Phylum – Chordata
Animals belonging to phylum Chordata are fundamentally
characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal
gill slits.
These are bilaterally
symmetrical, triploblastic, coelomate with
organ-system level of organisation. They
possess a post anal tail and a closed circulatory
system.
Phylum Chordata is divided into three
subphyla: Urochordata or Tunicata,
Cephalochordata and Vertebrata.
Subphyla Urochordata andCephalochordata are often referred to as
protochordates and are
exclusively marine. In Urochordata, notochord
is present only in larval tail, while in
Cephalochordata, it extends from head to tail
region and is persistent throughout their life.
Examples: Urochordata – Ascidia, Salpa,
Doliolum; Cephalochordata – Branchiostoma
(Amphioxus or Lancelet).
The members of subphylum Vertebrata
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. Besides the
basic chordate characters, vertebrates have a
ventral muscular heart with two, three or four
chambers, kidneys for excretion and
osmoregulation and paired appendages which
may be fins or limbs.
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 of 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 the 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 (Figure 4.20). 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 twochambered
(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 (Figure 4.21). 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. The heart is threechambered
(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 or reptum, to creep or crawl). They are mostly terrestrial
animals and their body is covered by dry and cornified skin, epidermal
scales or scutes (Fig. 4.22). 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
poikilotherms. 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 (Tree lizard),
Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator).
Hemidactylus (Wall lizard), Poisonous snakes – Naja (Cobra), Bangarus
(Krait), Vipera (Viper).
Class – Aves
The characteristic features of Aves (birds) are the presence of feathers
and most of them can fly except flightless birds (e.g., Ostrich). They possess
beak (Figure 4.23). 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 of birds has
additional chambers, the crop and gizzard. Heart is completely fourchambered.
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. Heart is fourchambered.
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:
👉Oviparous-Ornithorhynchus (Platypus)
Viviparous
👉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).
Phylum Chordata includes animals which possess a notochord either
throughout or during early embryonic life. Other common features observed in
the chordates are the dorsal, hollow nerve cord and paired pharyngeal gill slits.
Frogs can live both on land and in freshwater and belong to class
Amphibia of phylum Chordata. The most common species of frog found
in India is Rana tigrina.
Phylum
Classes comprising animals like fishes, amphibians, reptiles, birds
along with mammals constitute the next higher category called
Phylum. All these, based on the common features like presence
of notochord and dorsal hollow neural system, are included in
phylum Chordata. In case of plants, classes with a few similar
characters are assigned to a higher category called Division.
Man :Homo sapiens :Homo Hominidae: Primata ;Mammalia ;Chordata
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. Hemichordates have a rudimentary
structure in the collar region called stomochord, a structure
similar to notochord.
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 (Figure 4.15).
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
