Phylum Chordata: Characteristics of Each Subphylum and Class

The animals belonging to the class Chordata are characterized by the following characteristics at any stage (from larval to the adult stage) of life. These characteristics are:
1. Notochord
2. Single, dorsal, hollow nerve chord
3. Paired pharyngeal gill pouches/slits
4. Post anal tail and
5. Ventral heart
The first four characteristics are the hallmark of chordates. The heart is absent in Cephalochordates. Therefore, the ventral heart is not a universal characteristic of the chordates.
Chordates also share some characteristics with higher phyla of non-chordates. These common chordate and non-chordate characteristics are:
1. Presence of coelom
2. They are bilaterally symmetrical
3. They have metameric segmentation and
4. Cephalization

For the overall classification of chordates view the flow chart here.

The following sections will describe the characteristics of each subphylum and class

Subphylum: Urochordata (Tunicata)

  • Urochordata is derived from the Greek oura, tail, + Latin chorda, cord, + ata. Thus, Urochordate means organisms with notochord present in the larval tail only. They are also called Tunicata as they live in tough, nonliving tunics.
  • Habitat: They are exclusively marine, found in all seas from the near shoreline to great depths. Larvae are the free-living form, and adults are sessile.
  • The larval tail (which contains the notochord) disappears during metamorphosis. Also, the dorsal nerve chord is reduced and forms a single ganglion.
  • The incurrent and excurrent siphons are present. Water enters through the incurrent siphon, passes down the mouth, and to the brachial sac (pharynx/endostyle), where food gets trapped in the mucus. The entrapped food is then passed posteriorly to the esophagus. The incoming water is then out of the excurrent siphon.
  • The circulatory system consists of a ventral heart near the stomach and a pair of large vessels on either side of the heart. The odd feature found in no other chordate is that the heart drives the blood first in one direction for a few beats, then pauses, reverses, and drives the blood in the opposite direction.
  • Excretory system: Nephridium is present near the intestine.
  • Nervous system: A nerve ganglion is present, accompanied by a few nerves present on the dorsal side of the pharynx.
  • Reproduction: They are hermaphrodites. Solitary ascidians reproduce only sexually, where external and cross-fertilization occurs. Colonial forms reproduce both sexually and asexually. Asexual reproduction occurs by budding.
  • A few Urochordates (of class Larvacea and Thaliacea) are bioluminescent.
  • Urochordates are divided into three classes- Ascidiacea (sea squirts), Larvacea, and Thaliacea.
  • Examples: Ascidia, Salpa, and

Subphylum: Cephalochordata

  • Cephalochordata is derived from the Greek kephale, head, + Latin chorda, cord. The Cephalochordates are the organisms having notochord present from head to tail and throughout life.
  • They are marine and inhabit sandy bottoms of coastal waters.
  • Their Body is slender (thin), laterally compressed, translucent animals about 5 to 7 cm in length.
  • Water enters the mouth, is driven by cilia in the buccal cavity, and then passes through numerous pharyngeal slits in the pharynx, where food is trapped in mucus. Digestion occurs intracellularly in the midgut cecum (diverticulum).
  • The circulatory system is closed, although there is no heart. Their blood is colorless and lacks blood cells and hemoglobin.
  • The excretory system consists of paired nephridia.
  • The nervous system is centered around a hollow nerve cord above the notochord. The “brain” is a simple vesicle at the anterior end of the nerve cord.
  • The sexes are separate (dioecious). Sex cells are set free in the atrium, then pass out the atriopore to the outside, where (external) fertilization occurs. Development is indirect.
  • Example: Branchiostoma (Amphioxus or Lancelet).

Subphylum: Vertebrata (Craniata)

  • The term Vertebrata is derived from the Latin vertebratus, meaning backboned. The members of this subphylum have bony or cartilaginous vertebrae (which form the backbone) surrounding the spinal cord (vertebrae absent in agnathans); notochord only in embryonic stages (persisting in some fishes).
  • Other vertebrate characteristics are:
  1. The body plan is typical of the head, trunk, and tail. The neck persists in some terrestrial forms. Mammals have a thoracic cavity. The coelom is well-developed and contains the visceral system.
  2. Integument, an outer epidermis of stratified epithelium derived from ectoderm and an inner dermis of connective tissue derived from mesoderm; many skin modifications, such as glands, scales, feathers, claws, horns, and hair, are present in various classes.
  • The endoskeleton consists of a vertebral column (notochord persistent in jawless fishes, which lack vertebrae), limb girdles, two pairs of jointed appendages derived from somatic mesoderm, and a head skeleton called cranium (hence the name Craniata).
  1. The muscular, perforated pharynx, which possesses gills in fishes and tetrapods, is present only in the embryonic stage.
  2. Muscles attached to the skeleton provide movement.
  3. The digestive system is complete and ventral to the vertebral column. Digestive glands are associated with it.
  • The circulatory system is closed with a 2 to 4-chambered heart. Blood contains blood corpuscles and hemoglobin.
  • The excretory system consists of a pair of kidneys.
  1. The nervous system consists of a well-developed brain—10 or 12 pairs of cranial nerves.
  2. The endocrine system consists of many Endocrine (ductless) organs distributed throughout the Body.
  3. They are almost always dioecious with a pair of gonads with ducts to discharge egg and sperm. 

The subphylum Vertebrata is classified into two superclasses Agnatha (Cyclostomata) and Gnathostomata. The Agnathans are divided into classes: Myxine (hagfish) and Cephalaspidomorphi (Pteromyzones), and the superclass Gnathostomata is divided into many classes viz. Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves & Mammalia. The classes Chondrichthyes and Osteichthyes bear fins, and therefore, these are grouped as Pisces. The other Gnathostome classes (Amphibia, Reptilia, Aves & Mammalia) are together called Tetrapoda as they bear limbs (except a few).

Superclass: Agnatha (Cyclostomata)

  • The term Agnatha is derived from the Greek a, without + gnathos, jaw. The members of this superclass lack jaws.
  • Hagfishes are scavengers and predators, but lampreys are parasitic.
  • They also lack internal ossification, scales, paired fins, and an eel-like body form.
  • They have pore-like gill openings, 5 to 16 pairs of gills, a single pair of gill apertures in hagfishes, and seven pairs of gills in lampreys.
  • They have a fibrous and cartilaginous skeleton; the notochord is persistent and lacks vertebrae.
  • They have sucking and circular mouths. The digestive system is without a stomach.
  • They have a two-chambered heart (one atrium and one ventricle); a closed, single-circuit circulatory system.
  • The hagfishes are hermaphroditic, but gonads of only one sex are functional (i.e., functionally unisexual) and lack larval stage. In the lampreys, the sexes are separate and have a long larval stage. Both the groups (hagfish and lamprey) fertilize externally.
  • Lampreys migrate to fresh water for spawning, and the adults die soon after spawning. The larvae, after metamorphosis, migrate to the sea, if marine, or remain in freshwater.
  • Nonparasitic lampreys do not feed after metamorphosis since their alimentary canal degenerates to a non-functional strand of tissue. Within a few months and after spawning, they die.
  • Examples: Petromyzon, Myxine, Eptatretus.

Superclass: Gnathostomata

  • Gnathostomata is derived from the Greek gnathos, jaw, + stoma, mouth. All the members of this superclass have jaws and usually have paired appendages. They are classified into the classes described below.

Class: Chondrichthyes (cartilaginous fishes)

  • Chondrichthyes is derived from the Greek chondros, cartilage, + ichthys, a fish. They have cartilaginous endoskeletons and notochord persistent throughout life.
  • The mouth is ventrally placed; teeth (modified placoid scales) are not fused to jaws and are usually replaced. They are predators.
  • They bear five to seven pairs of gills with separate openings, no operculum, and no swim bladder or lungs.
  • They have a two-chambered heart (one atrium and one ventricle).
  • The senses of smell, vibration reception (lateral line system), and electroreception are well-developed.
  • The sexes are separate; internal fertilization; oviparous (Latin ovum, egg, + parere, to bring forth), ovoviviparous (Latin ovum, egg, + vivus, living, + parere, to bring forth), or viviparous (Latin vivus, living, + parere, to bring forth); direct development. In males, pelvic fins bear claspers.
  • Placoid scales present.
  • Stingrays have a whiplike, slender tails with spines that can inflict dangerous wounds. Electric rays have powerful electric organs, which can give severe shocks.
  • Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark), Trygon (Sting ray), Torpido (electric rays), etc.

Class: Osteichthyes (bony fishes)

  • Osteichthyes is derived from the Greek osteon, bone, + ichthys, a fish. They have bony endoskeletons and notochord replaced with backbone.
  • They have streamlined Bodies.
  • Their mouth is mostly terminal.
  • The skin with mucous glands and embedded dermal scales. The scales are of three types: ganoid, cycloid, or ctenoid; no placoid scales, some without scales.
  • Respiration is by four pair of gills which is supported by bony gill arches and covered by a common operculum. Sub-class Sarcopterygii has lungs.
  • A swim bladder is often present, which provides buoyancy.
  • Two chambered hearts.
  • Eels are catadromous (Greek kata, down, + dromos, running), i.e., they spend most of their lives in freshwater but migrate to the sea to spawn. On the other hand, Salmon are anadromous (Greek anadromous, running upward); i.e., they spend their adult lives at sea but return to freshwater to spawn.
  • They are dioecious (some hermaphroditic), gonads paired; mostly oviparous, but Some sharks are viviparous, and some aquarium fishes (guppies and mollies) are ovoviviparous; fertilization is usually external; mostly direct development but some species like (American eels, Anguilla rostrata) have larval forms which differ significantly from adults.
  • Examples: Marine – Exocoetus (Flying fish), Hippocampus (Sea horse);

           Freshwater – Labeo (Rohu), Catla (Katla),  Clarias (Magur);

           Aquarium – Betta (Fighting fish), Pterophyllum (Angelfish).

Class: Amphibia

  • Amphibia is derived from the Greek amphi, both + bios, life. They have double life (on land and in water) from where the group derives their name.
  • Most amphibians are quasiterrestrial, but a few are completely adapted to land. However, they cannot stay far from the moist condition.
  • Most have two pairs of limbs, frogs & toads (order Anura or Salientia) and salamanders (order Caudata or Urodela), but a few are limbless caecilians (order Apoda or Gymnophiona).
  • The skin of amphibians is moist, without scales (except a few caecilians where small scales are present), and porous, which is used as a primary or accessory breathing organ. The skin contains many glands, including poison glands in some; pigment cells are also present.
  • In addition to the skin, lungs are also present (absent in some salamanders). Gills are present in the larval stage only.
  • They are ectothermic (along with other lower classes and reptiles), and their body temperature varies with the environment.
  • The heart is a three-chambered heart, with two atria and one ventricle, double circulation through the heart.
  • The alimentary canal, urinary and reproductive tracts open in the cloaca, which opens to the exterior.
  • The eyes have eyelids. A tympanum (small hole) represents the ear near the eyes.
  • Excretion through a pair of kidneys. The main nitrogenous waste is urea (ureotelic).
  • They are dioecious, fertilization mostly internal in salamanders and caecilians and mostly external in frogs and toads. They are predominantly oviparous, some ovoviviparous or (rarely) viviparous; development is indirect but terrestrial species have direct development.
  • Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), and Ichthyophis (limbless amphibia).

Class: Reptilia

  • Reptilia is derived from the Latin repere or repto, to creep. These are the organisms that move by creeping.
  • They have varying body shapes. Some are elongated and compact in others.
  • Skin bears horny epidermal scales. Sometimes bony dermal plates are also present; integument with few glands.
  • The limbs are paired, usually with five toes, adapted for climbing, running, or paddling; limbs are absent in snakes and some lizards (a glass lizard, Ophisaurus sp).
  • Ossified endoskeleton. Ribs with the sternum (sternum absent in snakes).
  • Respiration by lungs, gills absent.
  • Three-chambered heart (four-chambered in crocodiles); systemic and pulmonary circuits are functionally separated.
  • Ectothermic
  • Excretion occurs by kidneys; the main nitrogenous waste is uric acid (uricotelic).
  • Snakes and lizards shed their scales as skin cast.
  • They do not have external ear openings. A small hole near the eyes called the tympanum represents the ear.
  • They are dioecious, fertilization is internal, oviparous (the tropical bushmaster and  American pit vipers are ovoviviparous, and Hydrophis is viviparous). Development is direct.
  • Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), and Alligator (Alligator). Hemidactylus (Wall lizard), Poisonous snakes – Naja (Cobra), Bangarus (Krait), Vipera (Viper), etc.

Class: Aves

  • The term Aves is derived from the Latin plural of avis.
  • Usually, a spindle-shaped Body is divided into four regions the head, neck, trunk, and tail; the neck is long for balancing and food gathering.
  • The digestive tract of birds has additional chambers, the crop, and the gizzard. The tongue is horn covered.
  • Feathers present to cover the epidermis is a hallmark of bird. Scales present on legs. Sweat glands are absent, but oil or preen gland at the base of the tail is present.
  • The limbs are paired. The forelimbs are usually modified into wings for flying; the posterior pair the foot with four toes (two or three toes in some) adapted for perching, walking, and swimming.
  • The Pinna of the ear is rudimentary.
  • Jaws covered with horny sheath forming a beak. Teeth absent.
  • Ribs with sternum present.
  • Fully ossified bone is present with small air cavities. These are called pneumatic bones, a flight adaptation to reduce body weight.
  • Only one bone in the middle ear.
  • Four-chambered heart. The red blood cells are nucleated and biconvex.
  • Endothermic (homoiothermous).
  • Respiration by lungs with thin air sacs; syrinx (voice box) near the junction of trachea and bronchi.
  • Excretion by the kidney; ureters open into cloaca; no urinary bladder; semisolid urine; uricotelic.
  • Dioecious; paired testes with vas deferens opening into the cloaca. Females with left ovary and oviduct only; copulatory organs present in ducks, geese, paleognathids (ostrich and kiwis), and few others.
  • Internal fertilization; oviparous; incubation of eggs is external; young active at hatching (precocial) or helpless and naked, i.e., no feathers (altricial); sex determination by females (females heterogametic; ZZ, males and ZW, females).
  • Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

Class: Mammalia

  • The term Mammalia is derived from the Latin mamma, They possess a mammary gland active in females after puberty, but in males, it is rudimentary.
  • Hairs are the hallmark characteristics of mammals. Some mammals, like bears, have dense hair, while others have very few hairs and are limited to some regions of the body and modified to serve many purposes. Vibrissae (whiskers) are sensory hairs that provide a tactile sense to many mammals; spines of porcupines, hedgehogs, echidnas, etc., provide protection.
  • Skin is two layered the epidermis and dermis. Many integumentary glands are present, which are of four kinds, the sweat, scent, sebaceous, and mammary glands.
  • Middle ear with three ossicles (malleus, incus, stapes).
  • The muscular diaphragm is present in mammals only.
  • Diphyodont dentition (two sets of teeth, milk or deciduous teeth, and permanent teeth) and heterodont dentition (more than one type of teeth).
  • External ear (ear pinnae) present.
  • Four pairs of limbs adapted for many functions.
  • The circulatory system is closed with a four-chambered heart. The mature RBCs are biconcave, lack nucleus and other organelles (except in Camel and Lamma).
  • A pair of lugs with alveoli where O2/CO2 exchange takes place. Separate air and food passages are present.
  • Excretion by a pair of kidneys and it opens in the urinary bladder through ureters arising from each kidney.
  • Endothermic or homoiothermic.
  • Unisexual and Internal fertilization.
  • All mammals are viviparous except a few. The monotremes, Ornithorhynchus (duck-billed Platypus) and Echidna lay eggs and hatched outside. In marsupials (pouched mammals like a kangaroo), gestation is brief, and a tiny young individual is reared in the pouch. All mammals provide lactation to the young one.
  • Examples: Ornithorhynchus (Platypus); Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion), etc.

References:

  • Linsenmayer, T. F., Gibney, E., & Schmid, T. M. (1986). Segmental appearance of type X collagen in the developing avian Notochord. Developmental Biology, 113(2), 467–473. https://doi.org/10.1016/0012-1606(86)90182-x
  • Kardong, K. V. (2011). Vertebrates: Comparative anatomy, function, evolution (6th ed.). McGraw Hill Higher Education.
  • Pechenik, J. A. (2014). Biology of the invertebrates (7th ed.). McGraw Hill Higher Education.
  • Moore, J. (2006). An introduction to the invertebrates (2nd ed.). Cambridge University Press.
  • Hickman, C. P., Roberts, L. S., & Larson, A. L. (2002). Animal Diversity (3rd ed.). McGraw Hill Higher Education.
  • Parker, T. J., & Haswell, W. A. (1962). Textbook of zoology: V. 2 (7th ed.). Macmillan.
  • Etymonline – Online Etymology Dictionary
  • Dictionary by Merriam-Webster: America’s most-trusted online dictionary

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