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Difference Between Phylum and Class

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  • Last Updated : 07 Sep, 2022
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Taxonomic hierarchy is also called Linnaean hierarchy because it was first proposed by Linnaeus. Hierarchy of categories is the classification of organisms in a definite sequence of categories (taxonomic categories) in a descending order starting from the kingdom and reaching up to species or in ascending order from species to kingdom. The hierarchical categories into which every organism gets classified are discussed in many details. In this article, we will focus on two major taxonomic categories Phylum and Class. These are two out of the various taxonomical categories used by taxonomists. The highest taxon is Kingdom. There are five kingdoms in the general system of classification. Each kingdom is divided into various phyla (term used for animals) or divisions (term used for plants). For example, Kingdom Animalia is divided into phyla like Porifera, Cnidaria Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata. Each phylum or division is divided into various classes. E.g: Phylum Chordata has various classes like Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia.

Taxonomic Categories

There are about 1.7-1.8 million species identified and studied by biologists now and many more are yet to be discovered. The biodiversity on earth is truly immense. This necessitates us to develop systems to make their study easier. There are 7 obligate and many smaller taxons into which organisms are classified. Organisms in the lower taxon share more similarities amongst themselves. As we move higher up, the number of common characteristics decreases. These are arranged in descending order as follows:

Taxonomical Hierarchy

 

  • Species: It is a group of organisms sharing fundamental similarities and can freely interbreed amongst themselves. They are reproductively isolated from any other species. Eg: Cat is a species.
  • Genus: It comprises a group of related species. For eg Lion, Tiger, and Leopard all belong to the same genus Panthera.
  • Family: Related genera on the basis of vegetative and reproductive features are grouped together in a family. They have fewer similarities as compared to genus and species. The plant family Solanaceae includes genera like Solanum, Petunia, and Datura.
  • Order: Families having similarities in floral and other characteristics are included in the same order. Order Carnivora includes families like Felidae (cat family) and Canidae (dog family).
  • Class: This taxonomic category includes related orders. Class Mammalia includes orders such as Primata and Carnivora.
  • Phylum/Division: The term phylum is used in the case of animals. For plants, we use the term division.
  • Kingdom: It is the highest category in the system of classification. It has very few similarities compared to all other categories. 
  Name Species  Genus   Family Order       Class Phylum Kingdom
1) Wheat Triticum Aestivum Poaceae Poales Monocotyledonae Angiospermae Plantae
2) Human Homo Sapiens Hominidae Primate Mammalia Chordata Animalia

Phylum/Division

It is a taxonomic category located below kingdom and above class in the system of classification. For plants, algae, and fungi the term division is preferred over phylum. For animals, the term phylum is used. The word phylum was coined by Ernst Haeckel in 1866. It is derived from the Greek word phylon which is related to the word phyle meaning tribe/clan. Phylum is a universal taxonomic category i.e. it is applicable to all organisms animals, plants, fungi, etc. Kingdom Animalia contains about 31 phyla, kingdom Plantae contains about 14 phyla and kingdom Fungi contains about 8 phyla. There are five major kingdoms Monera, Protista, Fungi, Plantae, and Animalia. Each kingdom is divided into various phyla/divisions like:

Kingdom Monera:  There are approximately 29 phyla in Eubacteria and 5 phyla in Archaea. Some well-known phyla of the 29 phyla in Eubacteria are:

  • Chlamydiae:  The members of this bacterial phylum are very diverse. They include pathogens of humans and animals, some have a symbiotic relation with protists, and some are marine forms. They lack peptidoglycan and some other important proteins. 
  • Cyanobacteria: They are also known as blue-green algae. Earlier they were placed under plants but now have been placed under kingdom Monera as they share more similarities with bacteria.
  • Actinobacteria: These are gram-positive bacteria. They may be terrestrial or marine. They are of great economic importance to humans because agriculture and forests depend on their contributions to soil systems. They help decompose the organic matter of dead organisms.
  • Firmicutes: These include gram-positive bacteria which have a tough cell wall. Some of them are pathogens while some produce energy through anoxygenic respiration. This phylum is also known as Bacillota.
  • Spirochetes: It is a group of spiral-shaped bacteria, some of which are serious pathogens for humans causing diseases such as syphilis, yaws, Lyme disease, and relapsing fever. They are chemoheterotrophs. They are characterized by a special type of flagella known as endoflagella.

Kingdom Protista: There are nearly over 15 phyla in this kingdom, some of which are:

  • Amoebozoa: It is a major taxonomic group containing about 2,400 described species of amoeboid protists. Amoebozoa includes many of the best-known amoeboid organisms such as Chaos, Entamoeba, Pelomyxa, and the genus Amoeba itself. They may be naked or shelled, and they may or may not have flagella. They exhibit pseudopodal movement.
  • Ciliophora: It includes the ciliates which possess cilia i.e. they are small hair-like structures and many in number. Cilia are variously used in swimming, crawling, attachment, feeding, and sensation.
  • Euglenozoa: They include a variety of common free-living species and a few important parasites, some of which infect humans. Most euglenozoa have two flagella. The group is characterized by the ultrastructure of the flagella. In addition to the normal microtubules or axonemes, each contains a rod (called paraxonemal), which has a tubular structure in one flagellum and a latticed structure in the other. 
  • Oomycota: They are filamentous and heterotrophic and can reproduce both sexually and asexually. Oomycetes were originally grouped with fungi due to similarities in morphology and lifestyle. However, molecular and phylogenetic studies revealed significant differences between fungi and oomycetes and they now come under the kingdom Protista.

Kingdom Fungi: There are 8 different phyla in this kingdom namely:

  • Ascomycota: Its members are commonly called sac fungi. It is the largest phylum of the kingdom Fungi. The defining feature of this fungal group is the ascus, a microscopic sexual structure in which nonmotile spores, called ascospores, are formed. Some eg include morels, truffles, bakers yeast, etc. They are major components of Lichens.
  • Basidiomycota: It is a large and diverse phylum of fungi that includes jelly and shelf fungi, mushrooms, puffballs; certain yeasts, and the rusts and smuts.  Most species reproduce sexually with the help of a club-shaped spore-bearing organ (basidium) that usually produces sexual spores (basidiospores). Basidia are borne on fruiting bodies (basidiocarps), 
  • Blastocladiomycota: It is parasitic on plants and animals, some are saprotrophic. Some are aquatic and terrestrial. They possess flagella. They alternate between haploid and diploid generations (zygotic meiosis)
  • Chytridiomycota: They consist of zoosporic organisms possessing a structure containing unreleased zoospores. They have a chitinous cell wall, a posterior whiplash flagellum, absorptive nutrition and use glycogen as an energy storage compound,  
  • Glomeromycota: It is a monophyletic group of soil-borne fungi which are among the most important microorganisms on Earth because they form intimate mycorrhizal associations with most land plants and also because they are believed to play a crucial role in the initial colonization of the terrestrial realm by plants. 
  • Microsporidia: They are a group of spore-forming unicellular parasites. They were once considered protozoans or protists, but are now known to be fungi. They are capable of infecting many species of animals. In humans, they can infect many organs including the eye.
  • Neocallimastigomycota: It is a phylum containing anaerobic fungi, which are symbionts found in the digestive tracts of larger herbivores.
  • Zygomycota: They are mostly terrestrial in habitat, live in soil or on decaying plant or animal material. Some are parasites of plants, insects, and small animals while others form symbiotic relationships with plants. Their hyphae are maybe coenocytic.

Kingdom Plantae: They have five phyla/divisions:

  • Thallophyta: It is a division of the plant kingdom including unicellular to large algae, fungi, and lichens showing a simple plant body without roots, stems, or leaves. They are non-embryophytes. Eg: Volvox, Fucus.
  • Bryophyta: They include mosses, liverworts, and hornworts. They lack true roots, stems, and leaves. They are non-vascular plants. They require water for reproduction and are hence called amphibians of the plant kingdom. Eg: Marchantia, Funaria.
  • Pteridophyta: They possess true roots, stems, and leaves. They are the first terrestrial plants. They are also the first vascular plants. They are seedless but reproduce through spores. Eg: Selaginella, Equisetum.
  • Gymnospermae: These are vascular plants that reproduce by means of exposed spores. The seeds of many gymnosperms are borne in cones and are not visible until maturity. Eg: Pinus, Cycas.
  • Angiospermae: These are called flowering plants. Their seeds are protected by fruits. Eg: Wolffia, Eucalyptus.

Kingdom Animalia: This is an extensive kingdom with over 30 animal phyla. Some of the phyla are:

  • Porifera: They are commonly known as sponges. They are generally asymmetrical and have a cellular level of organization. They have many minute holes called Ostia, one single large hole called osculum, and a central cavity called spongocoel. These structures help in osmoregulation. They are hermaphrodites, have intracellular digestion, and their skeleton is made of spongin and spicules. Eg: Sycon, Spongilla.
  • Cnidaria: They are aquatic, free-swimming, or sessile, radially symmetrical. They have cnidoblasts for anchorage and defence. They are diploblastic and exhibit a tissue level of organization. They may have either intracellular or extracellular digestion. Their central cavity is called the gastrovascular cavity. Cnidarians possess two body forms polyp(sessile) and medusa(free-swimming). Eg: Adamsia, Pennatula.
  • Platyhelminthes: They are dorsoventrally flattened, are generally endoparasites, and have hooks and suckers. They have flame cells for osmoregulation and excretion. They are hermaphrodites with internal fertilization and indirect development. Eg: Taenia, Fasciola.
  • Aschelminthes: They are commonly called roundworms. They exhibit organ system levels of the organization and are bilaterally symmetrical, triploblastic, and pseudocoelomate. They are dioecious, often females are longer than males, however, the latter possesses a curved tail. Eg: Ascaris, Ancyclostoma. 
  • Annelida: They possess organ system level of organization, are bilaterally symmetrical, triploblastic, metamerically segmented, and coelomate. They have nephridia for osmoregulation and excretion. Some are dioecious and others are monoecious. Eg: Nereis, Hirudinaria.
  • Mollusca: This is the 2nd largest phylum in the kingdom Animalia. They exhibit organ system level of organization, are bilaterally symmetrical, triploblastic, and coelomate. They have a calcareous shell, their body is divided into different heads, feet, and humps. Eg: Pila, Octopus.
  • Arthropoda: It is the largest phylum of the kingdom Animalia. These organisms exhibit organ system levels of the organization, and are bilaterally symmetrical, triploblastic, segmented, and coelomate. They have a chitinous exoskeleton. Their body is divided into head, thorax and abdomen. They have jointed appendages, their circulatory system is open. They have Malpighian tubules for excretion. They are mostly dioecious, have internal fertilization, direct or indirect development, and are generally oviparous. Eg: Limulus, Apis.
  • Echinodermata: They possess a calcareous endoskeleton, are exclusively marine, and exhibit organ system level of organization. Adults are radially symmetrical whereas larvae are bilaterally symmetrical. They are triploblastic and coelomate. Their digestive system is complete, they have a water vascular system for locomotion, respiration, capture, and transport of food. They are dioecious, have external fertilization, and have indirect development. Eg: Echinus, Asterias.
  • Chordata: They have a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. They are bilaterally symmetrical, triploblastic, coelomate and exhibit organ system level of organization. They have a post-anal tail and a closed circulatory system. Eg: Homo sapiens, Salpa.

Class

It is a taxonomic category located between phylum and order in the system of classification. This term was first introduced by the French botanist Joseph Pitton de Tournefort in his classification of plants in 1694. This taxonomic category includes related orders. Class Mammalia includes orders such as Primata and Carnivora. Each phylum is divided into various classes like:

Phylum Chordata: It has many classes, some of which are:

  • Cyclostomata: They are ectoparasites on fishes. They possess 6-15 gill slits for respiration, they have a sucking circular mouth without jaws for ingestion. They are devoid of scales and paired fins. They have a cartilaginous cranium and vertebral column and a closed circulatory system. They are exclusively marine. Eg: Petromyzon, Myxine etc.
  • Chondrichthyes: They are marine and have a streamlined body with a cartilaginous endoskeleton. They possess a ventral mouth, their gill slits are separate and without operculum. They have placoid scales. They lack air bladders and have to swim constantly. Their heart is two-chambered and they are poikilothermous. Sexes are separate, fertilization is internal and they are mostly viviparous. Eg: Scoliodon, Trygon.
  • Osteichthyes: They have a bony endoskeleton and a streamlined body. They possess a terminal mouth with four pairs of gills covered by an operculum. Their skin is covered with an operculum. They have air bladders for remaining afloat. Their heart is two-chambered and they are poikilothermous. Their sexes are separate, fertilization is usually external, development is direct, are mostly oviparous. Eg: Catla, Betta.
  • Amphibia: They are aquatic as well as terrestrial. They have two pairs of limbs and their body is divisible into head and trunk. They may have a tail. Tympanum represents ears. Respiration is through the gills, skin, and lungs. Their heart is 3 chambered and they are poikilothermous. Their sexes are separate, fertilization is external, development is indirect, are oviparous. Eg: Bufo, Rana.
  • Reptilia: They have a creeping/crawling motion. They are covered by dry and cornified skin. The tympanum represents ears and has two pairs of limbs. The heart is usually 3 chambered except for crocodiles, which are poikilothermous. Fertilization is internal, they are oviparous, and development is indirect. Eg: Chelone, Hemidactylus.
  • Aves: They possess feathers, and their forelimbs are modified into wings. Their endoskeleton is ossified, they have pneumatic bones. Their heart is 4 chambered, they are homothermous. Respiration is by the lungs. Their sexes are separate, fertilization is internal, and development is direct. Eg: Columba, Psittacula.
  • Mammalia: Their characteristic feature is the presence of mammary glands, hair on their body, and an external ear called the pinna. Their heart is 4-chambered. They are homothermous. They respire by lungs. Their sexes are separate, fertilization is internal, development is direct, are viviparous. Eg: Macropus, Rattus.

Phylum Arthropoda: It has various classes like:

  • Crustacea: They moult their exoskeleton, mostly free-living aquatic animals. They are distinguished from other groups of arthropods by the possession of biramous (two-parted) limbs. The scientific study of crustaceans is known as carcinology. Eg: Crabs, Shrimp etc.
  • Arachnida: They have 8 pairs of legs, mostly terrestrial some live in freshwater. Eg: Scorpions, Spiders etc.
  • Insecta: They are the largest group within the arthropod phylum. Insects have three pairs of jointed legs, two pairs of wings, compound eyes, and one pair of antennae. Eg: Butterfly, Beetle etc.
  • Myriapoda: They have a single pair of antennae, and they have simple eyes. Eg:  Millipedes, Centipedes.

Phylum Angiospermae: It is divided into two classes:

  • Monocotyledonae: They possess a single cotyledon in their seeds, parallel venation in their leaves, and trimerous flowers. Eg: Maize, Wheat etc.
  • Dicotyledonae: They have two cotyledons in their seeds, reticulate venation in their leaves, and tetramerous or pentamerous flowers. Eg: Beans, Pea.
                                 Phylum                                               Class
It is a taxonomic rank lower than kingdom but higher than class. It is a taxonomic rank lower than phylum but higher than order.
It has lesser number of similar characters compared to class.

Members belonging to a class have more similarities than organisms 

grouped in one phylum.

It has more general characters compared to class. It has fewer general characters compared to phylum.
It is a group of related classes. It is a group of related orders.

FAQs on Class and Phylum

Question 1: State the types of taxonomy.

Answer:

  • Alpha taxonomy:  It includes only morphology.    
  • Beta taxonomy: It considers morphology, genetics, anatomy, physiology.
  • Omega taxonomy: It is based on phylogenetic relationships.

Question 2: List a few codes of nomenclature.

Answer: 

  • ICBN: International Code of Botanical Nomenclature.  
  • ICZN: International Code of Zoological Nomenclature.
  • ICVN: International Code of Virus classification and Nomenclature.

Question 3: Who is the author of the book Systema Naturae?

Answer:

 Carolus Linnaeus is the author of the book Systema.

Question 4: For which taxonomic category is the suffix opsida used?

Answer:

Suffix opsida is used in class category of taxonomy.

Question 5: What is the difference between class and phylum?

Answer: 

Class is the group of related orders while phylum is the group of related classes.


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