Echinodermata Hemichordata Chordata

4.3.9 Phylum — Echinodermata

These animals have an endoskeleton of calcareous ossicles and, hence, the name (Echinos: spiny; derma: skin). All are marine with organ-system level of organisation. The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical. They are triploblastic and coelomate animals. Digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side. The most distinctive feature of echinoderms is the presence of a water vascular system which helps in locomotion, capture and transport of food and respiration. An excretory system is absent. Sexes are separate. Reproduction is sexual. Fertilisation is usually external. Development is indirect with free-swimming larva.

Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).

4.3.10 Phylum — Hemichordata

Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata. This phylum consists of a small group of worm-like marine animals with organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk. Circulatory system is of open type. Respiration takes place through gills. Excretory organ is proboscis gland. Sexes are separate. Fertilisation is external. Development is indirect.

Examples: Balanoglossus and Saccoglossus.

4.3.11 Phylum — Chordata

Animals belonging to phylum Chordata are fundamentally characterised by the presence of three features:

1. A notochord
2. A dorsal hollow nerve cord
3. Paired pharyngeal gill slits

They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation. They possess a post-anal tail and a closed circulatory system.

Chordates vs Non-Chordates

Subphyla of Chordata

Phylum Chordata is divided into three subphyla:

• Urochordata or Tunicata: notochord present only in larval tail. Examples: Ascidia, Salpa, Doliolum.
• Cephalochordata: notochord extends from head to tail and persists throughout life. Example: Branchiostoma (Amphioxus or Lancelet).
• Vertebrata: notochord is replaced by a vertebral column in adult; have ventral muscular heart with 2, 3 or 4 chambers, kidneys for excretion and paired appendages (fins or limbs).

Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine. In Vertebrata, all members have notochord during the embryonic period; the notochord is replaced by a vertebral column in adults.

Vertebrata = Chordata + vertebral column & brain

All vertebrates are chordates but all chordates are not vertebrates. Vertebrates possess notochord during the embryonic period. The notochord is replaced by a cartilaginous or bony vertebral column in the adult. Thus, all vertebrates are chordates but all chordates are not vertebrates.

Class Cyclostomata

All living members of the class Cyclostomata are ectoparasites on some fishes. They have an elongated body bearing 6–15 pairs of gill slits for respiration. Cyclostomes have a sucking and circular mouth without jaws. Their body is devoid of scales and paired fins. Cranium and vertebral column are cartilaginous. Circulation is closed type. Cyclostomes are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.

Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

Class Chondrichthyes

They are marine animals with streamlined body and have cartilaginous endoskeleton. Mouth is located ventrally. Notochord is persistent throughout life. Gill slits are separate and without operculum (gill cover). The skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed. Their jaws are very powerful. These animals are predaceous. Due to absence of air bladder, they have to swim constantly to avoid sinking. Heart is two-chambered (one auricle and one ventricle). Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon). They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature. Sexes are separate. In males, pelvic fins bear claspers. They have internal fertilisation and many of them are viviparous.

Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).

Class Osteichthyes

It includes both marine and fresh water fishes with bony endoskeleton. Their body is streamlined. Mouth is mostly terminal. They have four pairs of gills which are covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales. Air bladder is present which regulates buoyancy. Heart is two-chambered (one auricle and one ventricle). They are cold-blooded animals. Sexes are separate. Fertilisation is usually external. They are mostly oviparous and development is direct.

Examples: Marine — Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater — Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium — Betta (Fighting fish), Pterophyllum (Angel fish).

Class Amphibia

As the name indicates (Gr., Amphi: dual; bios: life), amphibians can live in aquatic as well as terrestrial habitats. Most of them have two pairs of limbs. Body is divisible into head and trunk. Tail may be present in some. The amphibian skin is moist (without scales). The eyes have eyelids. A tympanum represents the ear. Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca which opens to the exterior. Respiration is by gills, lungs and through skin. Heart is three-chambered (two auricles and one ventricle). These are cold-blooded animals. Sexes are separate. Fertilisation is external. They are oviparous and development is indirect.

Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

Class Reptilia

The class name refers to their creeping or crawling mode of locomotion (Latin, repere: to creep). They are mostly terrestrial animals and their body is covered by dry and cornified skin, epidermal scales or scutes. They do not have external ear openings. Tympanum represents ear. Limbs, when present, are two pairs. Heart is usually three-chambered, but four-chambered in crocodiles. Reptiles are poikilothermous. Snakes and lizards shed their scales as skin cast. Sexes are separate. Fertilisation is internal. They are oviparous and development is direct.

Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon, Calotes (Garden lizard), Crocodilus (Crocodile), Alligator, Hemidactylus (Wall lizard); poisonous snakes — Naja (Cobra), Bungarus (Krait), Vipera (Viper).

Class Aves

The characteristic features of Aves (birds) are presence of feathers and most of them can fly except flightless birds (e.g., Ostrich). They possess beak. The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches. Skin is dry without glands except the oil gland at the base of the tail. Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic). The digestive tract has additional chambers, the crop and gizzard. Heart is completely four-chambered. They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature. Respiration is by lungs. Air sacs connected to lungs supplement respiration. Sexes are separate. Fertilisation is internal. They are oviparous and development is direct.

Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

Class Mammalia

They are found in a variety of habitats — polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some of them have adapted to fly or live in water. The most unique mammalian characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished. They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying. The skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw (heterodont dentition). Heart is four-chambered. They are homoiothermous. Respiration is by lungs. Sexes are separate and fertilisation is internal. They are viviparous with few exceptions and development is direct.

Examples: Egg-laying — Ornithorhynchus (Platypus); pouched mammal — Macropus (Kangaroo); Flying — Pteropus (Flying fox/Bat); Aquatic — Balaenoptera (Blue whale); Predators — Panthera tigris (Tiger), Panthera leo (Lion); Domestic — Equus (Horse), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Delphinus (Dolphin); Highest — Homo sapiens (Human).

Quick Comparison — Vertebrate Classes

Worked Examples