Animal Phyla: Cnidaria, Platyhelminthes, Nematoda, Annelida Characteristics

Posted on Feb 16, 2026 in Biology

Phylum Cnidaria (Coelenterata)

Phylum Coelenterata (also known as Cnidaria) consists of about 10,000 species of simple aquatic animals. The name “Coelenterata” refers to their single central body cavity (the coelenteron), while “Cnidaria” refers to their specialized stinging cells.

General characters

• Habitat: Primarily marine (e.g., corals, jellyfish), with a few freshwater forms like Hydra.
• Level of organization: Tissue grade of organization.
• Germ layers: Diploblastic — body wall made of two layers: an outer epidermis and an inner gastrodermis, separated by a jelly-like layer called mesoglea.
• Symmetry: Mostly radial symmetry; some (like sea anemones) show biradial symmetry.
• Body forms: Two primary morphological forms:
  • Polyp: Sessile, cylindrical, usually asexual (e.g., Hydra, Adamsia).
  • Medusa: Free-swimming, umbrella-shaped, usually sexual (e.g., Aurelia or jellyfish).
• Stinging cells: Characterized by cnidocytes (cnidoblasts) containing stinging capsules called nematocysts, used for defense and capturing prey.
• Digestion: Mouth is the only opening (blind-sac body plan). Digestion is both extracellular (in the coelenteron) and intracellular.
• Nervous system: Primitive, consisting of a diffuse nerve net.

Classification (up to class level)

The phylum is divided into four main classes based on the dominance of the polyp or medusa stage.

Key concept: Metagenesis

Certain coelenterates like Obelia exhibit metagenesis, where an asexual polypoid generation alternates with a sexual medusoid generation. The polyp produces medusae asexually (by budding), and the medusae produce polyps sexually (through zygote formation).

Type Study: Obelia (Sea Fur)

Obelia is a colonial, marine hydrozoan known for its complex life cycle that exhibits a clear alternation of generations.

Structure and colony

• The colony is small (about 2 cm) and consists of two types of filaments: horizontal hydrorhiza (for attachment) and vertical hydrocaulus.
• It is polymorphic, containing different types of individuals (zooids):
  • Gastrozooids (hydranths): Feeding polyps with tentacles and a mouth to capture prey.
  • Gonozooids (blastostyles): Reproductive polyps that lack a mouth and tentacles; they produce medusa buds asexually.

Life cycle and metagenesis

• The life history involves metagenesis, an alternation between an asexual polyp stage and a sexual medusa stage.
• Asexual phase: The sessile polyp colony produces free-swimming medusae by budding.
• Sexual phase: Medusae are dioecious (separate sexes) and release eggs and sperm into the water for external fertilization.
• Development: The zygote develops into a hollow blastula, then a solid gastrula, and finally a free-swimming ciliated planula larva. This larva eventually attaches to a substrate to form a new polyp colony.

Phylum Platyhelminthes (Flatworms)

General characters

• Body form: Dorsoventrally flattened, leaf-like or ribbon-like bodies.
• Level of organization: Organ-system level.
• Symmetry and germ layers: Bilaterally symmetrical and triploblastic (ectoderm, mesoderm, endoderm).
• Coelom: Acoelomate (no true body cavity); the space between organs is filled with parenchyma tissue.
• Excretion: Performed by specialized flame cells (protonephridia).
• Reproduction: Mostly hermaphroditic (monoecious) with internal fertilization.

Classification (up to class level)

• Class Turbellaria: Mostly free-living, aquatic flatworms with a ciliated epidermis (e.g., planaria / Dugesia).
• Class Trematoda (flukes): Ecto- or endoparasitic with suckers for attachment and a thick tegument (e.g., Fasciola hepatica).
• Class Cestoda (tapeworms): Exclusively endoparasitic, long ribbon-like body divided into proglottids; lack a digestive system (e.g., Taenia solium).

Type Study: Fasciola hepatica (Liver Fluke)

• Habitat: An endoparasite found in the liver and bile ducts of definitive hosts like sheep, cattle, and occasionally humans.
• Structure: Leaf-shaped fluke with an anterior oral sucker (surrounding the mouth) and a ventral sucker (acetabulum) for attachment.
• Life cycle: A complex digenetic cycle requiring two hosts:
  • Definitive host: Sheep/cattle (where adult flukes live and reproduce).
  • Intermediate host: Freshwater snail (usually Lymnaea / Galba truncatula).
• Developmental stages:
  • Eggs: Passed out via host feces into water.
  • Miracidium: Ciliated larva that hatches from the egg and penetrates a snail.
  • Inside snail: Transforms into sporocyst → redia → cercaria.
  • Metacercaria: Cercariae leave the snail, encyst on water plants, and become the infective stage for the definitive host.

Phylum Aschelminthes (Nemathelminthes / Roundworms)

General characters

• Body form: Cylindrical, unsegmented, tapering at both ends.
• Coelom: Pseudocoelomate (body cavity present but not lined by mesoderm).
• Body wall: Covered by a thick, tough cuticle and a syncytial epidermis.
• Digestive system: Complete alimentary canal with a distinct mouth, muscular pharynx, and anus.
• Reproduction: Mostly dioecious (separate sexes), with males often smaller than females.

Classification (up to class level)

• Class Nematoda: The largest group, including many parasites and free-living forms (e.g., Ascaris, Wuchereria).
• Class Nematomorpha: Hairworms; adults are free-living in water while larvae are parasitic in arthropods (e.g., Gordius).
• Other classes: Some schemes include smaller groups like Rotifera, Gastrotricha, and Kinorhyncha.

Phylum Annelida

Phylum Annelida (from Latin annulus, meaning “little ring”) includes the segmented worms. They are distinguished from other worms by their true segmentation and the presence of a true coelom.

General characters of phylum Annelida

• Habit and habitat: Mostly aquatic (marine or freshwater like Nereis and leeches), some terrestrial (earthworms), and a few parasitic.
• Body organization: Triploblastic, bilaterally symmetrical, organ-system level of organization.
• Metamerism: Body divided into a linear series of similar segments called metameres or somites, separated internally by septa.
• Coelom: True coelom (schizocoelous), fluid-filled and acting as a hydraulic skeleton.
• Locomotion: Carried out by chitinous bristles called setae (chaetae) or fleshy appendages called parapodia.
• Circulatory system: Usually closed; respiratory pigment (haemoglobin) is often dissolved in the plasma.
• Excretion: Performed by coiled tubular structures called nephridia.
• Nervous system: Consists of a circumpharyngeal nerve ring and a double ventral nerve cord with segmental ganglia.

Classification (up to class level)

The classification is primarily based on the presence or absence of parapodia, setae, and clitellum.

Type Study: Earthworm (Pheretima posthuma)

Habitat and habits

• Habitat: Found in moist, humus-rich soil.
• Habits: Nocturnal (active at night). They are fossorial (burrowing) and move by contraction of body muscles. They are hermaphroditic but practice cross-fertilization.

External features & metamerism

• Body shape: Long, cylindrical, divided into about 100–120 metameres.
• Prostomium & peristomium: The first segment is the peristomium, which contains the mouth and is covered by a small fleshy lobe called the prostomium.
• Clitellum: A prominent circular band of glandular tissue found in segments 14, 15, and 16 in mature worms.

Digestive system

The alimentary canal is a straight tube:

• Mouth: 1st segment.
• Buccal cavity: 2nd–3rd segments.
• Pharynx: 4th segment; contains pharyngeal glands that secrete saliva with protease.
• Oesophagus: 5th–7th segments.
• Gizzard: 8th segment; a highly muscular organ used to grind soil and food.
• Stomach: 9th–14th segments; contains calciferous glands to neutralize soil acidity.
• Intestine: 15th segment to the last; contains an internal fold called the typhlosole (from the 26th segment onwards) to increase the absorptive surface area.

Circulatory system

• Closed type with red blood (haemoglobin in plasma).
• Hearts: Four pairs of “lateral hearts” (segments 7, 9, 12, and 13) pump blood from the dorsal to the ventral vessel.
• Dorsal vessel: Acts as the main collecting vessel; blood flows posterior to anterior.
• Ventral vessel: Acts as the main distributing vessel; blood flows anterior to posterior.

Nervous system

• Consists of a nerve ring (brain) around the pharynx.
• A double ventral nerve cord runs the length of the body, with a ganglion in each segment.

Reproductive system

Earthworms are monoecious (hermaphroditic) but are protandrous (testes mature before ovaries), which ensures cross-fertilization.

• Male: Two pairs of testes (segments 10, 11) and two pairs of seminal vesicles (11, 12). Male genital pores are on the 18th segment.
• Female: One pair of ovaries attached to the septum of segments 12/13. A single female genital pore is on the 14th segment.
• Spermathecae: Four pairs (segments 6–9) that store sperm received during copulation.
• Cocoon: Fertilization and development occur inside a cocoon secreted by the clitellum.

Circulatory system of earthworm

(See details above under Circulatory system.)