8.28.1 Lesson: Sponges and Cnidarians

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Sponges and Cnidarians


H.1 – Identify the characteristics of Kingdom Animalia and its various phyla.



Specialized cell that delivers a toxic sting

Collar Cells

Line the central cavity of sponges and create a current for the filtration of water


Outer cell layer; also called the epidermis


Inner cell layer; also called the gastroderm

Excurrent Pore

Used for the outward flow of waste following filtration

Incurrent Pore

Used for the flow of water and nutrients into a sponge


Jelly-like middle layer that separates the ectoderm and epidermis


Nearly 97% of identified animal species are invertebrates, which means that they do not have a backbone. Like all animals, however, they share common characteristics of being multicellular, lacking cell walls, and reproducing by the union of gametes.

If mammals, fish, birds, reptiles and amphibians all have vertebrae, then which organisms are left that can compose such a large percentage of animals without backbones? We will analyze several invertebrates to include sponges, jellyfish, worms, snails and insects. 

Phylum Porifera 

Phylum Porifera consists of sponges, all of which are found in aquatic environments. Most live in seawater, however, there are a few species that can survive in freshwater.


Sponges are the simplest of all animals based on body structure. These organisms don’t have a mouth, heart, muscles or brain. Additionally, they lack true tissues, and instead, have a more simplistic cell layer composition. A gel-like layer, called the mesenchyme, is sandwiched between two cell layers. The inner layer is known as the endoderm and the outer layer is called the ectoderm (or epidermis).


The ectoderm functions as a protective outer covering and is full of pores and channels. These small passageways aid in filter-feeding, the method used by sponges to obtain nutrition. Water from the environment is pumped into the sponge through tiny openings known as incurrent pores, bringing nutrients into the sponge’s central chamber. The central cavity is lined with collar cells, which have flagella to move about inside of the sponge, creating a current. This current aids in filtering food particles from the water. Waste is then pumped out of the top of the sponge or through the excurrent pore, known as the osculum.



Watch the video clip below to view an excellent example of how this mechanism works with the use of nontoxic dye.


Most sponges are attached to a reef and live their lives in a sessile (immobile) form; however, one characteristic of all animals is the ability to move. Sponges meet this requirement by being motile in the larval portion of their life cycle. Larvae are able to swim in water and once they land on a piece of rock or reef, they anchor themselves for the remainder of their lives.

Phylum Cnidaria

Another group of invertebrates is known as the Cnidarians. These organisms are also simple in structure but are composed of true tissue. Examples include jellyfish, sea anemones and corals, all of which exhibit radial symmetry.

The epidermis, or outer layer, contains distinctive structures known as cnidocysts, or stinging cells. Organisms such as jellyfish and corals possess these specialized cells in their tentacles and use them to deliver toxic stings to predators and prey.

The endoderm, which can also be called the gastrodermis, covers the inside of the organism. It is separated from the epidermis by a layer of mesenchyme, much like the composition of sponges discussed above.

Life Cycle

The jellyfish life cycle contains both sessile and motile forms. They begin their lives as free-swimming larvae before entering the polyp (sessile) phase when they attach to coastal reefs. Then, upon maturation, they re-enter a motile state known as the medusa phase. During this phase, the adult jellyfish can release eggs or sperm into the water. When the egg is fertilized, a new larva forms and the life cycle begins for the offspring. 


Watch the brief video clip below for an explanation of the jellyfish life cycle depicted in the diagram above.

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