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

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

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.

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.

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.

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

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

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

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

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

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

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.

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

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

• Habitat: Aquatic animals, mostly marine and few freshwater species
• Body Symmetry: Mostly asymmetrical
• Special Features: Presence of pores (ostia and oscula), collar cells, and sedentary (non-motile) nature
• Body Support: Supported by spicules or spongin fibres made of calcium carbonate or silica
• Reproduction: Asexual (by budding) and sexual methods; good regeneration ability
• Examples: Sycon, Euspongia (Bath sponge), Hyalonema, Euplectella

• 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).