Introduction to the Hemichordata
Four distinctive derived characteristics of chordates distinguish them from their ancestors: Hemichordates are a group of organisms that show an affinity to the . Also characteristic of the chordates are a tail that extends behind and above the anus, Chordates enter into a wide variety of symbiotic relationships and are other features that are common among the hemichordates and the chordates. Four chordate characters — dorsal hollow nerve cord, notochord, gill slits, and relationship between Urochordata and Vertebrata, with Cephalochordata as the.
Hemichordates have some characters of chordates. They contain pharyngeal gill slits in their collar. The stomochord is a semi-rigid, rod-like structure below the heart of hemichordates.
It resembles the notochord of chordates.
Difference Between Hemichordata and Chordata | Definition, Classification, Habitat, Characteristics
The circulatory system of hemichordates bears a heart-like, contractile vesicle. It is an open circulatory system with blood vessels and sinuses. The pharyngeal gill slits are used to capture small food particles by non-vertebrate chordates.Hemichordata (Animal Kingdom) Hindi Medium
A hemichordate is shown in figure 1. Hemichordate The three classes of hemichordates are Enteropneusta, Pterobranchia, and Planctosphaeroidea. Enteropneusts are acorn worms that live in shallow water. Pterobranchia contains colonial tube-like worms that live in an externally secreted encasement. Planctosphaeroidea contains planktonic organisms with spherical bodies covered with ciliary bands.
What is Chordata Chordata refers to an animal phylum that contains a notochord, dorsally situated central nervous system, and gill slits. Chordates can be found in freshwater, marine, and terrestrial habitats. All chordates possess a notochord, dorsal nerve cord, pharyngeal gill slits, and a post-anal tail at some point in their life. The notochord is a rod-like, elongated structure, which occurs dorsal to the gut and ventral to the nerve cord. The nerve cord is a dorsal and hollow.
The central nervous system of chordates consists of a brain and a spinal cord. The peripheral nervous system connects the body to the central nervous system.
Difference Between Hemichordata and Chordata
The pharyngeal gill slits occur in the pharyngeal portion of the gut. The notochord, nerve cord, and gill slits are shown in figure 2. Urochordate larva contains the notochord, nerve cord, and the post-anal tail. The adult urochordates are planktonic or sessile. The urochordate body occurs inside a tunic. The body of the Chelochordata is laterally compressed and transparent. Chelochordates present notochord, nerve cord, pharyngeal gill slits, and post-anal tail.
In Vertebrata, the notochord is covered by the vertebral column. The vertebral column provides axial support. The anteriorly modified skeleton produces the skull, which protects the brain. Both Hemichordata and Chordata are deuterostomes.
Both Hemichordata and Chordata are triploblasticcoelomates. Both Hemichordata and Chordata possess bilateral symmetry. Tissues and muscles In both cephalochordates and vertebrates, muscles used in locomotion are well developed and organized segmentally. The tail musculature of tunicates is simpler and without clear indications of segmentation. There is at least a small amount of musculature throughout the body of all chordates.
As jaws, limbs, and other body parts have evolved in vertebrates, so have the muscles that operate them. Nervous system and sense organs The anterior end of the main nerve cord in chordates is enlarged to form at least the suggestion of a brain, but a brain is well developed only in vertebrates.
Tunicate larvae have visual organs sensitive to light and sense organs responsive to the direction of gravity. Pigment spots and light receptors in the nerve cord of lancelets detect sudden changes in light intensity. The eyes and other sense organs of vertebrates are more elaborate and complex. The presence in cephalochordates and vertebrates of a nervous system with segmentally repeated nerves arising from the dorsal hollow nerve cord is suggestive of a common ancestry.
The tunicate nervous system does not have the segmentally repeated nerves. The brains of all vertebrates are greatly enlarged and subdivided into functionally specialized regions. Digestion and nutrition Both tunicates and cephalochordates are filter feeders of small particles of food suspended in the water.
Beating cilia hairlike cellular extensions on the gill slits draw a current of water into the mouth and through the pharynx, where a sheet of mucussecreted by the endostyle a glandular organ lying below the two rows of gill slitsfilters suspended food particles from the water.
Cilia lining the pharynx move the food-rich sheet of mucus upward over the gill slits, and it is then rolled up and transported to the posterior part of the gut. The water current passes into the atrium and exits through the atrial opening. The difference is that the food consists of somewhat larger particles that have been deposited on the bottom detritusand, instead of the feeding current being driven by cilia, the pharyngeal musculature pumps water and food particles across the gill slits.
The earliest fishes probably fed on detritus, and a sucking action is retained by their extant representatives lampreys and hagfishes. With the development of jaws, it became possible for the vertebrates to capture and seize larger food items.
The lower digestive tract of the primitive chordate is a simple tube with a saclike stomach. There are only indications of the specialized areas and of glandlike structures, such as the liver and pancreas, that occur in vertebrates.
Excretion The excretion of wastes and the control of the chemical composition of the internal environment are largely effected by kidneys, although other parts of the body, including the gills, may play an important role.
Tunicates and cephalochordates have a salt content essentially the same as seawater, but vertebrates, even marine species, have body fluids of low salt content, with the exception of hagfishes.
A possible explanation is that the vertebrates evolved in fresh water, but it seems reasonable that hagfishes branched off while still marine and that the freshwater form evolved later. Respiration A primitive chordate gill is present in tunicates and cephalochordates, where it serves in both respiration and feeding. The vertebrate gill may retain some role in feeding, although the current is now produced by the action of muscles, not cilia. The gills became reduced in number in various lineages, and they were strengthened by supporting elements, some of which evolved into jaws.
Lungs, already present in fishes, became the main respiratory organs of terrestrial vertebrates. Circulatory system The circulatory system in chordates has a characteristic pattern. In tunicates and vertebrates the blood is propelled by a distinct heart; in cephalochordates, by contraction of the blood vessels.
Unoxygenated blood is driven forward via a vessel called the ventral aorta. It then passes through a series of branchial arteries in the gills, where gas exchange takes place, and the oxygenated blood flows to the body, much of it returning to its origin via a dorsal aorta. The blood of vertebrates passes through the tissues via tiny vessels called capillaries. In tunicates and cephalochordates, capillaries are absent and the blood passes through spaces in the tissues instead.
Hormones In vertebrates, endocrine glands those of internal secretion produce hormones that regulate many physiological activities.
In tunicates and cephalochordates, organs have been identified that correspond in anatomical position to the pituitary gland of vertebrates, but which hormones, if any, they secrete is uncertain. In vertebrates, the thyroid gland produces thyroxine, an iodine-containing hormone that helps regulate metabolism. The thyroid is a modified endostyle, as can be illustrated by larval lampreys in which the thyroid still secretes mucus for use in feeding.
The endostyles of lancelets take up iodine and form thyroxine, but the thyroxine formed may not function as a hormone in the lancelets themselves. Features of defense and aggression Tunicates largely rely upon the passive defense afforded by their heavy tunic. Lancelets move rapidly through the substrate, and their well-developed locomotory apparatus evolved largely to provide a means of escaping predators.
Vertebrates have ceased to feed on detritus brought to them by water currents. They have shifted to consuming larger foodstuffs and to actively locating, pursuing, and subduing what they eat. Evolution and paleontology Many scientists maintain that chordates originated sometime earlier than million years ago; that is, they predate the fossil record. Such early representatives were soft-bodied and therefore left a poor fossil record.
The oldest known fossil chordate is Pikaia gracilens, a primitive cephalochordate dated to approximately million years ago. There is disagreement over whether older animals—such as Yunnanozoon lividum and Haikouella both of which date to million years ago and possess several chordate features —should be considered chordates. An extensive vertebrate fossil record begins about million years ago.
Embryological evidence places the phylum Chordata within the deuterostomes bilaterally symmetrical animals with undeterminate cleavage and whose mouth does not arise from the blastoporewhich also includes the phyla Hemichordata, Echinodermata, and Chaetognatha.
The closest relatives of the chordates are probably the hemichordates, since these animals possess gill slits and other features not found in other animal phyla. A slightly more remote relationship to the echinoderms is inferred on the basis of resemblances between the larvae in some groups of hemichordates and echinoderms. The derivation of chordates from certain fossil echinoderms has been argued on the basis of features such as what appear to be gill slits.
Theories that derive them from other phyla e. Whether the first ancestral chordate was more like a tunicate or a cephalochordate has been extensively debated. The classical theory is that the ancestor was like a cephalochordate and that one lineage became attached to hard surfaces and evolved into tunicates, whereas another remained unattached and evolved into vertebrates.
An alternative theory is that the ancestor was like a tunicate and that the other two subphyla arose by modification of the tadpole larva. There is some preference for the classical theory because it provides the most satisfactory way of accounting for the similarities between chordates and hemichordates of the subphylum Enteropneusta.
Within the chordates, the tunicates probably branched off before the common ancestor of cephalochordates and vertebrates arose, for the latter resemble each other in some details of neuroanatomy and biochemistry.
Classification Annotated classification Phylum Chordata Deuterostomatous eucoelomates; gill clefts; endostyle or its derivative in pharynx; notochord; hollow dorsal nerve cord; tail posterior and dorsal to anus.
Subphylum Tunicata or Urochordata; tunicates Notochord, when present, restricted to tail; body covered with tunic, but sometimes only cuticle; atrium, absent in Appendiculariadorsal and often paired in embryonic development; heart present; generally sessile attached as adults; see below Tunicates.
Class Ascidiacea sea squirts Sessile; benthic; solitary or colonial within a common tunic.
Introduction to the Hemichordata
Class Appendicularia larvacea Free-swimming; pelagic; resembles tadpole larvae of ascidians; 1 pair of gill slits; no distinct atrium. Class Thaliacea Pelagic; forms aggregations or colonies. Subphylum Cephalochordata or Acrania; lancelets Notochord extends entire body length, with tip anterior to nerve cord; atrium a single cavity with single, ventral opening; segments well developed; head poorly developed; no paired fins; no heart; see below Cephalochordates.
Subphylum Vertebrata or Craniata; vertebrates Notochord extends to the back of a well-developed head; no atrium; segments well developed; paired fins or limbs usually present; heart present; see below Vertebrates. Critical appraisal This outline gives the major groups of chordates. Modern systematic biology attempts to arrange groups of organisms in a way that suggests the genealogical relationships branching sequences and therefore presents an epitome of evolutionary history.