Chordates are neither the most diverse nor the largest of the animal phyla, although in terms of the number of species, they come in a respectable fourth behind arthropods, nematodes, and molluscs. Chordates consist of three groups of unequal size: cephalochordates (amphioxi or lancets), urochordates (tunicates or “sea squirts”), and the largest group, the vertebrates (fishes, amphibians, reptiles, and mammals). Tucked away within this phylum is a small family, the hominids, that includes humans. In part, our interest in chordates derives from the fact that humans belong to this phylum, so studying chordates brings topics concerning us close to home. Herdmania is also an example of urochordates.
Chordates have a fluid-filled internal body cavity termed a coelom. They are part of a major radiation within the Bilateria, animals built upon a bilateral, symmetrical body plan. Within the Bilateria, two apparently distinct and independent evolutionary lines are present. One line is the protostomes, which includes molluscs, annelids, arthropods, and many smaller groups. The protostome lineage itself divides into Lophotrochozoa and Ecdysozoa.
The other bilaterian line is the deuterostomes, which includes echinoderms, hemichordates, and chordates (figure 1). The distincti on between protostomes and deuterostomes was originally recognized on the basis of certain embryological characteristics. Recently, molecular studies have confirmed and clarified these two lines of evolution within the bilaterians. More will be said later about embryonic development, but here some general introductory features can help clarify the differences between protostomes and deuterostomes.
Embryonic Development – Early Cleavage
In Embryonic development the early cleavage In both bilaterian groups, the egg begins to divide repeatedly after fertilization, a process termed cleavage, until the very young embryo is made up of many cells formed from the original single-celled egg. In some animals, dividing cells of the embryo are offset from each other, a pattern known as spiral cleavage. In others, the dividing cells are aligned, a pattern termed radial cleavage. At this point, the embryo is little more than a clump of dividing cells that soon become arranged into a round, hollow ball, with cells forming the outer wall around a fluid-filled cavity within.
One wall of this ball of cells begins to indent and grow inward, a process called gastrulation. The opening into this indentation is the blastopore, and the indented cells themselves are destined to become the gut of the adult. Indentation continues until cells reach the opposite wall, where they usually break through, forming a second opening into the primitive gut (the blastopore being the first).
The now multicellular embryo is composed of three basic tissue layers: an outer ectoderm, an inner endoderm that forms the lining of the gut, and a mesoderm that forms the layer between the two. If a solid mass of mesodermal cells splits to form the body cavity within them, the result is a schizocoelom (figure 2a). If, instead, the mesoderm arises as outpocketings of the gut that pinch off to form the body cavity, the result is an enterocoelom (figure 2b).
Protostomes, that means “primitive mouth,” in which the mouth part arises from or near the blastopore. Additionally, they tend to have spiral cleavage, a schizocoelom, and a skeleton derived from the surface layer of cells (figure 2a). Deuterostomes, literally meaning “second mouth,” are animals in which the mouth arises not from the blastopore but secondarily at the opposite end of the gut as the blastopore itself becomes the anus (figure 2b). Additionally, embryonic development of deuterostomes includes radial cleavage, an enterocoelom, and a calcified skeleton, when present, derived generally from mesodermal tissues. These embryological characteristics shared by deuterostomes testify that they are more closely related to each other in an evolutionary sense than to any of the protostomes. Embryological characteristics, modern molecular phylogenies, and the fossil record all imply that there was an ancient and fundamental divergence between the protostomes and deuterostomes.
Chordates evolved within the deuterostomes. Their mouth forms opposite to the blastopore, their cleavage generally is radial, their coelom is an enterocoelom, and their skeleton arises from mesodermal tissues of the embryo. But we should be clear about the character of the chordates themselves. It is easy to forget that two of the three chordate taxa are technically invertebrates—the Cephalochordata and the Urochordata. Strictly speaking, the invertebrates include all animals except members of the vertebrates.
The earliest chordate fossils appear in the Cambrian period, about 530 million years ago. Although later chordates evolved hard bones and well-preserved teeth that left a substantial fossil testimony to their existence, ancestors to the first chordates likely had soft bodies and left almost no fossil trace of the evolutionary pathway taken from prechordate to chordate. Thus, to decipher chordate origins, we derive evidence from anatomical and molecular (codes of gene sequences) clues carried in the bodies of living chordates. In order to evaluate the success of our attempts at tracing chordate origins, we first need to decide what defines a chordate. We will then attempt to discover the animal groups that are the most likely evolutionary precursors of the chordates.
At first glance, the differences among the three chordate taxa are more apparent than the similarities that unite them. Most vertebrates have an endoskeleton, a system of rigid internal elements of bone or cartilage beneath the skin. The endoskeleton participates in locomotion, support, and protection of delicate organs. Some vertebrates are terrestrial, and most use jaws to feed on big food particles. But cephalochordates and urochordates are all marine animals, none are terrestrial, and all lack a bony or cartilaginous skeleton. However, their support system may involve rods of collagenous material. The suspension feeders are urochordates and cephalochordates which have a sticky sheet of mucus that strains small food particles from streams water passing over a filtering apparatus.
All three taxa, despite these superficial differences, share a common body design similar in at least five fundamental features: notochord, pharyngeal slits, endostyle or thyroid gland, dorsal hollow nerve cord forming the simple central nervous system, and postanal tail (figure 3a–c). These five features diagnose the chordates, and taken together, distinguish them from all other taxa. We look next at each characteristic separately.
The notochord is a slender rod that develops from the mesoderm in all chordates. It lies dorsal to the coelom but beneath and parallel to the central nervous system (brain and spinal cord). The phylum takes the name Chordata from this structure. Typically, the notochord is composed of a core of cells and fluid encased in a tough sheath of fibrous tissue (figure 4a). Sometimes the fluid is held within swollen cells called vacuolated cells; other times it resides between core cells of the notochord.
The notochord has the mechanical properties of an elastic rod, so it can be flexed laterally from side to side (figure 4c), but cannot be collapsed along its length like a telescope (figure 4b). This mechanical property results from the cooperative action of the outer fibrous sheath and the fluid core it encloses. If the fluid were drained, like letting air from a balloon, the outer sheath would collapse and form no useful mechanical device. Normally the fluid fills the notochord remains static and does not flow. In such structures, the outer wall encloses a fluid core, are called hydrostatic organs.
The notochord is a hydrostatic organ which has elastic properties that resist axial compression. It found along the body axis to allow lateral flexion but prevents collapse of the body during locomotion period (figure 4d). If you want to understand the mechanism of notochord, let’s imagine what would occur if one block of muscle contracted on one side of an animal without a notochord. As the muscle shortens, it shortens the body wall of which it is part and telescopes the body. A body with a notochord, the longitudinally incompressible cord resists the tendency of a contracting muscle to shorten the body of living.
Besides shortening the body, the contraction of the body muscle sweeps the tail to the side. Thus, upon contraction, the body’s segmentally arranged musculature acts upon the notochord to initiate swimming motions that produce lateral pressure against the surrounding substrate. Upon muscle relaxation, the springy notochord straightens the body. Thus, the notochord prevents the collapse or telescoping of the body and acts as the muscle’s antagonist in order to straighten the body. As a result, alternating side-to-side muscle contractions in partnership with the notochord generate lateral waves of body undulation. This form of locomotion may have been the initial condition that first favored the evolution of the notochord.
The notochord continues to be an important functional member throughout most groups of chordates. Only in later forms, such as in bony fishes and terrestrial vertebrates, is it largely replaced by an alternative functional member, the vertebral column. Even when replaced by the vertebral column, the notochord still appears as an embryonic structure, inducing the neural tube to develop above it into the brain and spinal cord and serving as a scaffold for the growing embryonic body. In adult mammals with a full vertebral column, the notochord is reduced to a remnant, the nucleus pulposus. This is a tiny core of gel-like material within each intervertebral disk that forms a spherical fling between successive vertebrae.
The pharyngeal slits (figure 3). The pharynx is a part of the digestive tract located immediately posterior to the mouth. During some point in the lifetime of all chordates, the walls of the pharynx are pierced, or nearly pierced, by a longitudinal series of openings, the pharyngeal slits (also called pharyngotremy, literally meaning “pharyngeal holes”).
The term gill slits is often used at the place of pharyngeal slits for each of these openings, but a “gill” proper is a specialized derived structure of fish and larval amphibians composed of tiny plates or folds that harbor capillary beds for respiration in water. In such vertebrates, gills form adjacent to these pharyngeal slits. The slits are openings only, often with no significant role in respiration. In many primitive chordates, these openings serve primarily in feeding, but in embryos they play no respiratory role; therefore gill slits is a misleading term.
Pharyngeal slits may appear early in embryonic development and persist into the adult stage, or they may be overgrown and disappear before the young chordate is born or hatched. Whatever their eventual embryonic or adult fate, all chordates show evidence of pharyngeal slits at some time in their lives.
The first evolved slits aided in feeding. As openings in the pharynx, they allowed the one-way flow of a water current—in at the mouth and out through the pharyngeal slits (figure 3). Secondarily, when the walls defining the slits became associated with gills, the passing stream of water also participated in respiratory exchange with the blood circulating through the capillary beds of these gills. Water entering the mouth could bring suspended food and oxygen to the animal. As it passed across the vascularized gills and then exited through the slits, carbon dioxide was given up to the departing water and carried away. Therefore, the current of water passing through pharyngeal slits can simultaneously support feeding and respiratory activities.
In gill-less primitive chordates, the pharynx itself is often expanded into a pharyngeal or branchial basket, and the slits on its walls are multiplied in number, increasing the surface area exposed to the passing current of water. Sticky mucus lining the pharynx snatches food particles from suspension. Sets of cilia, also lining the pharynx, produce the water current. Other cilia gather the food-laden mucus and pass it into the esophagus. This mucus and cilia system is especially efficient in small, suspension-feeding organisms, those that extract food floating in water. Such a feeding system is prevalent in primitive chordates and in groups that preceded them.
In the earliest vertebrates that depended upon gill respiration to support an active lifestyle, mucus and cilia served less well. Cilia are weak pumps, ineffective against gill resistance. In such vertebrates, a pharyngeal pump worked by muscles takes the place of cilia to now move the water that ventilates the gills. The muscular pump, in place of mucus and cilia, also becomes the basis for procurement and processing of large food items. Slits still serve as convenient exit portals for excess or spent water, while adjacent gill structures function in respiration. In fishes and aquatic amphibians, the pharyngeal slits that appear during embryonic development usually persist into the adult and form the exit channel through which water associated with feeding and respiration flows. In vertebrates that reside on land, however, the embryonic pharyngeal slits normally never open and thus do not give rise to any adult derivative.
Endostyle or Thyroid Gland
The endostyle is a glandular groove found in the floor of the pharynx that is involved in filter feeding. The thyroid gland is an endocrine gland that produces two major hormones. The thyroid gland, like the endostyle, arises embryologically from the floor of the pharynx. And the thyroid gland, like the endostyle, is involved in iodine metabolism, further suggesting a homology between the two, with the endostyle, being the phylogenetic predecessor of the thyroid. Supporting this, the jawless fish called lampreys have a true endostyle when they are young larvae, that becomes a true thyroid when they become adults. So, the all phylum of chordates have endostyles (urochordates, cephalochordates, larval lamprey) or thyroids (adult lamprey, all other vertebrates).
Dorsal and Tubular Nerve Cord
A dorsal hollow nerve cord derived from ectoderm (figure 5b). The central nervous system of all animals is ectodermal in embryonic origin, but only in chordates does the nerve tube typically form by a distinctive embryonic process, namely, by invagination. Future nerve tube cells of the early chordate embryo gather dorsally into a thickened neural plate within the surface ectoderm of the back. This neural plate of cells folds or rolls up and sinks inward from the surface (invaginates) as a tube to take up residence dorsally within the embryo, just above the notochord. A nerve cord produced from a thickened plate by invagination is also called a neurulated nerve cord.
In most non-chordate embryos, by contrast, the ectodermal cells destined to form the central nervous system do not amass as thickened surface plates (placodes); instead, cells individually move inward to assemble into the basic nervous system. More importantly, the major nerve cord in most invertebrates is ventral in position, below the gut, and solid. In chordates, however, the nerve cord lies above the gut and is hollow along its entire length; or more accurately, it surrounds the neurocoel, a fluid-filled central canal (figure 5b). The advantage of a tubular rather nerve cord besides a solid nerve cord is not known, but its distinctive feature is found only among chordates.
Chordates possess a postanal tail that represents a posterior elongation of the body extending beyond the anus. The tail is primarily an extension of the chordate locomotor apparatus, the segmental musculature and notochord. More will be said later about the role of the tail in swimming.
Chordate Body Plan
What is common to all chordates are these five primary features: notochord, pharyngeal slits, endostyle or thyroid, dorsal hollow nerve cord, and postanal tail. These characteristics may be present only briefly during embryonic development, or they may persist into the adult stage, but all chordates exhibit them at some point during their lifetimes. Taken together, they are a suite of characters found only among chordates. Chordates also show segmentation. Blocks of muscle, or myomeres, are arranged sequentially along the adult body and tail as part of the outer body wall. Sometimes the myomeres are straight or, more typically, V-shaped.
Now that we have an idea about the basic and secondary characteristics of chordates, let us turn our attention to the evolutionary origin of this group. Biologists interested in such questions often consult an assortment of primitive chordates and their immediate ancestors whose structure and design inform us about how and why the early chordate body plan arose. These animals are the protochordates.