Simple Invertebrates Section 2

CH 27 Phylum Cnidaria

Jelly-fish, coral, sea anemones, and hydras

Simple Invertebrates Section 2

Characteristics of Cnidarians• Cnidarians have two tissue layers and radial symmetry

– Epidermis,

– Gastrodermis• Mesoglea is the jelly – like substance between the layers.

• Cnidarians have two basic body forms– Medusa, free floating and are often umbrella shaped

– Polyp, tube - like and attach to a rock or some other object

Simple Invertebrates Section 2

Visual Concept: Comparing Medusa and Polyp Bodies

Simple Invertebrates Section 2

Differentiation of Cells

• Cnidarians have nerve cells arranged into nerve net

• Sensory cells

• Muscle - like cells

• Stinging cells (cnidocytes)

Simple Invertebrates Section 2

Stinging Cells: Cnidocytes

• Cnidocytes are located on fingerlike tentacles that surround the one opening.

• Each cnidocyte has a threadlike organelle called a nematocyst– That can contain toxins– Used for defense and to capture prey

Simple Invertebrates Section 2

Visual Concept: Cnidocytes

Simple Invertebrates Section 2

Reproduction

• Sexual and asexual reproduction occurs in both forms

• In Obelia, the life cycle alternates between a polyp and a medusa stage.

• Once the gametes fuse, a planulae forms as a free-swimming larvae.

Simple Invertebrates Section 2

Obelia Life Cycle

Simple Invertebrates Section 2

Groups of Cnidarians

• The three main groups of cnidarians are;

– Class Hydrozoa (Colony, Portuguese man – of – war)

– Class Scyphozoa (Common jelly fish)

– Class Anthozoa (“flowering” animals, sea anemone)

Simple Invertebrates Section 2

Phylum Ctenophora: Comb Jellies

Simple Invertebrates Section 2

Phylum Ctenophora: Comb Jellies• “comb – holder”

– 8 comb – like rows of cilia

• Move by beating their cilia• Most do not have cnidocytes, instead colloblasts

– Secrete sticky substance on two tentacles

• Apical organ, direction in water• Hermaphroditic• Bioluminescent