Phylum Cnidaria: Characteristics

• Phylum Cnidaria includes soft-bodied stinging animals like corals, sea anemones, and jellyfish
• The name Cnidaria comes from the Greek word "cnid," meaning nettle (a stinging plant)
• Cnidarians inhabit many aquatic environments
• They range in size from tiny to graceful giants with trailing tentacles several meters long
• Cnidarians are diploblastic eumetazoans with a tissue grade of organization
• Cnidarians are characterized by cnidocytes and polyp/medusa forms

General Characteristics of Phylum Cnidaria

• Most are marine, but some (like Hydra) live in freshwater
• Many are colonial (e.g., corals)
• Some are solitary (e.g., sea anemones)
• Body is radially symmetric, but sea anemones show biradial symmetry
• Polyp (hydroid) is sessile with mouth-up
• Medusa is umbrella/bell-shaped with mouth-down and swims by constricting the bell
• The body wall has an outer epidermis and an inner gastrodermis with mesoglea in between
• Mesoglea consists of amoeboid cells from ectoderm and is thin in polyps and thick (for buoyancy) in medusa
• The body wall contains stinging cnidocytes containing a fluid-filled capsule called cnida used for defense and prey capture
• The gastrovascular cavity (coelenteron) is a blind sac that opens via a mouth surrounded by tentacles used for ingestion and egestion
• In medusa, the coelenteron is specialized into a stomach, radial canals, and a ring canal for digestion and circulation
• Digestion begins extracellularly in the coelenteron and finishes intracellularly in gastrodermis cells
• Gas exchange and excretory waste removal occur by diffusion through the body wall
• Neurons form a pair of nerve nets, one in the epidermis and the other in the gastrodermis, joined by neurons crossing the mesoglea
• Nerve impulse conduction is diffuse and can travel in any direction
• Medusae have nerve rings and ganglia around the bell margin
• Sensory structures like statocysts occur in the medusoid form
• Asexual reproduction occurs by budding, fission, and fragmentation
• Cnidarians are mostly unisexual, but some are bisexual with external fertilization and holoblastic cleavage
• Development is indirect, including a free-swimming ciliated larval stage called planula
• Species with both polyp and medusa phases alternate asexually reproducing polyp and sexually reproducing medusa forms (metagenesis)
• Cnidarians have remarkable regeneration abilities

Classification of Phylum Cnidaria

• Phylum Coelenterata/Cnidaria has about 10,000 species in three classes: Hydrozoa, Scyphozoa, and Anthozoa

Class I: Hydrozoa

• Mostly marine, but some live in freshwater
• Chiefly colonial but can be solitary
• Medusa stage absent in some
• Medusa is craspedote (has a velum)
• Polyps have an undivided coelenteron
• Mesoglea is acellular
• Cnidocytes are restricted to the epidermis
• Gonads occur in the epidermal region
• Colonies are polymorphic with gastrozooids (feeding), dactylozooids (defense), and gonozooids (reproduction)

Class II: Scyphozoa

• All marine
• Medusa stage is predominant and acraspedote (no velum)
• The mouth is surrounded by four oral arms
• Mesoglea is cellular and contains amoebocytes
• Cnidocytes are in the epidermis and gastrodermis
• Gonads are in the gastrodermal region
• Polyps are solitary or colonial with a polyp stage called a scyphistoma which forms juvenile medusa (ephyrae) by strobilation; the ephyra grows into an adult medusa

Class III: Anthozoa

• All marine
• Solitary or colonial
• All are sedentary polyp forms, with no medusa stage
• Mouth is oval, surrounded by tentacles resembling a flower
• The mouth leads to a stomodaeum (tubular pharynx) that opens into the coelenteron, which is divided by mesenteries
• Cnidocytes occur in the epidermis and gastrodermis
• Gonads occur in the gastrodermis

Cnidarian Body Structure

• Body layers surround the gastrovascular cavity, which extends into hollow tentacles
• Cnidarians generally have radial symmetry
• Cnidarians have polyp and medusa structural forms.
• The polyp is a cylinder/bag with a mouth and tentacles at the top, attached to a surface and has a thin mesoglea
• The medusa is umbrella-shaped with a mouth and tentacles hanging down, motile, and has a thick mesoglea

Cnidocytes and Nematocysts

• Cnidarians have stinging cells called cnidocytes containing a nematocyst
• A nematocyst is a coiled, tubular harpoon-like structure (thread and bag) that is inverted
• Upon stimulation by touch or chemoreception, the nematocyst fires outward and injects venom
• Each nematocyst is used only once, replaced by new cnidocytes and they are specific to different species

Feeding and Support

• Cnidarians are carnivorous predators
• Jellyfish capture small animals with cnidocyte-filled tentacles
• Sessile coral polyps and sea anemones sting prey, grasp it with tentacles, and put it in their mouth
• Venom potency differs among species; some are toxic to humans
• Sea anemones and jellyfish use a hydrostatic skeleton and fill the gastrovascular cavity with water for support
• Coral polyps have a hydrostatic skeleton but often sit in a hard limestone skeleton

Other Systems

• Cnidarians lack organs and respiratory/circulatory systems
• Cells get oxygen directly from the water and nutrients pass through the liquid between cells
• Cnidarians have a simple nervous system (nerve net) for responding to stimuli through impulses to muscle cells
• They have mechanoreception (touch/pressure) and chemoreception (chemical stimuli, taste/smell)
• They also have photoreception (light intensity); box jellies have image-forming eyes, and most jellyfish have a statocyst for sensing direction

Movement

• Cnidarians respond to stimuli via muscle cells; vertical bands shorten the body, and circular muscles close the mouth
• Jellyfish use an umbrella shape of mesoglea; muscle contraction forces water out and propels them forward
• Most jellyfish are poor swimmers and therefore classified as plankton

Reproduction

• Cnidarians reproduce sexually and asexually.
• Some are simultaneous hermaphrodites, releasing egg-sperm bundles
• Others are either male or female
• Many coral species reproduce externally via broadcast spawning triggered by environmental cues
• Some cnidarians have internal fertilization
• After fertilization in broadcasting spawners, a larva grows with cilia; these are also considered plankton
• Cnidarians can reproduce asexually by budding or fragmentation to form large colonies
• They can regenerate lost or damaged parts; some can form entire new organisms from detached tissue