Cnidaria and Ctenophora

Used to be just one phylum -Coelenterata (hollow gut).

1) Cnidaria - hydra and jellyfishes

2) Ctenophora - comb jellies


Common Features of two phylum:

1) All have radial or biradial symmetry.

2) Good tissue level of organization with very few organs, therefore no true organ systems.

3) diploblastic (mostly endoderm and ectoderm); some spp. having mesoderm, therefore referred to as tripoblastic.

4) Internal body cavity (gastrovascular cavity-GVC) lined with specialized tissue (gastroderm). There is only one opening that (serves mouth and anus) is intake and excretion of materials through the same opening,

5) Extracellular digestion within GVC and intracellular digestion within gastrodermal cells. Intercellular transport digested (soluble) nutrients.

6) Tentacles/extensible projections which usually surround mouth and primarily aid in food capture and secondarily in defense.

7) Have nerve cells of primitive/simple type (protoneurons) arranged as nerve net with no true central nervous system (CNS).

8) Do posses some specialized sense organs, though simple.

a) Statocysts : equilibrium sensation.

b) Ocelli : photo sensation ("eyes") contain photo-sensitive chemicals and allow sensation of light and orientation with regard to light direction.

9) Locomotion in some spp. by muscular contractions or ciliary combs (Ctenophora members). Both phylum adapted to free floating than swimming due to their radial/ biradial symmetry.

10)Appearance of polymorphic forms within certain spp.

a) sessile (polyp) form : "hydra-like" .

b) mobile (free-swimming or floating) medusae form.

c) sp. of Cnidaria can exist as a polyp form.

11) Specialized , unique features

a) Nematocysts : stinging organelles in specialized cells of Cnidaria spp.

b) Colloblasts : adhesive organelles in Ctenophora spp.

c) Ciliary comb plates : used for locomotion and food capture in Ctenophora.


Phylum Cnidaria

(Gr. knide = nettle: stinging structures; L. aria = pl. suffix like/connection with)

9,000 spp. All have specialized stinging cells (cnidocytes) containing stinging organelles (nematocysts).

Specific characteristics:

1) Entirely aquatic; mostly marine, some freshwater spp. do exist.

2) Radial symmetry (biradial some spp.) Defined oral/aboral ends (anterior/posterior).

3) Polyps and medusae in nearly all Cnidarian spp.

4) Exoskeleton/endoskeleton one of usually three types:

a) Chitinous material : derived from polysaccharide chitin which is polymer modified glucose (glucosamine) glucose -> NH2 -> amino group.

b) Calcareous (calcium-carbonate) skeletal material sometimes making up Cnidarian corals (colomoid sessile forms).

c) Protein material.

5) Mostly diploblastic with mesoglea. Some tripoblastic spp.

6) GVC (mouth) surrounded by cnidocyte containing tentacles.

7) Hemocysts within cnidocytes. Used for food capture and defense.

8) Nerve net.

9) Locomotion by muscular contractions and movement (swimming) using the tentacles.

10) Reproduction either asexually or sexually.

a) asexual: budding (polyp form).

b) sexual: fusion of true gametes -> zygote. free-swimming larva -metamorphose-> mature adult.

11) No true excretory/respiratory system: excretion of wastes. simple diffusion in gas exchange.

12) No true coelomic cavity. Thus name change. Instead have GVC.


Form and Function

1) Polymorphism:

a) sessile polyp form (hydroid) : adapted to sedimentary (within sediments) or sessile (attached to substrata).

b) mobile medusae form (jelly-fish-shaped) free-floating and/or free-swimming.

2) Nematocysts discharge:

a) cnidocyte : stinging cell.

b) nematocysts : stinging organelle.

1) Undischarged nematocyst:

Cnidocil - extraorganellular and extracellular nettle-like extension which triggers discharge of nematocyst. Operculum - lid like covering which closes nematocyst and opens upon discharge of nematocyst. Barb - sharp, thread-like structure; folded up inside undischarged nematocyst. Filament - long, thread- like structure; tightly coiled and compacted within undischarged nematocyst.

2) Mechanism of discharge:

a) Buildup of osmotic pressure (turgor pressure) due to osmosis of water. Provides the potential energy, which is later converted into kinetic energy which discharges the barb and the filament. 140 atmospheres of osmotic pressure within undischarged nematocyst.

b) Triggering and increase in hydrostatic pressure.

1) contact of cnidocil with object or organism.

2) increase in nematocyst membrane permeability to water (allows water influx to increase hydrostatic pressure). Original high osmotic pressure within nematocyst. Must have low H2O and high solutes = high osmotic pressure. Q: how does high solute occur? A: active transport - energy requiring transport of solutes against a gradient.

c) Changes occur within nematocyst membrane which cause water to be taken up very quickly and solute active transport increases rapidly to facilitate osmosis into the organelle.

d) High hydrostatic pressure.



1) Class Hydrozoa (water + animal); Genus Hydra; Species Hydra sp. (one), Hydra spp. (many).

All cnidaria are aquatic (some marine, some freshwater). Hydra - freshwater hydrozoan.

a) body plan polar : distinct anterior and posterior regions.

1) Posterior - basal disc for attachment to substrata (polyp form-sessile).

2) Anterior - hypostome - oral opening into GVC; located below tentacles.

Hydroid colonies:

a) For hydra, asexual individual will arise and remain attached to parent. If all offspring remain attached along with subsequent offspring colony of many genetically identical hydra individuals.

b) Some cases of sexually-derived, genetically distinct colonies can occur if a hydra bearing eggs within an ovary, is fertilized.

c) Some individual hydra buds break off and live independently or can start their own colony.

d) Polyp forms

1) Medusae (freshwater forms) typically reproduce sexually. Some spp. tend exist primarily in medusal form (preferred by some spp. Environmental factors and geological location play a role in which form is favored.

2) Some asexual reproduction in medusal spp. results in "floating colonies"; aggregate of polyp and medusal forms.


2) Class Scyophozoa (true jelly-fishes) - medusal form predominates.

a) Major representatives include larger, cap-shaped (bell-shaped) jelly fishes.

3) Class Cubozoa (cube-shaped, jelly-fish-like cnidarians) - medusal form predominates.

4) Class Anthozoa - polyp form predominates.

a) sea anemones.

5) Class Anthozoa - flower-like animal

a) Polyp form predominates.

b) Sessile, pigmented organism with many pigmented tentacles.

c) Larger than hydra with thicker body wall.

d) Some with true triploblastic tissue arrangement and/or mesoderm (mesoglea still persists w/ some specialized cells embedded w/in gelatinous matrix).

e) Some will form calcareous stony corals. Many coral reefs formed by these.


Phylum Ctenophora

General characteristics

Comparison with Cnidaria

Contrast of Phyla Cnidaria and Ctenophora