DESCRIPTION, GENERAL CHARACTERISTICS AND CLASSIFICATION OF CHORDATES MODULE 1
MODULE 1 DESCRIPTION, GENERAL CHARACTERISTICS AND CLASSIFICATION OF CHORDATES
Unit 1 Principal features of chordates There are five principal features (hallmarks) that characterize the chordates and have played an important role in the evolution of the phylum Chordata, these are: the notochord, dorsal tubular nerve cord, pharyngeal gill slits, endostyle and post-anal tail. 1. The Notochord: it is a flexible, rod-like structure extending the length of the body. It is present at some stage in the life of all chordates and persists throughout the life cycle of some invertebrate chordates (protochordates) and in primitive vertebrates. It is located below the nerve cord and forms on the dorsal side of the primitive gut. The notochord is an axis for muscle attachment (provides a base for myomeric muscles), and because it can bend without shortening, it permits undulatory movement of the body. Notochord provides longitudinal stiffening of the main body axis. It provides the main support for the body in primitive chordates. 2. Dorsal tubular nerve cord: a single, hollow tubular cord runs just beneath the dorsal surface of the animal. The anterior end becomes enlarged to form the brain. Among the vertebrates, the nerve cord lies in the neural arches of the vertebrae and the anterior brain is surrounded by a bony or cartilaginous cranium. 3. Pharyngeal gill slits: these are perforated slit-like openings that connect the pharynx, a muscular tube, to the outside. In most vertebrates, the slits do not actually connect to the outside and are termed pharyngeal pouches. Pharyngeal pouches are present in the embryos of all vertebrates but are lost later in the development of terrestrial vertebrates. 4. Endostyle: the endostyle and its derivative, thyroid gland occurs in all chordates. It was initially not recognized as one of the main chordate characteristics. The endostyle is a ventral groove of the pharynx; and secretes mucus that aid in feeding by trapping food particles in the pharyngeal cavities of lower chordates. Some cells in the endostyle secrete or sequesters iodinated proteins, therefore, it is thought to be the forerunner in protochordates of the thyroid gland of vertebrates.
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DESCRIPTION, GENERAL CHARACTERISTICS AND CLASSIFICATION OF CHORDATES MODULE 1
5. Post-anal tail: this is a structure that extends beyond the anus, at least during the embryonic stage of chordates, the post-anal tail, together with the swimming musculature and the stiffening notochord, provides motility that larval tunicates and amphioxus need for their free-swimming existence.
Figure 1: Five principal features of chordates as shown in a generalized embryo. Source: Adapted from Raven and Johnson, 1999: p. 864
In addition to these five principal features, a number of other characteristics distinguish the chordates fundamentally from other animals. Chordates have more or less segmented body plan and distinct blocks of muscles. Most chordates have an internal skeleton (endoskeleton) against which muscles work. Chordates have paired limbs; pectoral and pelvic appendages are present in most vertebrates in the form of paired fins or legs. Chordates have living endoskeleton as compared with the dead, non-cellular exoskeleton of arthropods. The advantages of endoskeleton over exoskeleton are as follows: 1. It permits continuous growth without moulting, therefore, they can attain great size. 2. It provides ample surface for muscle attachment. 3. More muscle tissue necessitates greater development of body systems. 4. It allows for development and specialization of higher animals.
Unit 2
Classification Phylum Chordata (L. chorda, string or cord) Members include the protochordates or sea-squirts, cephalochordates (lancelets) and the vertebrates.
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DESCRIPTION, GENERAL CHARACTERISTICS AND CLASSIFICATION OF CHORDATES MODULE 1
Group Protochordata (Acrania) – invertebrate chordates Subphylum Urochordata (Tunicata) Subphylum Cephalochordata Group Craniata Subphylum Vertebrata – vertebrate chordates Superclass Agnatha Superclass Gnathostomata
Figure 2: Chordate family tree Source: https://www.carlsonstockart.com/photo/chordate-chordata-nerve-cord-notochord-family-tree-illustration/
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DESCRIPTION, GENERAL CHARACTERISTICS AND CLASSIFICATION OF CHORDATES MODULE 1
Conclusion
All chordates share some basic characteristics at least at one stage in their life cycle. These chordate features are: the notochord, dorsal tubular nerve cord, pharyngeal gill slits, endostyle and post-anal tail. Chordates also have living endoskeleton (in comparison with the dead, non-cellular exoskeleton of arthropods).
References/Further Reading
Raven PH, Johnson GB (1999). Biology. 5th edition. McGraw-Hill. 1284pp.
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