Phylum Chordata, Echinodermata & Hemichordata | Biology for JAMB PDF Download

Phylum Chordata

Introduction to Animal Kingdom

• Animals are part of the Animalia kingdom and are heterotrophs, relying on other organisms for nutrition.
• They are multicellular organisms capable of various functions.
• Animals can move from one place to another, unlike plants.
• They communicate using nerve cells, except in sponges where nerve cells are absent.
• While a central vacuole is absent, a small vacuole may be present.

Phylum Chordata Overview

• Chordata is one of the Phyla in the Animalia kingdom, coined by Georges Leopold Cuvier in 1869.
• Chordates include organisms with a notochord at some stage of their development.
• They exhibit a dorsal hollow nerve cord and pharyngeal slits at some point in their lives.
• Chordates are further classified into subphyla such as Vertebrata and Urochordata.

Chordata

Diagnostic Characters of Chordates

• Notochord: The notochord is a solid, unjointed, stiff but flexible rod-like structure located on the mid-dorsal side between the central nervous system and the alimentary canal. While some protochordates retain the notochord throughout their lives, in vertebrates, it is present only in the embryo stage and is later replaced by the vertebral column in adults.
• Dorsal Hollow Nerve Cord: This structure is always hollow and is positioned dorsally to the notochord.
• Paired Pharyngeal Gill Slits: At some point in their life cycle, all chordates possess a series of paired narrow openings, known as gill slits, located on the lateral sides of the pharynx.
• Tail: The tail represents the postanal part of the body in chordates. While absent in many adult chordates, it serves various functions in different species.

Classification of Phylum Chordata

• Phylum Chordata can be classified into three sub-phyla: Urochordata, Cephalochordata, and Vertebrata.

Subphylum Urochordata (Tunicata)

• Subphylum Urochordata, also known as Tunicata, features adult organisms enclosed in a leathery test made of a cellulose-like substance called tunicin.
• In this subphylum, only the larval tail contains the notochord, which is later replaced by a dorsal ganglion in the adult stage.
• While the larval form possesses a dorsal tubular nerve cord, it regresses into a small ganglion in the adult phase.
• Organisms in this subphylum are hermaphrodites.
• The larva, known as a tadpole, undergoes retrogressive metamorphosis, transitioning from a more developed larva to a less developed adult. Examples include Herdmania, Ascidia, Botryllus (a colonial urochordate), Molgula, Doliolum, Salpa, Pyrosoma (colonial urochordate), and Oikopleura.
• Typically found in marine environments, Urochordata species can be solitary or colonial. Examples of these organisms include Oikopleura, Herdmania, Pyrosoma, Doliolum, and Salpa.

Subphylum Cephalochordata

• The defining characteristic of the subphylum Cephalochordata is the extension of the notochord up to the anterior end of the body.
• In Cephalochordates, the notochord persists throughout their entire lifespan.
• These organisms possess numerous and well-developed pharyngeal gill slits.
• An atrium is present in the anatomy of Cephalochordates.
• Amphioxus features a unique wheel organ, also known as the ciliated organ of the Muller or rotatory organ. Posterior to this structure lies a circular ring-like formation called the velum, which connects to the pharynx.

Amphioxus

• The tail is a persistent feature throughout the life cycle of Amphioxus (Branchiostoma or Lancelet).

acraniaprotochordate

• Cephalochordates, along with Urochordates, are collectively referred to as acrania due to the absence of a cranium or brain box. They are also known as protochordates, indicating their status as primitive chordates.
• Examples:
• Cephalochordates exhibit separate sexes.
• Illustrative examples of Cephalochordates include Amphioxus or Branchiostoma (Lancelet).

Subphylum Vertebrata or Craniata

• In the embryonic stage, the notochord is present. It is later replaced by the vertebral column in adult forms.
• Vertebral Column
• Vertebrates typically have no more than two pairs of appendages.
• Cephalization, which is the development of distinct head structures, is observable.
• Cephalization
• The epidermis is composed of multiple layers of cells and may feature an exoskeleton like scales, feathers, or hair.
• The coelom, a body cavity, is well-developed.
• The complete digestive tract is ventral to the central nervous system.
• The endoskeleton is primarily made of cartilage or a combination of cartilage and bone.
• Ventrally located hearts and a hepatic portal system are present.
• There is a closed circulatory system that includes both blood vascular and lymphatic systems. Lymph, similar to blood but colorless, circulates. Blood contains red and white blood cells, with red cells containing hemoglobin.
• Gills, skin, buccopharyngeal cavity, and lungs serve as respiratory organs.
• Excretion occurs with the aid of a pair of kidneys.
• The nervous system comprises the central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves), and autonomic nervous system (sympathetic and parasympathetic nervous systems).
• There are typically 8, 10, or 12 pairs of cranial nerves.
• All vertebrates possess endocrine glands.
• Virtually all vertebrates are unisexual, with the exception of Hagfish, which is bisexual. They do not reproduce asexually.

Classification of Sub-Phylum Vertebrata

• Subphylum Vertebrata comprises two main sections: Agnatha and Gnathostomata.

Agnatha (Jawless Fishes)

• Agnatha are early vertebrates characterized by a lack of true jaws but possess vertebral columns and cranium.
• They have a suctorial mouth and lack paired appendages or fins.

Characteristics of Agnatha

• Long, eel-like body structure.
• Soft, slimy, scaleless skin.
• Round, suctorial mouth without jaws.
• Single median nostril for respiration.
• Respiration occurs through gills in pouches.
• Presence of cartilaginous endoskeleton and cylindrical notochord.
• Two-chambered heart and single gonad.
• External fertilization with direct or indirect development.
• They are found in aquatic environments, both marine and freshwater, and can be free-living or ectoparasites on fishes.

Examples of Agnatha:

• Petromyzon (lamprey)
• Myxine (hagfish)
• Bdellostoma (hagfish)

Gnathostomata (Jawed Vertebrates)

• Gnathostomata are vertebrates with true jaws and paired appendages.
• They exhibit a wide range of adaptations for various lifestyles.

Characteristics of Gnathostomata

• Presence of true jaws and paired appendages.
• Diverse adaptations for different ecological niches.
• Varied modes of reproduction and development.
• Examples include sharks, birds, mammals, and reptiles.

Gnathostomata

• Vertebrates with jaws and paired appendages.
• This subphylum is divisible into the following six classes: class Chondrichthyes and class Osteichthyes, which together form Pisces.

Class 1. Chondrichthyes:

• Chondrichthyes are marine fishes with a completely cartilaginous endoskeleton. They have a ventral mouth, tough skin coated with minute placoid scales, and respire through gills. These fishes have fins for locomotion and balance, with tail fins being heterocercal. Examples include Scoliodon, Sphyrna, Torpedo, etc.

Class 2. Osteichthyes:

• Osteichthyes consist of marine and freshwater fish with a partly or wholly bony endoskeleton. Their bodies are generally spindle-shaped, with skin that is either naked or covered in cycloid or ctenoid scales. They respire through gills, lay eggs, and fertilization is external. Examples include Labeo, Hippocampus, Anabas, etc.

Class 3. Amphibia:

• Amphibia include frogs, toads, newts, and salamanders. These creatures inhabit both freshwater and land, typically moist areas. Their skin is smooth or rough, often moist, slimy, glandular, and mostly lacks scales. They have a three-chambered heart, with double circulation. Amphibia are ectothermic (cold-blooded).

Class 4. Reptilia:

• Cold-blooded, terrestrial or aquatic vertebrates with a body coated with dry water-proof skin having horny epidermal scales or dermal scute plates. The body ranges in form and is usually divisible into the head, neck, trunk, and tail. The tympanum is small and depressed. Teeth are present in all reptiles except tortoises and turtles. Respiration occurs through the lungs only, with no gills present. Fertilization is internal.
• Most reptiles are oviparous, laying eggs with tough coverings that do not require a water environment. However, a few reptiles like lizards and snakes are viviparous. There is no aquatic larval stage in reptiles.
• Examples of reptiles include Kachuga, Chelone, Uromastix, and Draco.

Class 5. Aves:

• Warm-blooded tetrapod vertebrates (birds) varying in size from the tiny hummingbird to the large ostrich. Birds have horny scales on their feet and are predominantly covered in feathers. They lack cutaneous glands and have a body divided into head, neck, trunk, and tail. Wings, adapted for flight, emerge from modified forelimbs. Some birds like Kiwis have vestigial wings. Hind limbs typically have four clawed digits and are specialized for walking, perching, or swimming. Birds exhibit parental care and undergo internal fertilization. They are oviparous.
• Examples of birds include Gallus, Passer, Corvus, Columba, Psittacula, and Pavo.

Class 6: Mammalia:

• Mammals are warm-blooded creatures with bodies divided into the head, neck, trunk, and tail. They possess movable eyelids and females have milk-producing mammary glands for nursing their young. External fleshy ears (pinnae) are present, along with two pairs of limbs with five digits each. Limbs are adapted for various functions like walking, running, climbing, burrowing, swimming, or flying. Respiration in mammals occurs solely through the lungs.