Asymmetric Animals: Animals which cannot be divided by any plane into equal halves. e.g. Mostly sponges (Spongilla); some sponges show radial symmetry.
Radial Symmetry: Animals which can be divided by more than one plane into equal halves. e.g. Coelenterata, Ctenophora, Echinodermata.
Bilateral Symmetry: Animals which can be divided by a single plane only. e.g. Platyhelminthes to Chordata.
Germ Layers
Diploblastic: Two germ layers present — Ectoderm and Endoderm, with Mesoglea present at the place of Mesoderm. e.g. Coelenterata to Ctenophora.
Triploblastic: Three germ layers present — Ectoderm, Mesoderm, Endoderm. Mesoderm develops for the first time in Platyhelminthes. e.g. Platyhelminthes to Chordata.
Coelom — Body Cavity Classification
Coelom is the internal space/cavity formed between the outer body wall and gut wall, and forms the basis for classification of animals into:
Acoelomate: Coelom absent (Diploblastic / Acoelomate). e.g. Coelenterata, Ctenophora, Platyhelminthes — body layers consist of Ectoderm, Mesoderm, Endoderm with no cavity.
Pseudocoelomate: e.g. Round worm (Aschelminthes) — body layers are Ectoderm, Mesoderm, Endoderm with a Pseudocoelom present.
Coelomate (Eucoelom): True coelom formed by splitting of mesodermal tissue (Schizocoelom) — e.g. Annelida, Arthropoda, Mollusca. True coelom formed by outpocketing of mesodermal tissue (Enterocoelom) — e.g. Echinodermata.
Pattern of Development
Protostome: Mouth forms first from the Blastopore of the Embryo. e.g. Annelida, Arthropoda, Mollusca.
Deuterostome: Anus forms first from the Blastopore of the Embryo. e.g. Echinodermata, Hemichordata, Chordata.
Notochord
Non-Chordata: Notochord absent.
Chordata: Notochord present — a rod-like supporting structure. In some Chordata the notochord is replaced by a Vertebral Column. Such Chordata are called Vertebrata. Note: All Chordates are not Vertebrata.
Phylum Porifera
They are generally marine; some also present in fresh water. Mostly show Asymmetric condition; some also show radial symmetry (e.g. Sycon and Euspongia).
They are primitive multicellular organisms and have cellular level of organisation.
Structure
The body wall has Pinacocytes (outer layer), Choanocytes/Collar cells (lining the spongocoel/canal system), Archeocytes (totipotent cells), and is supported by a skeleton made of Spicules or Spongin Fibres. Water enters through pores called Ostia into the central cavity (Spongocoel) and goes out through the Osculum. The body is generally sessile, attached to a solid surface.
Functions
Sponges have a water transport (canal) system. This transport is helpful in food gathering, respiration and removal of waste, and also in fertilization.
Choanocytes (collar cells) line the spongocoel/canal and help create water current.
Digestion is intracellular.
Reproduction
Sexes are not separate (Hermaphrodite) — eggs and sperms are produced by the same individual.
Sponges reproduce asexually (by Budding, Fragmentation) and by formation of Gametes (sexual).
Fertilization is internal.
Development is Indirect — having a larval stage which is morphologically distinct from the adult.
They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name Cnidaria is derived from Cnidoblast or Cnidocyte — containing the stinging capsules called Nematocysts (which contain poison).
Structure
Cnidoblasts are present on Tentacles and Body — used for Anchorage, Defense, and Capture of prey. Body parts: Hypostome, Mouth, Tentacles with Cnidoblast/Cnidocil, Gastrovascular cavity, Basal disc.
Features
Cnidaria exhibit Tissue level of organisation.
They are Diploblastic, with a single Gastrovascular cavity (mouth on Hypostome, body apolar with Basal disc).
Digestion is both Extracellular and Intracellular.
Some corals have a CaCO₃ skeleton.
Nerve net (Incomplete) is poorly developed in coelenterates.
Body Forms
Polyp: Cylindrical form, attached with solid surface (sessile), reproduces by Asexual reproduction. e.g. Hydra, Adamsia, Gorgonia.
Medusa: Umbrella-like, free-swimming form, reproduces by Sexual reproduction. e.g. Aurelia (Jelly fish).
Cnidarians which exist in both forms exhibit alternation of generations (Metagenesis) — i.e. polyps produce medusa asexually, and medusa forms polyps sexually. e.g. Life cycle of Obelia. This phenomenon (asexual stage alternating with sexual stage) is also seen in Hydra and Physalia.
Reproduction
Asexual Reproduction: known as budding in some cnidarians (e.g. Hydra).
Sexual Reproduction: involves Male Gametes and Female Gametes, leading to fertilization and an indirect (Larva) development.
"Comb Bearing Animals" — Ctenophores, commonly known as Sea Walnuts or Comb Jellies, are exclusively marine, radially symmetrical, diploblastic organisms with Tissue level of organisation.
The body bears eight external rows of Ciliated Comb Plates, which help in locomotion.
Bioluminescence (the property of emitting light) is well marked.
Sexes are not separate — reproduction takes place only by sexual means, with external fertilization, and indirect development (larval stage).
Examples: Pleurobrachia, Ctenoplana.
Note — Monoecious vs Dioecious: If an organism is Bisexual (has both male and female reproductive organs on the same individual), it is called Monoecious (Hermaphrodite/Bisexual). If an organism is Unisexual, it is called Dioecious.
Phylum Platyhelminthes
They have dorsoventrally flattened body, hence are called Flatworms. They are mostly endoparasites found in animals including human beings.
They are Bilaterally symmetrical, with Organ level of organisation, Triploblastic and Acoelomate.
Ladder Like Nervous System is present.
Hooks and Suckers are present in the parasitic forms, which help in attachment and absorb nutrients from the host directly.
Structure (Liver Fluke)
Body parts include: Suckers (for attachment and ingestion of food), Budding zone, Proglottides (22-32, for attachment), Ventral sucker, Flame cells/Protonephridia (excretory cells), Mouth, Worm body, Excretory pore.
Excretion and Reproduction
Specialised cells called Flame cells (Protonephridia) help in osmoregulation and excretion.
Reproduction may be Sexual (Self or Cross fertilization). Some members like Planaria have high regeneration power.
Development involves a larval stage (Indirect development).
Life Cycle of Liver Fluke (Fasciola)
This is a Diheterogenetic life cycle — completed in two hosts:
Primary Host (e.g. Sheep/Goat): Sexual life — Adult worm in Bile duct; sexual reproduction & fertilization occurs here; Egg → Miracidium Larva (1st larva, infective stage for secondary host).
Secondary Host (Snail): Asexual life — Sporocyst → Redia (5-6, by asexual reproduction) → Cercaria Larva (15-16) → Metacercaria (infective stage for primary host).
Male Ascaris: Smaller in size, posterior end curved with one pair of Penial setae (helps in copulation). Single testis (Monodelphic). Cloaca = Pore + Genital pore.
Female Ascaris: Large and longer, posterior end straight. Penial setae absent. Has Oviduct + Uterus, with separate Genital pore (didelphic / amphidelphic arrangement referenced).
Life Cycle (Ancylostoma / Filariasis)
Ancylostoma — Monogenetic life cycle, Human is the host, infection through embryonated eggs from contaminated food and water (causing intestinal blockage etc.).
Wuchereria (Elephantiasis): Diheterogenetic life cycle — Primary Host: Human, Secondary Host: Mosquito (Culex). Causes inflammation in lymph nodes.
Phylum Annelida
Segmented body — Metameric Segmentation is present (their body surface is distinctly marked into segments, hence the name).
They may be aquatic — Marine (Nereis), Fresh water/parasitic (Leech), Terrestrial (Earthworm).
Free living and sometimes parasitic. They exhibit Organ system level of organisation and Bilateral symmetry.
They possess Longitudinal and Circular muscles which help in Locomotion in Earthworm.
Parapodia — present in aquatic annelids like Nereis, which help in swimming.
Oral sucker — present in Hirudinaria (Leech), used for attachment.
Circulatory and Excretory System
Nereis possesses a closed Circulatory system and Nephridia (Excretory organs).
Leech has an Open circulatory system.
Nervous System and Reproduction
Nervous system consists of paired Ganglia (sing. Ganglion) connected by Lateral nerves to a double, solid, ventral Nerve Cord.
Nereis is dioecious, but Earthworm and Leeches are monoecious.
Reproduction is Sexual.
Phylum Arthropoda
"Arthro" = Joint, "Poda" = Leg/Foot.
This is the Largest Phylum in Animalia — two-thirds (70%) of all named species on earth are Arthropods, and this includes Insects.
They have Organ system level of organisation. They are Bilaterally symmetrical, Coelomate (Schizocoelomate).
The body of Arthropods is covered by a Chitinous Exoskeleton, and the body is segmented into Head, Thorax, and Abdomen (all three present in all Insecta).
Respiration and Circulation
Respiratory organs include Gills, Book Lungs, and Tracheal system.
Circulation is of Open type (Haemocoel), with simple (without respiratory pigment in some).
Other example: Scorpion (with soft, spongy skin/exoskeleton mention).
Phylum Mollusca
This is the second largest animal phylum.
Molluscs are terrestrial or aquatic (Marine or Fresh water).
They have Organ system level of organisation, are Bilaterally symmetrical, Triploblastic and Coelomate (Schizocoelomate).
Body is segmented with a distinct Head, Muscular Foot, and Visceral Hump, and is covered by a Calcareous shell.
Mantle and Mantle Cavity
A soft and spongy layer (Mantle) forms over the Visceral Hump. The space between the hump and the mantle is called the Mantle Cavity, in which feather-like gills are present — these have respiratory and excretory functions.
Feeding and Reproduction
The interior head region has Sensory Tentacles.
The mouth contains a file-like rasping organ called the Radula.
They are usually Dioecious and Oviparous, with Indirect development.
Shell
Shell is secreted by the Mantle — mostly External, sometimes Internal (e.g. Cuttlefish), and Absent in Octopus.
Chordata are characterised by the presence of a Notochord, a Dorsal Hollow Nerve cord, and Post-anal Gill slits.
Feature
Non-Chordata
Chordata
Notochord
Absent
Present (Origin: Mesodermal)
Nerve Cord
Ventral, Solid, Single
Dorsal, Hollow, Double (Origin: Ectodermal)
Gill Slits
Absent
Present (in some higher chordates too)
Heart Position
Dorsal
Ventral
Notochord — Details
Notochord: Origin — Mesodermal. Present only in Chordata. It is Solid, Elastic, Rod-like, present between the Nerve cord and the Alimentary canal, below the Nerve Cord. In Vertebrates, the Notochord is replaced by the Vertebral Column. Post-anal part helps in Balancing.
General Chordate Features
Bilaterally Symmetrical.
Organ system level of organisation.
They possess a Closed Circulatory system.
CNS is Hollow.
Pharynx is perforated by Gill slits.
Heart is Ventral.
Post Anal Tail is present (helps in balancing) — in some, the Brain box is part of this.
Sub-Phyla of Chordata
Urochordata: Notochord present only in Larva (not in adult). e.g. Ascidia, Doliolum.
Cephalochordata (Euchordata): Notochord extends from head to tail and persists throughout life. e.g. Lancelet.
Vertebrata: Notochord present in embryo, replaced by Vertebral column in adults.
Classes of Vertebrates
Division of Vertebrata (on the basis of Jaws)
Agnatha (Cyclostomata): Jawless. e.g. Petromyzon (Lamprey), Myxine (Hagfish).
Gnathostomata (Bears Jaws): Includes Superclass Pisces (Chondrichthyes + Osteichthyes) and Superclass Tetrapoda (on the basis of Locomotory Limbs) — Amphibia, Reptilia, Aves, Mammalia.
Class Cyclostomata
Cyclostomes have a body devoid of scales and paired fins.
Cyclostomes are marine but migrate to fresh water for spawning. After spawning, they die within a few days. Their larvae return to the marine/ocean water as Adults after metamorphosis (Ammocoete larva → Adult).
Class Chondrichthyes (Cartilaginous Fish)
They are marine animals with a streamlined body and Cartilaginous endoskeleton.
Notochord persists throughout life. Mouth is located Ventrally.
Gill slits are separate and without an Operculum (gill cover).
The skin is tough, containing minute Placoid Scales. Teeth are modified Placoid scales which are backwardly directed and very powerful (Predacious).
Some of them have Electric organs (e.g. Torpedo), and some possess a poison sting (e.g. Trygon).
Due to absence of Air Bladder, they have to swim constantly to avoid sinking.
Sexes are separate (Dioecious). In males, the Pelvic fin bears Claspers (male copulatory organ).
They are Cold Blooded (Poikilothermous) animals. Fertilization is internal, and many of them are Viviparous.
Hepatic Portal System: Present in all Chordata. Renal Portal System: Present in Lower Chordates (Fish & Amphibia). Absent in Higher Chordates (Reptilia to Mammalia).
Semicircular Canals — Comparison
Group
Semicircular Canals
Function
Hagfish
Single
Balancing
Lamprey
Two
Balancing
Fish
Three
Balancing
Amphibia
Three
Balancing + Hearing, Tympanum with single ear ossicle
Cloaca — common pore for opening of Digestive, Urinary and Reproductive tracts. Present in Cartilaginous fish, Amphibia, Reptilia, Aves, and some Mammals. Absent in Bony Fish and most Mammals (which have separate openings).
Class Osteichthyes (Bony Fish)
Includes both Marine and Fresh water fishes. Their body is streamlined.
Mouth is Terminal. Notochord persistent.
Gills are present on each side, covered by an Operculum.
Skin is covered with Cycloid scales.
An Air Bladder is present which regulates buoyancy.
Heart is Two Chambered (one Auricle, one Ventricle).
They are Cold Blooded animals.
Sexes are separate (Dioecious), Oviparous. Fertilization is usually External. Development is Direct.
The characteristic feature of birds is the presence of Feathers. Most of them can fly, except a few (e.g. Ostrich).
They possess a beak. The forelimbs are modified into Wings. Hind limbs generally have scales and are modified for walking or clasping the tree branches.
Skin is dry without glands, except an oil gland (Uropygial gland) present at the base of the tail.
Endoskeleton is fully ossified. Long bones are hollow with air cavities (Pneumatic bones).
The digestive tract of birds has additional chambers — Crop and Gizzard (present in all birds without exception).
Heart is completely Four Chambered.
They are Warm Blooded (Homoiothermous) animals — able to maintain a constant body temperature.
Respiration aided by Air sacs connected to Lungs.
Sexes are separate (Dioecious). Fertilisation is internal. They are Oviparous, and development is Direct.
They are found in a variety of habitats — forests, grasslands, deserts, mountains, polar ice and dark caves. Some of them have adapted to flying or aquatic life.
The most unique mammalian characteristic is the presence of milk-producing glands (Mammary glands) by which young ones are nourished.
They have two pairs of limbs adapted for walking, running, burrowing, swimming or flying.
The skin of mammals is unique in possessing Hair. External ears (Pinna) are present.
Different types of teeth are present in the jaw.
Heart is Four Chambered. They are Homoiothermous (Warm Blooded). Sexes are separate (Dioecious).
Explanation: Coelenterates exhibit Tissue level of organisation and are Diploblastic, having two germ layers — Ectoderm and Endoderm — with Mesoglea present at the place of Mesoderm. Porifera shows Cellular level organisation, while Platyhelminthes and Annelida are Triploblastic with Organ/Organ-system level organisation.
Q2. A true coelom formed by splitting of mesodermal tissue is called:
Explanation: Schizocoelom is a true coelom that forms by the splitting of mesodermal tissue blocks, seen in Annelida, Arthropoda and Mollusca. Enterocoelom, in contrast, forms by outpocketing of mesodermal tissue (e.g. Echinodermata). Pseudocoelomate animals (e.g. round worms) have a body cavity not completely lined by mesoderm, and acoelomates lack a coelom altogether.
Q3. The infective stage of Liver Fluke (Fasciola) for the primary host is:
Explanation: The life cycle of the liver fluke is diheterogenetic, completed in two hosts. In the snail (secondary host), asexual stages occur in order: Sporocyst → Redia → Cercaria → Metacercaria. The Metacercaria is the encysted infective stage that, when ingested by the primary host (e.g. sheep/goat), develops into the adult fluke residing in the bile duct.
Q4. Which phylum is the largest in Animalia, comprising about 70% of all named species (including Insects)?
Explanation: Phylum Arthropoda ("Arthro" = joint, "Poda" = leg) is the largest phylum in Animalia, with insects accounting for the vast majority of named species — roughly two-thirds (70%) of all described animal species. Mollusca is the second largest phylum.
Q5. Which group of fishes lacks an Air Bladder, requiring them to swim constantly to avoid sinking?
Explanation: Chondrichthyes (cartilaginous fishes) lack an Air Bladder, which in Osteichthyes (bony fishes) regulates buoyancy. Due to this absence, cartilaginous fishes such as sharks must swim continuously to avoid sinking. Chondrichthyes also have a Two Chambered heart, separate gill slits without an operculum, and tough skin with placoid scales.
Q6. The presence of a Tympanum representing the ear, moist skin without scales, and a common Cloaca chamber are features of which class?
Explanation: Class Amphibia ("Amphi" = dual, "Bios" = life) is characterised by the ability to live both in aquatic and terrestrial habitats. Their skin is moist and without scales, eyes have eyelids, and a Tympanum represents the ear. A common chamber called Cloaca opens to the exterior, through which the alimentary, urinary and reproductive tracts open. The heart is Three Chambered, and they are Cold Blooded.
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