circulatory system, the wall of the gut, wall of the human body. Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process of differentiation; the mesoderm forms: muscle, cartilage, connective tissue, adipose tissue, circulatory system, lymphatic system, genitourinary system, serous membranes, notochord.
The ectoderm generates the outer layer of the embryo, it forms from the embryo's epiblast. The ectoderm develops into the surface ectoderm, neural crest, the neural tube; the surface ectoderm develops into: epidermis, nails, lens of the eye, sebaceous glands, tooth enamel, the epithelium of the mouth and nose. The neural crest of the ectoderm develops into: peripheral nervous system, adrenal medulla, facial cartilage, dentin of teeth; the neural tube of the ectoderm develops into: brain, spinal cord, posterior pituitary, motor neurons, retina. Note: The anterior pituitary develops from the ectodermal tissue of Rathke's pouch; because of its great importance, the neural crest is sometimes considered a fourth germ layer. It is, derived from the ectoderm. Histogenesis
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. Their closest relatives 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; until it was thought that all species lived in the sediment on the seabed, subsisting as deposit feeders or suspension feeders. However, the last decade has seen the description of a new family, the Torquaratoridae, evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites, it is assumed that the ancestors of acorn worms used to live in tubes like their relatives Pterobranchia, but that they started to live a safer and more sheltered existence in sediment burrows instead. Some of these worms may grow to be long.
Due to secretions containing elements like iodine, the animals have an iodoform-like smell. Most acorn worms range from 9 to 45 centimetres in length, with the largest species, Balanoglossus gigas, reaching 1.5 metres or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, a long, worm-like trunk; the creature's mouth is located at the collar behind the proboscis. The skin is covered with cilia as well as glands; some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms move only sluggishly. Many acorn worms are detritus feeders, extracting organic detritus. Others feed on organic material suspended in the water, which they can draw into the mouth using the cilia on the gill bars. A groove lined with cilia lies just in front of the mouth and directs suspended food into the mouth and may allow the animal to taste; the mouth cavity is tubular, with a narrow diverticulum or stomochord extending up into the proboscis.
This diverticulum was once thought to be homologous with the notochord of chordates, hence the name "hemichordate" for the phylum. The mouth opens posteriorly into a pharynx with a row of gill slits along either side; the remainder of the digestive system consists of intestine. In some families there are openings in the dorsal surface of the oesophagus connecting to the external surface, through which water from the food can be squeezed, helping to concentrate it. Digestion occurs in the intestine, with food material being pulled through by cilia, rather than by muscular action. Acorn worms breathe by drawing in oxygenated water through their mouth; the water flows out the animal's gills which are on its trunk. Thus, the acorn worm breathes about the same way as fish. Acorn worms have an open circulatory system. A dorsal blood vessel in the mesentery above the gut and delivers blood to a sinus in the proboscis that contains a muscular sac acting as a heart. Unlike the hearts of most other animals, this structure is a closed fluid-filled vesicle whose interior does not connect directly to the blood system.
Nonetheless, it does pulsate, helping to push blood through the surrounding sinuses. From the central sinus in the collar, blood flows to a complex series of sinuses and peritoneal folds in the proboscis; this set of structures is referred to as a glomerulus and may have an excretory function, since acorn worms otherwise have no defined excretory system. From the proboscis, blood flows into a single blood vessel running underneath the digestive tract, from which smaller sinuses supply blood to the trunk, back into the dorsal vessel; the blood of acorn worms is acellular. Acorn worms continually form new gill slits as they grow in size, with some older individuals having more than a hundred on each side; each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft and to the exterior through a dorso-lateral pore. Cilia push water through the slits; the tissues surrounding the slits are well supplied with blood sinuses. A plexus of nerves lies underneath the skin, is concentrated into both dorsal and ventral nerve cords.
While the ventral cord runs only as far as the collar, the dorsal cord reaches into the proboscis, is separated from the epidermis in that region. This part of the dorsal nerve cord is hollow, may well be homologous with the brain of vertebrates. In acorn worms, it seems to be involved with coordinating muscular action of the body during burrowing and crawling. Acorn worms have no eyes, ears or other special sense organs, except for the ciliary organ in front of the mouth, which appears to be involved in filter feeding and taste. There are, numerous nerve endings throughout the skin. Acorn worms are considered more specialised and advanced than other shaped worm-like creatures, they have a circulatory system with a heart that functions as a kidney. Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates; some have a postanal tail which may be homologous to the post-anal tail of vertebrates.
An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neur
In biology, phylogenetics is the study of the evolutionary history and relationships among individuals or groups of organisms. These relationships are discovered through phylogenetic inference methods that evaluate observed heritable traits, such as DNA sequences or morphology under a model of evolution of these traits; the result of these analyses is a phylogeny – a diagrammatic hypothesis about the history of the evolutionary relationships of a group of organisms. The tips of a phylogenetic tree can be living organisms or fossils, represent the "end", or the present, in an evolutionary lineage. Phylogenetic analyses have become central to understanding biodiversity, evolution and genomes. Taxonomy is the identification and classification of organisms, it is richly informed by phylogenetics, but remains a methodologically and logically distinct discipline. The degree to which taxonomies depend on phylogenies differs depending on the school of taxonomy: phenetics ignores phylogeny altogether, trying to represent the similarity between organisms instead.
Usual methods of phylogenetic inference involve computational approaches implementing the optimality criteria and methods of parsimony, maximum likelihood, MCMC-based Bayesian inference. All these depend upon an implicit or explicit mathematical model describing the evolution of characters observed. Phenetics, popular in the mid-20th century but now obsolete, used distance matrix-based methods to construct trees based on overall similarity in morphology or other observable traits, assumed to approximate phylogenetic relationships. Prior to 1950, phylogenetic inferences were presented as narrative scenarios; such methods are ambiguous and lack explicit criteria for evaluating alternative hypotheses. The term "phylogeny" derives from the German Phylogenie, introduced by Haeckel in 1866, the Darwinian approach to classification became known as the "phyletic" approach. During the late 19th century, Ernst Haeckel's recapitulation theory, or "biogenetic fundamental law", was accepted, it was expressed as "ontogeny recapitulates phylogeny", i.e. the development of a single organism during its lifetime, from germ to adult, successively mirrors the adult stages of successive ancestors of the species to which it belongs.
But this theory has long been rejected. Instead, ontogeny evolves – the phylogenetic history of a species cannot be read directly from its ontogeny, as Haeckel thought would be possible, but characters from ontogeny can be used as data for phylogenetic analyses. 14th century, lex parsimoniae, William of Ockam, English philosopher and Franciscan friar, but the idea goes back to Aristotle, precursor concept 1763, Bayesian probability, Rev. Thomas Bayes, precursor concept 18th century, Pierre Simon first to use ML, precursor concept 1809, evolutionary theory, Philosophie Zoologique, Jean-Baptiste de Lamarck, precursor concept, foreshadowed in the 17th century and 18th century by Voltaire and Leibniz, with Leibniz proposing evolutionary changes to account for observed gaps suggesting that many species had become extinct, others transformed, different species that share common traits may have at one time been a single race foreshadowed by some early Greek philosophers such as Anaximander in the 6th century BC and the atomists of the 5th century BC, who proposed rudimentary theories of evolution 1837, Darwin's notebooks show an evolutionary tree 1843, distinction between homology and analogy, Richard Owen, precursor concept 1858, Paleontologist Heinrich Georg Bronn published a hypothetical tree to illustrating the paleontological "arrival" of new, similar species following the extinction of an older species.
Bronn did not propose a mechanism responsible for precursor concept. 1858, elaboration of evolutionary theory and Wallace in Origin of Species by Darwin the following year, precursor concept 1866, Ernst Haeckel, first publishes his phylogeny-based evolutionary tree, precursor concept 1893, Dollo's Law of Character State Irreversibility, precursor concept 1912, ML recommended and popularized by Ronald Fisher, precursor concept 1921, Tillyard uses term "phylogenetic" and distinguishes between archaic and specialized characters in his classification system 1940, term "clade" coined by Lucien Cuénot 1949, Jackknife resampling, Maurice Quenouille, precursor concept 1950, Willi Hennig's classic formalization 1952, William Wagner's groundplan divergence method 1953, "cladogenesis" coined 1960, "cladistic" coined by Cain and Harrison 1963, first attempt to use ML for phylogenetics and Cavalli-Sforza 1965 Camin-Sokal parsimony, first parsimony criterion and first computer program/algorithm for cladistic analysis both by Camin and Sokal character compatibility method called clique analysis, introduced independently by Camin and Sokal and E. O. Wilson 1966 English translation of Hennig "cladistics" and "cladogram" coined 1969 dynamic and successive wei
Embryonic development embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe stages. Embryonic development starts with the fertilization of the egg cell by a sperm cell. Once fertilized, the ovum is referred to a single diploid cell; the zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo. Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals; the egg cell is asymmetric, having an "animal pole" and a "vegetal pole". It is covered with different layers; the first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.
Fertilization is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation; the fertilized egg cell is known as the zygote. To prevent more than one sperm fertilizing the egg, fast block and slow block to polyspermy are used. Fast block, the membrane potential depolarizing and returning to normal, happens after an egg is fertilized by a single sperm. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering. Cell division with no significant growth, producing a cluster of cells, the same size as the original zygote, is called cleavage.
At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. Depending on the amount of yolk in the egg, the cleavage can be holoblastic or meroblastic. Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta or milk, such as might be secreted from a marsupium. On the other hand, meroblastic cleavage occurs in animals; because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms: The end of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription.
In amniotes, the cells of the morula are at first aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, an inner cell mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, thus the morula is converted into a vesicle, called the blastodermic vesicle; the inner cell mass remains in contact, with the trophoblast at one pole of the ovum. After the 7th cleavage has produced 128 cells, the embryo is called a blastula; the blastula is a spherical layer of cells surrounding a fluid-filled or yolk-filled cavity Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass, distinct from the surrounding blastula. The blastocyst must not be confused with the blastula. In the mouse, primordial germ cells arise from a layer of cells in the inner cell mass of the blastocyst as a result of extensive genome-wide reprogramming.
Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, results in cellular totipotency. Before gastrulation, the cells of the trophoblast become differentiated into two strata: The outer stratum forms a syncytium, termed the syncytiotrophoblast, while the inner layer, the cytotrophoblast or "Layer of Langhans", consists of well-defined cells; as stated, the cells of the trophoblast do not contribute to the formation of the embryo proper. On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and assumes the form of a small sac, called the yolk sac. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic c
Echinoderm is the common name given to any member of the phylum Echinodermata of marine animals. The adults are recognizable by their radial symmetry, include such well-known animals as sea stars, sea urchins, sand dollars, sea cucumbers, as well as the sea lilies or "stone lilies". Echinoderms are found from the intertidal zone to the abyssal zone; the phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are the largest phylum that has no freshwater or terrestrial representatives. Aside from the hard-to-classify Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian. One group of Cambrian echinoderms, the cinctans, which are close to the base of the echinoderm origin, have been found to possess external gills used for filter feeding, similar to those possessed by chordates and hemichordates; the echinoderms are important both geologically. Ecologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans.
Most echinoderms are able to reproduce asexually and regenerate tissue and limbs. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, can provide valuable clues as to the geological environment, they were the most used species in regenerative research in the 20th centuries. Further, it is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Along with the chordates and hemichordates, echinoderms are deuterostomes, one of the two major divisions of the bilaterians, the other being the protostomes. During the early development of the embryo, in deuterostomes, the blastopore becomes the anus whereas in the protostomes, it becomes the mouth. In deuterostomes, the mouth develops at a stage, at the opposite end of the blastula from the blastopore, a gut forms connecting the two; the larvae of echinoderms have bilateral symmetry but this is lost during metamorphosis when their bodies are reorganised and develop the characteristic radial symmetry of the echinoderm pentamerism.
The characteristics of adult echinoderms are the possession of a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree. There are a total of about 7,000 extant species of echinoderm as well as about 13,000 extinct species, they are found in habitats ranging from shallow intertidal areas to abyssal depths. Two main subdivisions are traditionally recognised: the more familiar motile Eleutherozoa, which encompasses the Asteroidea, Ophiuroidea and Holothuroidea; these consist of the extinct blastoids and Paracrinoids. A fifth class of Eleutherozoa consisting of just three species, the Concentricycloidea, were merged into the Asteroidea; the fossil record includes a large number of other classes which do not appear to fall into any extant crown group. All echinoderms are marine and nearly all are benthic; the oldest known echinoderm fossil may be Arkarua from the Precambrian of Australia.
It is a disc-like fossil with radial ridges on the rim and a five-pointed central depression marked with radial lines. However, no stereom or internal structure showing a water vascular system is present and the identification is inconclusive; the first universally accepted echinoderms appear in the Lower Cambrian period, asterozoans appeared in the Ordovician and the crinoids were a dominant group in the Paleozoic. Echinoderms left behind an extensive fossil record, it is hypothesised that the ancestor of all echinoderms was a simple, bilaterally symmetrical animal with a mouth and anus. This ancestral stock adopted an attached mode of life and suspension feeding, developed radial symmetry as this was more advantageous for such an existence; the larvae of all echinoderms are now bilaterally symmetrical and all develop radial symmetry at metamorphosis. The starfish and crinoids still attach themselves to the seabed while changing to their adult form; the first echinoderms gave rise to free-moving groups.
The evolution of endoskeletal plates with stereom structure and of external ciliary grooves for feeding were early echinoderm developments. The Paleozoic echinoderms were globular, attached to the substrate and were orientated with their oral surfaces upwards; the fossil echinoderms had ambulacral grooves extending down the side of the body, fringed on either side by brachioles, structures similar to the pinnules of a modern crinoid. It seems probable that the mouth-upward orientation is the primitive state and that at some stage, all the classes of echinoderms except the crinoids reversed this to become mouth-downward. Before this happened, the podia had a feeding function as they do in the crinoids today, their locomotor function came after the re-orientation of the mouth when the podia were in contact with the substrate for the firs