Animal Phyla: Porifera and Cnidaria

Before starting this lesson, download the Animal Phyla Fact Sheet in the sidebar. It will aid in collecting and organizing information about organisms from the animal kingdom.

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Phylum Porifera: Sponges

There are between 5,000 – 10,000 different species of sponges. 88% are marine (salt water habitat). Freshwater sponges are smaller and less brightly colored than marine sponges.

Watch the following movie clip to learn more about sponges, the most ancient animals, from The Shape of Life. While viewing, record important ideas on your Fact Sheet.

Sponges are sessile and have no symmetry (asymmetrical).

Sponges have a skeleton composed of a flexible protein material called spongin and hard fibers (spicules) composed of calcium carbonate or silicon dioxide. The body of a sponge is filled with holes or pores through which water enters their hollow bodies.

Although they lack the tissue level of organization, they do have some specialized cells that live in close association with each other to make up the sponge’s body:

  • Choanocytes are specialized cells that line pores in a sponge and have a flagellum that beats to create a flow of water and food through the sponge.
  • Collar cells at the base of choanocytes capture plankton & start digesting it.
  • Amebocytes are specialized cells that carry food to all other parts of a sponge’s body.

Sponges are filter feeders; they must maintain a constant flow of water to get oxygen and to trap their food, microscopic plankton. They take in water and food through tiny pores and expel wastes and excess water through the osculum, or top opening.

Most sponges sexually reproduce and are hermaphrodites (produce both eggs and sperm.) Sponges reproduce sexually by dispensing eggs and sperm into the water. Sponges reproduce asexually by internal or external buds and by fragmentation whenever a piece of the sponge breaks off. Each piece can form a new sponge.

Sponges do not have a nervous system, a digestive tube or a circulatory system.

Phylum Cnidaria: Jellyfish, Hydra and Anemones

The Phylum Cnidaria includes three well-known groups as well as several other lesser-known classes:

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  1. Class Anthozoa, the sea anemones and corals,
  2. Class Hydrozoa, the hydra, and
  3. Class Scyphozoa, the jellyfish.

The distinguishing feature of this phylum is its cnidocytes, also called nematocysts. These are specialized stinging cells on their tentacles that are poisonous. They shoot tiny, harpoon-like structures that kill or paralyze prey.

Watch the following movie clip from The Shape of Life to learn more about Cnidarians.While viewing, record important ideas on your Fact Sheet.

All Cnidaria are marine except Hydra, which is a freshwater organism. Cnidarians have radial symmetry and are carnivorous. They use tentacles that surround their mouth to get food.

Cnidarians exhibit two body forms – the sessile polyp with tentacles and mouth at the top or the motile medusa with tentacles and mouth on the bottom. Cnidarians may exist in one of these two stages or go through both stages in their life cycle such as the jellyfish.

Cnidarians have 2 tissue layers: epidermis and endodermis (or gastroderm)

  1. Cnidarians have a hollow gastrovascular cavity on the inside lined with endodermis.
  2. Epidermis covers the outside and a jelly-like material called mesogleais between the layers. Mesoglea is thin in polyp forms but thick in medusa forms.

Cnidarians have a two-way digestive system (food enters through the mouth to the gastrovascular cavity and waste leaves through the same opening).

Cnidarians have muscles and a simple nerve net. There is no central control for the network of nerves, so Cnidarians react to touch anywhere as if they were being touched everywhere!

Cnidarians reproduce asexually by budding or sexually whenever males release sperm and females release eggs into the water.

Cnidarians, like coral, build a limestone case that makes an underwater reef.

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