General Characteristics and Basis of Classification of Non-Chordates
Prepared by Dr Bhabesh Nath
Assistant Professor
Department of Zoology
B N College, Autonomous, Dhubri
1. Introduction
The animal kingdom (Kingdom Animalia) is broadly divided into two major groups on the basis of the presence or absence of a notochord: the Chordata and the Non-Chordata (Invertebrata). Non-chordates comprise all animals that never possess a notochord at any stage of their life history. They represent the overwhelming majority of animal diversity — more than 95% of all described animal species — and range in complexity from unicellular protozoans to highly organised, organ-system-grade animals such as insects and cephalopods.
Non-chordates are classified into a large number of phyla (traditionally about 20–33 depending on the scheme followed), each defined by a characteristic body plan (Bauplan), grade of organisation, and pattern of embryonic development. This note summarises the general features that distinguish non-chordates from chordates, the criteria used by taxonomists to classify them, and a concise outline of the major non-chordate phyla, followed by a question bank for examination preparation.
2. General Characteristics of Non-Chordates
Although the non-chordate phyla are structurally very diverse, they share the following negative and positive features that collectively distinguish them from the Chordata:
2.1 Absence of Notochord
Non-chordates never develop a notochord — the flexible, rod-like, mesodermally derived axial support structure that defines the Chordata — at any stage of embryonic or adult life. Skeletal support, where present, is achieved instead by an exoskeleton (Arthropoda), a shell (Mollusca), a hydrostatic skeleton (Annelida, Cnidaria), or a mesodermal endoskeleton of calcareous ossicles unique to Echinodermata.
2.2 Nature of the Nerve Cord
The central nervous system of non-chordates, when present, is typically solid and ventral in position (e.g., the ventral nerve cord with segmental ganglia in Annelida and Arthropoda), in contrast to the single, dorsal, hollow neural tube characteristic of chordates. In radially symmetrical forms such as Cnidaria, the nervous system is a diffuse nerve net rather than a defined cord.
2.3 Position of the Heart
Where a heart or contractile pumping structure is present, it is typically dorsal in position relative to the gut (as in insects and other arthropods), whereas in chordates the heart is ventral to the alimentary canal.
2.4 Absence of Pharyngeal Gill Slits
Non-chordates never possess pharyngeal gill slits perforating the wall of the pharynx, a feature otherwise diagnostic of chordates (even though it may be lost in adult amniotes).
2.5 Absence of a Post-anal Tail
A muscular tail extending posterior to the anus, found in chordates for locomotion, is absent in non-chordates; any tail-like extension in invertebrates (e.g., the pygidium of annelids) is not a post-anal, muscular locomotory tail in the chordate sense.
2.6 Grade of Body Organisation
Non-chordates display every grade of organisation found in the animal kingdom — protoplasmic (Protozoa), cellular (Porifera), tissue (Cnidaria, Ctenophora), organ (Platyhelminthes), and organ-system grade (Annelida, Arthropoda, Mollusca, Echinodermata) — whereas all chordates are uniformly of organ-system grade.
2.7 Habitat and Diversity
Non-chordates occupy virtually every habitat on Earth — marine, freshwater, terrestrial, and parasitic/symbiotic modes of life — and show the greatest range of body plans, modes of nutrition (autotrophic in a few protists, holozoic, saprophytic, and parasitic), and reproductive strategies (asexual and sexual) of any group of animals.
2.8 Development
Development in most non-chordate phyla (Annelida, Mollusca, Arthropoda) follows the protostome pattern — spiral and determinate cleavage, and the blastopore contributing to the mouth — whereas Echinodermata (a non-chordate phylum) and Hemichordata share the deuterostome pattern of development with the Chordata, reflecting their closer evolutionary affinity to chordates.
3. Basis of Classification of Non-Chordates
Non-chordates are classified into phyla and lower taxonomic categories using a combination of morphological, developmental, and organisational criteria. The principal bases used are described below.
3.1 Grade of Organisation
This refers to the level of structural and functional complexity of the body:
• Protoplasmic grade: The entire body consists of a single cell performing all vital functions (Protozoa, e.g., Amoeba, Paramecium).
• Cellular grade: The body is composed of many cells with some division of labour, but cells are not organised into true tissues (Porifera).
• Cellular-tissue (tissue) grade: Cells are organised into definite tissues that perform specific functions (Cnidaria, Ctenophora).
• Organ grade: Tissues are further organised into organs (Platyhelminthes).
• Organ-system grade: Organs work together as coordinated organ systems (Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and all Chordata).
3.2 Symmetry
Symmetry describes the arrangement of body parts about an axis:
• Asymmetrical: No plane of symmetry divides the body into equal halves (e.g., Amoeba, most sponges).
• Spherical symmetry: Body parts are arranged concentrically around a central point; any plane through the centre divides it equally (some Radiolaria/Heliozoa among Protozoa).
• Radial symmetry: Body parts are arranged concentrically around a central oral-aboral axis; any vertical plane through the axis divides the body into similar halves (Cnidaria, adult Echinodermata).
• Biradial symmetry: A modification of radial symmetry with only one or two planes producing mirror-image halves, owing to a single structure such as a pharynx or tentacle pair (Ctenophora).
• Bilateral symmetry: Only one plane (the mid-sagittal plane) divides the body into right and left mirror-image halves; associated with cephalisation and directional locomotion (Platyhelminthes and all higher phyla, including Chordata).
3.3 Germ Layers (Diploblastic vs. Triploblastic Organisation)
• Diploblastic animals: The body wall develops from two primary germ layers — an outer ectoderm and an inner endoderm — separated by a non-cellular or sparsely cellular mesogloea (Porifera, Cnidaria, Ctenophora).
• Triploblastic animals: A true third germ layer, the mesoderm, develops between the ectoderm and endoderm, giving rise to muscles, connective tissue, and (in most phyla) the coelomic lining (Platyhelminthes onward, including all Chordata).
3.4 Body Cavity (Coelom)
The nature of the fluid-filled body cavity between the body wall and the gut is a key criterion distinguishing the triploblastic phyla:
• Acoelomate: No body cavity is present between the gut and body wall; the space is filled with mesodermal parenchyma (Platyhelminthes).
• Pseudocoelomate (Blastocoelomate): A body cavity is present but is derived from the persistent embryonic blastocoel and is not lined by mesodermal peritoneum (Aschelminthes/Nematoda, Rotifera).
• Eucoelomate (true coelomate): A body cavity arises within the mesoderm and is completely lined by a mesodermal epithelium (peritoneum); the gut is thus suspended by mesenteries (Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata).
3.5 Segmentation (Metamerism)
Metameric segmentation is the repetition of similar body segments (metameres) along the antero-posterior axis, each segment typically containing a serial repetition of organs. It is a major classificatory criterion distinguishing:
• Unsegmented (non-metameric) phyla: e.g., Platyhelminthes, Aschelminthes, Mollusca (largely unsegmented, though some show pseudometamerism).
• Metamerically segmented phyla: Annelida (true metamerism, both externally and internally) and Arthropoda (metamerism often modified by tagmosis — fusion of segments into functional body regions such as head, thorax, and abdomen).
3.6 Nature of the Alimentary Canal
• Incomplete digestive system: A single opening (mouth) serves for both ingestion and egestion; the gut is sac-like (Cnidaria, Platyhelminthes).
• Complete digestive system: A tubular gut with two separate openings, a mouth and an anus, permitting unidirectional flow of food and greater regional specialisation of the gut (Aschelminthes onward).
3.7 Type of Circulatory System
• Open (lacunar) circulatory system: Blood (haemolymph) flows partly through vessels and partly into open spaces called sinuses or haemocoel, as in most Arthropoda and Mollusca (except cephalopods).
• Closed circulatory system: Blood flows entirely within a continuous system of vessels, as in Annelida and cephalopod Mollusca.
3.8 Pattern of Embryonic Development
On the basis of the fate of the blastopore and the pattern of cleavage, the eucoelomate (bilaterian) phyla are grouped into two major evolutionary lineages:
• Protostomia: Spiral and (typically) determinate cleavage; the blastopore develops into or near the mouth; the coelom generally arises by schizocoely (splitting of mesodermal bands). Includes Annelida, Mollusca, and Arthropoda.
• Deuterostomia: Radial and (typically) indeterminate cleavage; the blastopore develops into or near the anus, with the mouth forming secondarily; the coelom generally arises by enterocoely (outpocketing of the archenteron). Includes Echinodermata, Hemichordata, and Chordata.
3.9 Nature of the Skeleton
Skeletal type also aids classification: hydrostatic skeletons (Cnidaria, Annelida), chitinous exoskeletons with jointed appendages (Arthropoda), calcareous shells (Mollusca, many Protozoa), and the unique mesodermal endoskeleton of calcareous ossicles bound by connective tissue (Echinodermata).
3.10 Locomotory and Other Organs
Presence and type of specialised locomotory or feeding structures — pseudopodia and flagella (Protozoa), parapodia and setae (Polychaete annelids), jointed appendages (Arthropoda), a muscular foot (Mollusca), and the unique water vascular system with tube feet (Echinodermata) — further help delimit phyla and classes.
4. Outline Classification of Major Non-Chordate Phyla
A concise, traditional outline of the principal non-chordate phyla, with their salient diagnostic features and representative examples, is given below.
4.1 Phylum Protozoa
Microscopic, mostly unicellular (acellular) organisms of protoplasmic grade; body may be naked or covered by a pellicle, shell, or test; locomotion by pseudopodia, flagella, or cilia; nutrition holozoic, saprozoic, or parasitic; reproduction asexual (binary/multiple fission) and sexual (conjugation, syngamy). Examples: Amoeba, Euglena, Paramecium, Plasmodium.
4.2 Phylum Porifera (Sponges)
Multicellular, cellular-grade, mostly marine, sessile animals with a perforated body wall (ostia and osculum) and a unique system of water canals driven by flagellated choanocytes; body supported by a skeleton of calcareous or siliceous spicules and/or spongin fibres; diploblastic with an intervening mesenchyme (mesohyl); digestion entirely intracellular; asexual (budding, gemmules) and sexual reproduction; possess remarkable powers of regeneration. Examples: Sycon, Spongilla, Euspongia.
4.3 Phylum Cnidaria (Coelenterata)
Diploblastic, radially (or biradially) symmetrical, mostly aquatic animals with a sac-like body enclosing a single gastrovascular cavity (coelenteron) with one opening; possess characteristic stinging cells called cnidoblasts/cnidocytes bearing nematocysts, used in defence and prey capture; exhibit polymorphism, alternating polyp (asexual) and medusa (sexual) forms in many groups; nervous system a diffuse nerve net; tissue-grade organisation. Examples: Hydra, Obelia, Aurelia, Physalia.
4.4 Phylum Ctenophora (Sea Walnuts/Comb Jellies)
Diploblastic, biradially symmetrical, exclusively marine, mostly free-swimming animals bearing eight rows of ciliary comb plates used for locomotion; possess adhesive cells called colloblasts (not nematocysts) for capturing prey; tentacles present in most; bioluminescent in many forms. Examples: Pleurobrachia, Ctenoplana.
4.5 Phylum Platyhelminthes (Flatworms)
Triploblastic, acoelomate, bilaterally symmetrical, dorsoventrally flattened worms; organ-grade organisation; digestive system incomplete or absent (in tapeworms); excretion by flame cells (protonephridia); hermaphroditic; many members are parasitic with complex life cycles involving one or more intermediate hosts. Examples: Planaria (free-living), Fasciola hepatica (liver fluke), Taenia solium (tapeworm).
4.6 Phylum Aschelminthes (Nemathelminthes/Nematoda)
Triploblastic, pseudocoelomate, bilaterally symmetrical, usually elongated, cylindrical, unsegmented worms with a complete digestive tract; body covered by a tough, resistant, non-cellular cuticle; sexes usually separate (dioecious) with marked sexual dimorphism; free-living as well as important parasites of plants and animals. Examples: Ascaris lumbricoides, Wuchereria bancrofti, Rotifers (Rotifera, now often treated as a separate phylum).
4.7 Phylum Annelida (Segmented Worms)
Triploblastic, eucoelomate, bilaterally symmetrical, and the first phylum in this outline to show true metameric segmentation, both externally (annulation) and internally (repeated coelomic compartments, nephridia, and ganglia); closed circulatory system; nervous system with a nerve ring and a solid ventral nerve cord with segmental ganglia; locomotion by chaetae (bristles) and/or parapodia. Examples: Nereis, Pheretima (earthworm), Hirudinaria (leech).
4.8 Phylum Arthropoda (Joint-legged Animals)
The largest and most diverse animal phylum; triploblastic, eucoelomate (coelom reduced, body cavity mainly a haemocoel), bilaterally symmetrical, segmented, with segments grouped into functional tagmata (head, thorax, abdomen); body covered by a chitinous exoskeleton that is periodically moulted (ecdysis); paired, jointed appendages; open circulatory system; respiration by gills, tracheae, or book lungs. Examples: Palaemon (prawn), Periplaneta (cockroach), Apis (honeybee), Limulus (king crab).
4.9 Phylum Mollusca (Soft-bodied Animals)
Triploblastic, eucoelomate (coelom reduced, largely replaced by haemocoel), bilaterally symmetrical (secondarily asymmetrical in gastropods due to torsion) unsegmented animals with a soft body typically differentiated into head, muscular foot, and visceral mass covered by a mantle that secretes a calcareous shell (reduced or absent in some); respiration by gills (ctenidia) in aquatic forms or a mantle cavity acting as a lung in terrestrial forms; feeding aided by a unique rasping organ, the radula (absent in bivalves). Examples: Pila (apple snail), Unio (freshwater mussel), Sepia (cuttlefish), Octopus.
4.10 Phylum Echinodermata (Spiny-skinned Animals)
Triploblastic, eucoelomate, exclusively marine animals; larvae are bilaterally symmetrical but adults typically show secondary pentaradial (five-part radial) symmetry; possess a unique mesodermal endoskeleton of calcareous ossicles often bearing spines; diagnostic water vascular system with external tube feet (podia) used for locomotion, feeding, and respiration; development is deuterostomous, aligning this phylum evolutionarily with the Chordata despite being classified as a non-chordate. Examples: Asterias (starfish), Echinus (sea urchin), Holothuria (sea cucumber), Antedon (sea lily).
4.11 Phylum Hemichordata
Triploblastic, eucoelomate, marine, worm-like animals with a body divided into proboscis, collar, and trunk; possess pharyngeal gill slits and a deuterostomous pattern of development, showing closer affinity to chordates; possess a short, dorsal, anterior, hollow outgrowth of the buccal cavity called the stomochord, formerly (and now no longer) regarded as homologous to a notochord — hence Hemichordata is now classified as a separate phylum closely allied to, but distinct from, the Chordata, and is often treated as the sister group of Echinodermata within the clade Ambulacraria. Example: Balanoglossus (acorn worm).
4.12 Minor Phyla (brief mention)
Several smaller non-chordate phyla are also recognised, including Nemertea (ribbon worms, with a unique protrusible proboscis), Rotifera (microscopic pseudocoelomate wheel animalcules), Bryozoa/Ectoprocta (colonial, sessile, coelomate lophophorates), and Brachiopoda (bivalved, lophophorate marine animals superficially resembling molluscs). These groups, though comparatively less speciose, are important in illustrating the diversity of body plans and evolutionary lineages within the non-chordates.
5. Summary Table: Comparative Features of Major Non-Chordate Phyla
Phylum | Symmetry | Germ Layers | Body Cavity | Segmentation | Example |
Porifera | Asymmetrical/radial | Diploblastic | Absent | Absent | Sycon |
Cnidaria | Radial | Diploblastic | Absent | Absent | Hydra |
Ctenophora | Biradial | Diploblastic | Absent | Absent | Pleurobrachia |
Platyhelminthes | Bilateral | Triploblastic | Acoelomate | Absent | Fasciola |
Aschelminthes | Bilateral | Triploblastic | Pseudocoelomate | Absent | Ascaris |
Annelida | Bilateral | Triploblastic | Eucoelomate | Present (true) | Pheretima |
Arthropoda | Bilateral | Triploblastic | Eucoelomate (reduced) | Present (tagmatised) | Periplaneta |
Mollusca | Bilateral (mostly) | Triploblastic | Eucoelomate (reduced) | Absent | Pila |
Echinodermata | Pentaradial (adult) | Triploblastic | Eucoelomate | Absent | Asterias |
Hemichordata | Bilateral | Triploblastic | Eucoelomate | Absent | Balanoglossus |
6. Question Bank
Section A — Very Short Answer Type (1–2 marks)
1. Define the term 'non-chordate'.
2. What is metamerism? Name one phylum showing true metamerism.
3. Define coelom. What is a pseudocoelomate animal?
4. What is meant by diploblastic organisation? Give one example.
5. Name the phylum that shows pentaradial symmetry in adults.
6. What are cnidoblasts?
7. Define protostome and deuterostome.
8. What is a radula? In which phylum is it found?
9. Name the excretory organs found in Platyhelminthes.
10. What is the significance of the stomochord in Hemichordata?
Section B — Short Answer Type (5 marks)
1. Distinguish between acoelomate, pseudocoelomate, and eucoelomate body plans with suitable examples.
2. Differentiate between radial and bilateral symmetry, giving examples of phyla showing each type.
3. Explain the difference between an open and a closed circulatory system, citing examples from non-chordate phyla.
4. Describe the levels (grades) of organisation seen among non-chordates.
5. What are the diagnostic features that distinguish Protostomia from Deuterostomia?
6. Write short notes on the water vascular system of Echinodermata.
7. Explain why Hemichordata, though sharing some chordate-like features, is classified as a non-chordate phylum.
Section C — Long Answer / Essay Type (10–15 marks)
1. Describe the general characteristics of non-chordates and explain how they differ fundamentally from the chordates.
2. Discuss in detail the various criteria used as the basis of classification of non-chordates, with suitable examples for each criterion.
3. Give an outline classification of the non-chordate phyla with their salient diagnostic features and one example of each.
4. Write an essay on the grades of organisation and types of symmetry observed in the animal kingdom, with special reference to non-chordates.
5. Trace the evolutionary significance of the coelom and describe the three grades of coelomic organisation with examples.
Section D — Multiple Choice Questions (MCQs)
A. Which of the following is NOT a feature of chordates? (a) Notochord (b) Dorsal tubular nerve cord (c) Ventral solid nerve cord (d) Post-anal tail
B. The body cavity that is not lined by mesoderm on all sides is called: (a) Coelom (b) Pseudocoelom (c) Haemocoel (d) Schizocoel
C. Biradial symmetry is characteristic of: (a) Porifera (b) Cnidaria (c) Ctenophora (d) Echinodermata
D. Which phylum shows true metameric segmentation? (a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Mollusca
E. The water vascular system is a diagnostic feature of: (a) Mollusca (b) Echinodermata (c) Arthropoda (d) Annelida
F. Spiral and determinate cleavage is typical of: (a) Deuterostomes (b) Protostomes (c) Diploblastic animals (d) Radiate animals
G. Choanocytes (collar cells) are characteristic of: (a) Porifera (b) Cnidaria (c) Ctenophora (d) Platyhelminthes
H. The stinging cells of Cnidaria are called: (a) Colloblasts (b) Cnidoblasts (c) Choanocytes (d) Flame cells
7. References
1. Barnes, R. D. (1980). Invertebrate Zoology (4th ed.). Saunders College Publishing.
2. Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Invertebrate Zoology: A Functional Evolutionary Approach (7th ed.). Cengage Learning.
3. Jordan, E. L., & Verma, P. S. (2018). Invertebrate Zoology. S. Chand Publishing.
4. Kotpal, R. L. (2017). Modern Text Book of Zoology: Invertebrates. Rastogi Publications.
5. Ekambaranatha Ayyar, M., & Ananthakrishnan, T. N. Manual of Zoology, Vol. I: Invertebrata. S. Viswanathan Printers & Publishers.
6. Hickman, C. P., Roberts, L. S., Keen, S. L., Larson, A., I'Anson, H., & Eisenhour, D. J. (2020). Integrated Principles of Zoology (18th ed.). McGraw-Hill Education.
7. Pechenik, J. A. (2015). Biology of the Invertebrates (7th ed.). McGraw-Hill Education.
8. Brusca, R. C., Moore, W., & Shuster, S. M. (2016). Invertebrates (3rd ed.). Sinauer Associates.
9. Storer, T. I., Usinger, R. L., Stebbins, R. C., & Nybakken, J. W. General Zoology (6th ed.). McGraw-Hill.
10. UGC Model Curriculum / NEP-2020 Zoology Syllabus, Gauhati University, Department of Zoology.
Vosit zoologys.co.in for more information
0 Comments