Nonchordates Up to Phyla Level

The term Nonchordata refers to all animals that lack a notochord at any stage of their life cycle. In contrast, chordates have a notochord either throughout their life or during embryonic development. Nonchordates are multicellular, eukaryotic, and heterotrophic organisms. They may live freely or as parasites and exhibit a wide range of body organisations—from simple, cell-level organisation in sponges to complex organ systems in arthropods and molluscs.

1.0Classification of Nonchordates

The Animal Kingdom (Kingdom Animalia) is divided into two major subkingdoms:

1. Nonchordata – Animals without a notochord.
2. Chordata – Animals with a notochord.

The subkingdom Nonchordata is further divided into various phyla based on body symmetry, segmentation, level of organisation, and other anatomical features.

2.01. Phylum Porifera (Sponges)

Porifera, commonly known as sponges, are the simplest multicellular animals.

Characteristics:

• The body has numerous pores: ostia for water entry and oscula for water exit.
• Sessile (attached to surfaces).
• Asymmetrical body.
• Cellular level of organisation.
• Skeleton made of spicules or spongin fibres.
• Reproduction: Both asexual (budding) and sexual.

Examples: Sycon, Spongilla, Euspongia (bath sponge).

3.02. Phylum Coelenterata (Cnidaria)

Also known as Cnidaria, these are aquatic animals that exhibit tissue-level organisation.

Characteristics:

• Radial symmetry.
• The body wall has two layers: ectoderm and endoderm (diploblastic).
• Presence of cnidoblasts or nematocysts (stinging cells) for defence and prey capture.
• Body forms: Polyp (sessile) and Medusa (free-swimming).
• Reproduce both sexually and asexually.

Examples: Hydra, Obelia, Aurelia (jellyfish), Coral.

4.03. Phylum Ctenophora (Comb Jellies)

These are marine, transparent, and gelatinous animals.

Characteristics:

• Biradial symmetry.
• Diploblastic organisation.
• Have eight rows of ciliary comb plates for locomotion.
• Exhibit bioluminescence (ability to emit light).
• Exclusively marine and free-swimming.

Examples: Pleurobrachia, Ctenoplana.

5.04. Phylum Platyhelminthes (Flatworms)

These are bilaterally symmetrical, triploblastic, and acoelomate animals.

Characteristics:

• Dorsoventrally flattened body.
• Organ-level of organisation.
• Many are parasitic, while some are free-living.
• The digestive system is incomplete (no anus).
• Reproduction is mainly sexual, often hermaphroditic.

Examples: Planaria (free-living), Taenia (tapeworm), Fasciola hepatica (liver fluke).

6.05. Phylum Aschelminthes (Nematoda / Roundworms)

Commonly known as roundworms, these are unsegmented, cylindrical, and pseudocoelomate animals.

Characteristics:

• Bilateral symmetry.
• Triploblastic body.
• Pseudocoelomate (false body cavity).
• Complete digestive system with a mouth and anus.
• Most are parasitic, but some are free-living.

Examples: Ascaris lumbricoides (roundworm), Ancylostoma (hookworm), Wuchereria bancrofti (causes filariasis).

7.06. Phylum Annelida (Segmented Worms)

Annelids are segmented, coelomate animals with a true body cavity and a high degree of organisation.

Characteristics:

• Bilateral symmetry and triploblastic.
• Body segmentation is visible externally and internally.
• Possess a closed circulatory system.
• Have setae or bristles for movement.
• Show metameric segmentation (repetition of body segments).

Examples: Earthworm (Pheretima), Leech (Hirudinaria), Nereis (marine worm).

8.07. Phylum Arthropoda

Arthropoda is the largest phylum in the animal kingdom, including insects, crustaceans, and arachnids.

Characteristics:

• The segmented body is divided into head, thorax, and abdomen.
• Exoskeleton made of chitin.
• Jointed appendages (hence the name “Arthropoda” = jointed legs).
• Open circulatory system.
• Well-developed nervous and sensory systems.

Examples: Cockroach, Butterfly, Crab, Spider, Prawn.

9.08. Phylum Mollusca

Molluscs are soft-bodied animals, often protected by a calcareous shell.

Characteristics:

• Bilateral symmetry.
• Triploblastic and coelomate.
• The body is divided into the head, visceral mass, and foot.
• Covered by a mantle, which secretes the shell.
• Possess a radula (a rasping organ) in many species.

Examples: Pila (apple snail), Octopus, Unio (mussel), Sepia (cuttlefish).

10.09. Phylum Echinodermata

These are marine animals with a spiny skin and a unique water vascular system.

Characteristics:

• Triploblastic and coelomate.
• Radial symmetry in adults, but bilateral symmetry in larvae.
• Possess a water vascular system for locomotion and food capture.
• Endoskeleton made of calcareous plates.
• No excretory organs.

Examples: Starfish (Asterias), Sea urchin (Echinus), Sea cucumber (Holothuria), Brittle star.

11.010. Phylum Hemichordata

Hemichordates are a small group of marine animals that form a link between nonchordates and chordates.

Characteristics:

• Bilateral symmetry, triploblastic, and coelomate.
• The body is divided into proboscis, collar, and trunk.
• Have pharyngeal gill slits but lack a true notochord.
• Mostly tubicolous (live in tubes) and marine.

Examples: Balanoglossus, Saccoglossus.

12.0Summary of Nonchordate Phyla