Revision: Term - 2 >> Basis of Classification Science SSLC (English Medium) Class 7 Tamil Nadu Board of Secondary Education

Classes comprising animals like fish, amphibians, reptiles, birds, and mammals constitute the next higher category called phylum. All these, based on common features like the presence of the notochord and dorsal hollow neural system, are included in the phylum Chordata. In the case of plants, classes with a few similar characters are assigned to a higher category called division.

1. You have seen earlier that categories like species, genus, and families are based on a number of similar characters.
2. Generally, order and other higher taxonomic categories are identified based on the aggregates of characters. Order, being a higher category, is the assemblage of families that exhibit a few similar characters.
3. The similar characters are fewer in number as compared to the different genera included in a family. Plant families like Convolvulaceae, Solanaceae are included in the order Polymoniales, mainly based on their floral characters.
4. The animal order, Carnivora, includes families like Felidae and Canidae.

This category includes related orders. For example, order Primata, comprising monkeys, gorillas, and gibbons, is placed in class Mammalia, along with order Carnivora, which includes animals like tigers, cats, and dogs. Class Mammalia has other orders as well.

A taxon is a taxonomic group at any level. It represents a group of organisms at any level.

Genus comprises a group of related species that have more characters in common in comparison to species of other genera. We can say that genera are aggregates of closely related species. For example, potato and brinjal are two different species, but both belong to the genus Solanum. Lions (Panthera leo), leopards (P. pardus), and tigers (P. tigris), with several common features, are all species of the genus Panthera. This genus differs from another genus, Felis, which includes cats.

1. The next category, family, has a group of related genera with a smaller number of similarities as compared to genus and species. Families are characterised on the basis of both vegetative and reproductive features of plant species.
2. Among plants, for example, three different genera, Solanum, Petunia, and Datura, are placed in the family Solanaceae. For example, the genus Panthera, comprising lions, tigers, and leopards, is put along with the genus  Felis (cats) in the family Felidae. Similarly, if you observe the features of a cat and a dog, you will find some similarities and some differences as well.
3. They are separated into two different families – Felidae and Canidae, respectively.

Thallus refers to a plant body that is not differentiated into roots, stems, and leaves. Example: Algae.

Small pores through which water enters the body of a sponge are called ostia.

A large opening through which water exits the body of a sponge is called an osculum.

Special types of cells present in sponges that help in generating water current and food collection are called collar cells.

Coelom: Coelom is the internal body cavity between visceral organs and body wall in which well-developed organs can be accommodated. In some animals pseudocoelom is present (e.g. nematodes) which is a body cavity not lined by mesoderm. True coelom is lined by mesoderm and first appears in annelids. Examples of animals in which true coelom is present are Earthworm, cockroach, snail, all chordates such as birds, humans, etc.

A cylindrical body form in coelenterates, such as Hydra, which is usually attached to a surface, is called a Polyp.

An umbrella-shaped body form in coelenterates, such as Aurelia (jellyfish), is called a Medusa.

Nomenclature is the system of assignment of names to organic compounds.

The theoretical study of classification which includes its basic principles, procedures and rules.

1. Meaning - All single-celled eukaryotes act as a link between plants, animals and fungi; mostly aquatic; have a nucleus and membrane-bound organelles.

2. Chrysophytes - Includes diatoms and golden algae; cell wall has silica; deposits form Diatomaceous Earth (used in polishing and filtration); chief producers in oceans.

3. Dinoflagellates - Mostly marine, photosynthetic; cell wall of cellulosic plates; have two flagella (one longitudinal, one transverse); rapid multiplication of red dinoflagellates (e.g. Gonyaulax) causes red tides.

4. Euglenoids - Have a pellicle instead of a cell wall; photosynthetic in sunlight but become heterotrophs in its absence. e.g. Euglena.

5. Slime Moulds - Saprophytic; feed on decaying matter; undergo syngamy to form a plasmodium that spreads over several feet.

6. Protozoans - 4 types:

• Amoeboid - move by pseudopodia. e.g. Amoeba, Entamoeba
• Flagellated - have flagella; cause sleeping sickness. e.g. Trypanosoma
• Ciliated - have cilia; have a gullet for feeding. e.g. Paramecium
• Sporozoans - no locomotory organelle; all parasitic. e.g. Plasmodium

1. Meaning - Heterotrophic organisms; body made of thread-like hyphae; network of hyphae = mycelium; cell wall made of chitin; grow in warm and humid places; except yeasts, all are multicellular.

2. Nutrition - Can be saprophytic (feed on dead matter), parasitic or symbiotic (as lichens and mycorrhiza).

3. Reproduction - Asexual by spores (conidia, zoospores, aplanospores); sexual reproduction involves 3 steps - plasmogamy → karyogamy → meiosis; Ascomycetes and Basidiomycetes undergo a dikaryotic stage.

4. 4 Classes of Fungi:

• Phycomycetes - Mycelium aseptate, coenocytic; spores endogenous in sporangium. e.g. Rhizopus, Albugo
• Ascomycetes (Sac fungi) - Asexual spores = conidia; sexual spores = ascospores in ascus. e.g. Aspergillus, Neurospora
• Basidiomycetes - No asexual spores; sexual spores = basidiospores (exogenous). e.g. Agaricus, Ustilago, Puccinia
• Deuteromycetes (Imperfect Fungi) - Sexual form unknown; saprophytic/parasitic. e.g. Alternaria, Colletotrichum

5. Classification basis - Based on morphology, mode of spore formation and fruiting bodies.

6. Important examples - Puccinia (wheat rust), Penicillium (antibiotic), Agaricus (mushroom), Neurospora (genetic research).

1. Meaning- All eukaryotic, chlorophyll-containing organisms; cell wall made of cellulose; mostly autotrophic; some are insectivorous (e.g. Venus fly trap, Bladderwort) or parasitic (e.g. Cuscuta).

2. Members - Includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

3. Alternation of generations - Life cycle has two phases - diploid sporophyte and haploid gametophyte - which alternate with each other.

4. Plant groups at a glance:

• Algae - Simple, aquatic, no true roots/stems/leaves. e.g. Seaweed, Green algae
• Bryophytes - Non-vascular, moist environments. e.g. Mosses, Liverworts
• Pteridophytes - Vascular, no seeds, reproduce by spores. e.g. Ferns
• Gymnosperms - Naked seeds in cones, no flowers/fruits. e.g. Pine, Fir
• Angiosperms - Seeds enclosed in fruits, the most diverse group. e.g. Rose, Lily

5. Key features - Have prominent chloroplasts for photosynthesis; membrane-bound organelles; the most diverse group among all kingdoms.

• Meaning - Heterotrophic, multicellular eukaryotes; cells lack a cell wall; cannot produce their own food.
• Nutrition - Mode of nutrition is holozoic (food obtained by ingestion); food is digested in an internal cavity; food is stored as glycogen or fat.
• Reproduction - By sexual reproduction - copulation of male and female followed by embryological development.
• Other features - Follow a definite growth pattern; higher forms have elaborate sensory and neuromotor mechanisms (nervous system and sensory organs); most are capable of locomotion.
• Cell structure - Has a nucleus and membrane-bound organelles; no chloroplasts (hence cannot photosynthesise); no cell wall (unlike plants and fungi).

• Algae are simple, chlorophyll-containing, autotrophic organisms, mostly found in aquatic habitats.
• They have a thallus body without true roots, stems, and leaves.
• Algae show different forms, such as unicellular (Chlamydomonas), colonial (Volvox), and filamentous (Spirogyra).
• Reproduction occurs by vegetative, asexual, and sexual methods, including fragmentation and spore formation.
• Algae play an important role in CO₂ fixation and oxygen production, supporting aquatic ecosystems.
• They have economic importance, providing food, agar, algin, carrageenan, and protein-rich supplements like Spirulina.

• Bryophytes are called amphibians of the plant kingdom as they live on land but need water for reproduction.
• They are found in damp, humid, and shaded places and lack true roots, stems, and leaves.
• The plant body is thallus-like and attached to the substratum by rhizoids.
• The gametophyte is the dominant phase, which produces male (antheridia) and female (archegonia) gametes.
• Fertilisation requires water, forming a zygote that develops into a sporophyte (foot, seta, capsule).
• Reproduction occurs by vegetative (fragmentation), asexual (gemmae), and sexual methods.
• Economic importance: prevents soil erosion, acts as a pioneer species, provides peat (fuel), and has medicinal uses.

• Pteridophytes are vascular plants with true roots, stems, and leaves; the sporophyte is the dominant phase.
• They are the first terrestrial plants with vascular tissues (xylem and phloem).
• Leaves may be microphylls (small), as in Selaginella, or macrophylls (large), as in ferns.
• Sporangia produce spores and are present on sporophylls, often forming cones (strobili).
• Spores germinate into a haploid gametophyte (prothallus), which is small, independent, and photosynthetic.
• Fertilisation requires water, and the zygote develops into a new sporophyte.
• Some pteridophytes show heterospory (microspores and megaspores), which is an important step towards a seed habit.

• Angiosperms are flowering plants with seeds enclosed in fruits.
• They show great variation in size, from small (Wolffia) to large trees (Eucalyptus).
• Classified into monocots and dicots.
• Double fertilisation occurs, forming a zygote and a triploid endosperm.
• After fertilisation, the ovary becomes a fruit, and the ovule becomes a seed.
• Life cycle is diplontic with a dominant sporophyte and a reduced gametophyte.

• Naked seeds - Gymnosperms have ovules not enclosed by the ovary wall; seeds remain exposed before and after fertilisation.
• Vegetative features - Stem branched (Pinus) or unbranched (Cycas); leaves are needle-like with thick cuticle and sunken stomata to reduce water loss; roots are tap roots.
• Special roots - Pinus has mycorrhiza (fungal association); Cycas has coralloid roots with N₂-fixing cyanobacteria.
• Reproduction - Reproductive structures are strobili (cones); plants are heterosporous - male strobilus has microsporophylls (produce pollen); female strobilus has megasporophylls (bear ovules); pollination is by air (wind).
• Fertilisation - Pollen tube carries male gametes to archegonium → fuses with egg → zygote → embryo; ovule develops into seed; life cycle is diplontic (sporophyte dominant).
• 4 groups - Cycads (Cycas), Conifers (Pinus, Cedrus), Ginkgoales (Ginkgo biloba), Gnetales (Ephedra, Gnetum).
• Economic importance - Ephedrine (from Ephedra) for respiration; Taxol (from Taxus) for cancer; sago from Cycas; timber, resins, turpentine and paper pulp from conifers.

• Meaning - Sponges; primarily marine (except Spongilla - freshwater); body is asymmetrical; cellular level of organisation.
• Water canal system - Water enters through ostia (pores) → spongocoel (body cavity) → exits through osculum; helps in feeding, respiration and waste removal; canals lined by choanocytes (collar cells).
• Digestion - Intracellular; body supported by spicules and spongin fibres.
• Reproduction - Hermaphrodite (produces both eggs and sperm); asexual by fragmentation; sexual by gamete formation; fertilisation is internal with indirect development (distinct larval stage).
• Examples - Sycon, Spongilla (freshwater), Euspongia (bath sponge), Euplectella (Venus' Flower Basket).

• Body Shape: Cylindrical (Polyp) or umbrella-like (Medusa)
• Habitat: Mostly marine; few are freshwater dwellers
• Body Structure: Radially symmetrical and diploblastic
• Special Feature: Tentacles with cnidoblasts used for capturing prey and injecting toxins
• Examples: Hydra, Adamsia (Sea anemone), Physalia (Portuguese man-of-war), Aurelia (Jellyfish), Corals

• Body Shape: Slender, flat like a leaf or strip (called flatworms)
• Habitat: Mostly endoparasitic; few are free-living and aquatic
• Body Structure: Acoelomate, bilaterally symmetrical, and triploblastic
• Reproduction: Hermaphrodite – both male and female systems in one body
• Examples: Planaria, Liver fluke, Tapeworm

• Body Shape: Long, thread-like or cylindrical (called roundworms)
• Habitat: Free-living (aquatic or terrestrial) or endoparasitic
• Body Structure: Triploblastic, pseudocoelomate, non-segmented, with a tough cuticle
• Reproduction: Unisexual – male and female are separate
• Examples: Ascaris (intestinal worm), Filarial worm, Loa loa (eye worm)

• Body Structure: Long, cylindrical, metamerically segmented
• Habitat: Mostly free-living (marine, freshwater, or terrestrial); some are ectoparasites
• Body Type: Triploblastic, bilaterally symmetrical, and eucoelomate
• Locomotion: Movement with setae, parapodia, or suckers
• Reproduction: Hermaphrodite or unisexual
• Examples: Earthworm, Leech, Nereis

• Body Structure: Segmented, triploblastic, eucoelomate, and bilaterally symmetrical
• Appendages: Jointed appendages with a chitinous exoskeleton
• Habitat: Found in all types of environments – aquatic, terrestrial, and aerial
• Reproduction: Unisexual
• Examples: Crab, spider, cockroach, butterfly, honey bee

• Body Structure: Soft, non-segmented, triploblastic, eucoelomate body
• Symmetry: Bilateral (except in some, like snail)
• Body Division: Head, foot, and visceral mass covered by a mantle
• Shell: Hard, calcareous shell (may be external, internal, or absent)
• Reproduction: Unisexual
• Examples: Bivalve, Snail, Octopus

• Body Structure: Triploblastic, eucoelomate, radially symmetrical in adults and bilaterally symmetrical in larvae
• Habitat: Exclusively marine
• Locomotion: Tube feet are used for movement and capturing prey; some are sedentary
• Skeleton: Made of calcareous spines and ossicles
• Reproduction: Mostly unisexual
• Examples: Starfish, sea urchin, sea cucumber, brittle star

• Body Structure: Divided into proboscis, collar, and trunk.
• Notochord: Present only in the proboscis region, hence the name hemichordates.
• Habitat: Marine animals that live in burrows in sand.
• Respiration: Possess one to many pharyngeal gill slits.
• Reproduction: Mostly unisexual, though some may be hermaphrodite.
• Examples: Balanoglossus, Saccoglossus.
• Evolutionary Importance: Balanoglossus is considered a connecting link between non-chordates and chordates.

• Notochord: Replaced by vertebral column
• Head: Well developed
• Brain Protection: Enclosed in a cranium
• Endoskeleton: Cartilaginous or bony
• Jaw Type: Some are jawless (Agnatha), others have jaws (Gnathostomata)

• The IUPAC system provides a unique, systematic way to name carbon compounds based on structure, replacing confusing common names.
• An IUPAC name has three parts: prefix, parent, and suffix, reflecting the carbon chain and functional group.
• The parent name is based on the longest carbon chain, and its ending changes to –ane, –ene, or –yne depending on the number of bonds.
• Functional groups are shown as prefixes or suffixes, and the chain is numbered to give them the lowest possible number.
• If the suffix begins with a vowel, the final ‘e’ in the parent alkane name is dropped (e.g., propane → propanone).