Invertebrates

About 97% of all animals are invertebrates. Invertebrates are animals which do not

have a backbone. There are nine phyla of invertebrates:

Porifera, Cnidaria, Platyhelminthes, Nematoda, Rotifera, Mollusca, Annelida, Arthropoda, & Echinodermata.

Invertebrates

The phylum Porifera are sponges.

There are about 800 different species of sponges, and 88% are marine.

Freshwater sponges are smaller and less brightly colored than marine sponges.

Porifera - Sponges

Sponges are filter feeders. This means that they use their body as a

filter to trap their food, microscopic plankton. Sponges are asymmetrical and live attached

to one spot as adults making them sessile animals.

Sponges have a skeleton composed of a flexible protein material called spongin & hard fibers called 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.

Sponges lack the tissue level of organization but they do have some specialized cells.

Choanocytes are specialized cells that line pores in a sponge and have a flagellum that spins to pull in water and food.

Sponge Bodies

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.

Wastes and excess water leave a sponge through an opening at the top called the osculum.

Food Digestion

Sponges reproduce asexually by internal or external buds and by fragmentation.

Each piece can form a new sponge.

This is how sponges form colonies.

Reproduction

Sponges are hermaphrodites that produce both eggs and sperm.

They do not fertilize their own eggs, but they exchange sperm with other sponges.

Sponges reproduce sexually by dispensing eggs and sperm into the water.

If the freshwater supply evaporates, freshwater sponges become dormant and form an internal bud or gemmule [jem-yool] which is released when the sponge dies.

The gemmule is a small freshwater sponge covered with hardened mucus which prevents it from drying out.

When the freshwater returns, the gemmule becomes an active sponge.

The phylum Cnidaria include sea anemones, hydra, corals and jellyfish.

All Cnidaria are marine except hydra, which is a freshwater organism.

Cnidarians

Sea anemone

Cnidarians have radial symmetry and are carnivorous using tentacles that surround their mouth to get food.

Cnidarians exhibit two body forms - the sessile polyp with tentacles & mouth at the top or the motile medusa with tentacles & 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.

Cnidarian - Body

Hydra

Cnidarians have 2 layers of cells --- gastrodermis & epidermis.

Cnidarians have a hollow gastrovascular cavity on the inside lined with gastrodermis.

Epidermis covers the outside and a jellylike material called mesoglea is between the layers.

Mesoglea is thin in polyp forms but thick in medusa forms.

Coral

Cnidarians have stinging cells called nematocysts or cnidocytes on their tentacles that are poisonous & shoot out like a harpoon to kill or paralyze prey.

Their mouth is the only opening to their body so they have a two-way digestive system. They also have a simple nerve net.

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

Some cnidarians like coral build a limestone case that makes an underwater reef.

Reproduction

The phylum Platyhelminthes are dorsoventrally flattened and have a definite anterior and posterior end giving them bilateral symmetry.

Their bodies are solid so they are said to be acoelomate [ey-see-luh-meyt] .

Platyhelminthes -Flatworms

Fluke

Some flatworms are parasites, while others are free-living carnivores or scavengers.

Examples of parasitic flatworms are flukes and tapeworms.

Flatworms also have only a mouth for both food and wastes.

Tapeworm

Their nervous system is composed of a nerve net and sometimes light-sensitive eyespots at the anterior end.

Specialized flame cells help get rid of wastes.

The planarian is the most common free-living flatworm found in water or moist places.

They are hermaphrodites producing both eggs and sperm, but they exchange sperm with each other during sexual reproduction.

Planarians also reproduce asexually by fragmentation.

Flukes and tapeworms often live in their host's digestive tract resistant to the host's enzymes.

They do not have a digestive system allowing the host to digest their food.

Tapeworms are divided into sections called proglottids that each have a complete reproductive system producing fertilized eggs.

Tapeworms are hermaphroditic (one body having both sexual parts), and they fertilize their own eggs.

Ripe proglottids with their eggs pass out with the host's feces.

Tapeworms anterior end is called the scolex and is modified with both hooks and suckers to attach to the host's intestines.

Humans most often get tapeworms from infected, undercooked pork, beef, or fish.

Tapeworm eggs can withstand boiling water so it is important to cook these meats well enough to destroy the eggs.

Children sometimes get tapeworms by playing with the feces in the litter box of a cat, getting the eggs on their hands, and placing their hands or fingers in their mouth.

The longest tapeworm ever passed by a person was 39 meters.

Flukes have complex life cycles that involve more than one host.

A fluke causes Schistosomiasis, a disease that affects 250 million people world wide.

This blood fluke attacks the kidneys, liver, and intestines causing progressive weakness.

It often takes 20 years to die from Schistosomiases, & there is no cure.

The phylum Nematoda are the roundworms.

Roundworms are cylindrical in shape, tapered at both ends, and vary in length from being microscopic to 20 inches long.

Roundworms are pseudocoelomate [soo-doh-see-luh-meyt] having a body cavity that is not completely lined.

Nematoda-Roundworms

Pinworm

The body cavity or pseudocoel [soo-doh-seel] serves as a hydrostatic skeleton against which muscles can contract.

Unlike flatworms, roundworms have a complete gut.

This means that they have a one-way digestive tract with a gut that begins with a mouth and ends with an anus.

Therefore, they are usually able to digest food.

Roundworms have no blood or heart.

Nutrients are distributed by a non-blood fluid which is not pumped.

Most roundworms are parasites and are found in all habitats.

They are bilaterally symmetrical and unsegmented.

Hookworm

Although they are cylindrical in shape, they usually taper at both ends.

They are covered with a thick protective cuticle that is flexible and can be molted.

They have separate sexes generally and reproduce sexually.

The roundworm Trichinella, causes the disease called trichinosis.

People get trichinosis from eating infected, undercooked pork.

Trichinella gets into muscles and leaves calcium deposits which affect muscle contraction.

Trichinosis can affect the heart.

Trichinella

Another roundworm, Ascaris, parasitizes human lungs.

The Filaria worm attacks the lymphatic system causing great swelling.

Hookworms and pinworms are also roundworms which parasitize humans.

Ascaris

The phylum Rotifera includes microscopic worms found in aquatic and soil habitats.

They have a crown of cilia at their head end surrounding their mouth for movement and feeding.

Their bodies are covered with an external layer of chitin[kahy-tin].

Rotifera-Rotifers

Having separate sexes, they reproduce sexually.

Some species contain only females and reproduce by parthenogenesis (unfertilized eggs developing into females).