Classification of Non-Chordates

The classification of non-chordates covers the ten animal phyla that never form a notochord, from Porifera to Hemichordata. Each phylum is identified by its level of organisation, symmetry, coelom and a few distinctive features. Examples are the canal system of sponges, the cnidoblasts of coelenterates and the water vascular system of echinoderms. These notes on the classification of non-chordates list every feature and example in exam order. NEET often tests them through match-the-list and statement questions.

1. Phylum Porifera

• Members are commonly known as sponges.
• They are generally marine and mostly asymmetrical animals.
• They are primitive multicellular animals with a cellular level of organisation.
• ★ Exam imp Sponges have a water transport or canal system. The path of water is:

1. Water enters through minute pores, the ostia, in the body wall.
2. It reaches a central cavity, the spongocoel.
3. It goes out through the osculum.

• This pathway helps in food gathering, respiratory exchange and removal of waste.

• Digestion is intracellular.
• The body is supported by a skeleton of spicules or spongin fibres.
• Sexes are not separate (hermaphrodite): eggs and sperms are produced by the same individual.
• Asexual reproduction is by fragmentation; sexual reproduction is by formation of gametes.
• ★ Exam imp Fertilisation is internal, and development is indirect, with a larval stage morphologically distinct from the adult.
• Examples: Sycon (Scypha), Spongilla (fresh water sponge), Euspongia (bath sponge).

2. Phylum Coelenterata (Cnidaria)

• They are aquatic, mostly marine, sessile or free-swimming and radially symmetrical.
• ★ Exam imp The name cnidaria comes from the cnidoblasts or cnidocytes, present on the tentacles and the body.
• Cnidoblasts contain the stinging capsules, the nematocysts.
• Cnidoblasts are used for anchorage, defence and the capture of prey.

• Cnidarians show the tissue level of organisation and are diploblastic.
• They have a central gastro-vascular cavity with a single opening, the mouth, on the hypostome.
• Digestion is both extracellular and intracellular.
• Some cnidarians, such as corals, have a skeleton of calcium carbonate ($\mathrm{CaC}{\mathrm{O}}_3$).
• Cnidarians show two basic body forms, the polyp and the medusa:

• Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan) and Meandrina (brain coral).

3. Phylum Ctenophora

• Ctenophores are commonly known as sea walnuts or comb jellies.
• They are exclusively marine, radially symmetrical and diploblastic, with the tissue level of organisation.
• ★ Exam imp The body bears eight external rows of ciliated comb plates, which help in locomotion.
• Digestion is both extracellular and intracellular.
• Bioluminescence, the property of a living organism to emit light, is well marked in ctenophores.
• Sexes are not separate. Reproduction takes place only by sexual means.
• Fertilisation is external, with indirect development.
• Examples: Pleurobrachia and Ctenoplana.

4. Phylum Platyhelminthes

• ★ Exam imp They have a dorso-ventrally flattened body, hence the name flatworms.
• Most are endoparasites found in animals, including human beings.
• Flatworms are bilaterally symmetrical, triploblastic and acoelomate, with the organ level of organisation.
• Hooks and suckers are present in the parasitic forms.
• Some absorb nutrients from the host directly through their body surface.

• Sexes are not separate. Fertilisation is internal, and development is through many larval stages.
• Some members, like Planaria, possess a high regeneration capacity.
• Examples: Taenia (tapeworm), Fasciola (liver fluke).

5. Phylum Aschelminthes

• The body is circular in cross-section, hence the name roundworms.
• They may be free-living, aquatic and terrestrial, or parasitic in plants and animals.
• Roundworms have the organ-system level of body organisation.
• ★ Exam imp They are bilaterally symmetrical, triploblastic and pseudocoelomate.
• The alimentary canal is complete, with a well-developed muscular pharynx.
• An excretory tube removes body wastes from the body cavity through the excretory pore.
• Sexes are separate (dioecious): males and females are distinct. Often females are longer than males.
• Fertilisation is internal. Development may be direct (the young ones resemble the adult) or indirect.
• Examples: Ascaris (roundworm), Wuchereria (filaria worm), Ancylostoma (hookworm).

6. Phylum Annelida

• They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic.
• They show the organ-system level of organisation and bilateral symmetry.
• ★ Exam imp They are triploblastic, metamerically segmented and coelomate.
• The body surface is marked out into segments or metameres, hence the name Annelida (Latin annulus: little ring).
• Longitudinal and circular muscles help in locomotion.
• Aquatic annelids like Nereis have lateral appendages, the parapodia, which help in swimming.
• A closed circulatory system is present.
• Nephridia (sing. nephridium) help in osmoregulation and excretion.
• The neural system has paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.
• ★ Exam imp Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious.
• Reproduction is sexual.
• Examples: Nereis, Pheretima (earthworm), Hirudinaria (blood-sucking leech).

7. Phylum Arthropoda

• ★ Exam imp This is the largest phylum of Animalia and includes the insects.
• Over two-thirds of all named species on earth are arthropods, so they are the most abundant group of animals.
• They have the organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic, segmented and coelomate.
• The body is covered by a chitinous exoskeleton.
• The body consists of the head, thorax and abdomen.
• They have jointed appendages (arthros: joint, poda: appendages).
• Respiratory organs are gills, book gills, book lungs or the tracheal system.
• The circulatory system is of the open type.
• Sensory organs include antennae, eyes (compound and simple) and statocysts or balancing organs.
• ★ Exam imp Excretion takes place through malpighian tubules.
• They are mostly dioecious. Fertilisation is usually internal. They are mostly oviparous.
• Development may be direct or indirect.

Examples to Remember

8. Phylum Mollusca

• ★ Exam imp This is the second largest animal phylum.
• Molluscs are terrestrial or aquatic (marine or fresh water), with the organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic and coelomate.
• The soft body is covered by a calcareous shell. It is unsegmented, with a distinct head, a muscular foot and a visceral hump.
• Mantle: a soft and spongy layer of skin that covers the visceral hump.
• Mantle cavity: the space between the hump and the mantle. It contains feather-like gills.
• The gills have respiratory and excretory functions.
• The anterior head region has sensory tentacles.

• Molluscs are usually dioecious and oviparous, with indirect development.

Examples to Remember

9. Phylum Echinodermata

• These animals have an endoskeleton of calcareous ossicles, hence the name Echinodermata (spiny bodied). Echinoderms possess a spiny skin.
• All are marine, with the organ-system level of organisation.
• ★ Exam imp The adults are radially symmetrical, but the larvae are bilaterally symmetrical.
• They are triploblastic and coelomate.
• The digestive system is complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side.

• ★ Exam imp An excretory system is absent.
• Sexes are separate. Reproduction is sexual. Fertilisation is usually external.
• Development is indirect, with a free-swimming larva.

Examples to Remember

10. Phylum Hemichordata

• Hemichordata was earlier considered a sub-phylum under phylum Chordata.
• ★ Exam imp It is now placed as a separate phylum under non-chordata.
• Hemichordates have a rudimentary structure in the collar region, the stomochord, which is similar to the notochord.
• The phylum is a small group of worm-like marine animals with the organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic and coelomate.
• The body is cylindrical, with an anterior proboscis, a collar and a long trunk.

• The circulatory system is of the open type.
• Respiration takes place through gills.
• The excretory organ is the proboscis gland.
• Sexes are separate. Fertilisation is external. Development is indirect.
• Examples: Balanoglossus and Saccoglossus.

11. Non-chordate Phyla Compared

• The two tables bring together the salient features of the ten non-chordate phyla.

(a) Body plan

(b) Organ systems and distinctive features

12. Exam Essentials: Pairs, Exceptions and Numbers

Numbers to Remember

• 8 external rows of comb plates in ctenophores.
• Over two-thirds of all named species are arthropods (largest phylum).
• Second largest phylum: Mollusca.
• 2 body forms of cnidarians: polyp and medusa.
• 1 opening (mouth on the hypostome) to the gastro-vascular cavity of cnidarians.
• 3 body regions of arthropods: head, thorax, abdomen.
• 3 body regions of hemichordates: proboscis, collar, trunk.

Examples to Remember

Pairs to Match

13. Quick Revision

• Porifera: generally marine, mostly asymmetrical, cellular level; ostia, spongocoel, osculum; choanocytes; spicules or spongin.
• Sponges: intracellular digestion, hermaphrodite, fragmentation, internal fertilisation, larval stage.
• Coelenterata: cnidoblasts with nematocysts; tissue level, diploblastic; mouth on hypostome.
• Polyp (sessile, cylindrical) and medusa (umbrella-shaped, free-swimming); metagenesis in Obelia.
• Ctenophora: comb jellies, 8 comb-plate rows, bioluminescence, only sexual reproduction, external fertilisation.
• Platyhelminthes: flattened, acoelomate, organ level, hooks and suckers, flame cells, Planaria regenerates.
• Aschelminthes: pseudocoelomate, muscular pharynx, excretory pore, dioecious, females longer.
• Annelida: metameres, coelom, parapodia, closed circulation, nephridia, double ventral nerve cord.
• Arthropoda: largest phylum, chitin, head-thorax-abdomen, jointed appendages, open circulation, malpighian tubules.
• Mollusca: second largest, shell, head, foot, visceral hump, mantle cavity with gills, radula.
• Echinodermata: ossicles, water vascular system, no excretory system, radial adults, bilateral larvae.
• Hemichordata: stomochord, proboscis-collar-trunk, open circulation, gills, proboscis gland.
• Segmentation: only annelids and arthropods among non-chordates.

14. Solved Examples

15. Practice Questions

Common Mistakes to Avoid