Invertebrates of Illinois

What are invertebrates?
They are animals that do not have a bony, internal skeleton or a backbone.

Where do invertebrates live in Illinois?
They are found in every habitat in the state.

How many kinds of invertebrates live in Illinois?
So far about 29,000 different types of invertebrates have been identified in Illinois. There are many more species waiting to be discovered and studied.

How big are the invertebrates that are found in this state?
They range from tiny species (some mites, some insects and others) that can barely be seen with your eyes to individuals that may be several feet long (tapeworms).

Why are there so many invertebrates?
Most invertebrates can reproduce quickly and have many offspring when they do so. Some of them can lay eggs that do not need to be fertilized in order to develop.

The Kingdom Animalia contains 35 phyla in the world. In Illinois, 16 of these phyla are represented.

Of these, 15 phyla contain animals with no backbone.

Phylum Porifera (Sponges)
Commonly known as sponges, these “pore animals” live in water. While many sponges are native to marine environments, 14 species of freshwater sponges live in Illinois. Sponges are sessile as adults and filter food from the water.

Phylum Cnidaria (Hydras and Freshwater Jellyfishes)
Cnidarians in Illinois include 10 species of hydras and one species of freshwater jellyfish. They are sessile as adults and have stinging cells to help them catch prey.

Phylum Platyhelminthes (Flatworms)
These invertebrates are the flatworms. Their body is flattened from top to bottom along its length. One group is free-living with most of those species being aquatic. The other types of flatworms are parasites. There are about 400 species of flatworms in Illinois including planarians, tapeworms and flukes.

Phylum Nemertea (Ribbon Worms)
There is one species of ribbon worm in Illinois. These animals are elongated and flattened. They have a proboscis, developed from the body wall, that can be extended to help them catch their prey.

Phylum Nematoda (Roundworms)
The number of roundworm species in Illinois is not known. The body of roundworms is not segmented and comes to a point at each end. They live on land, in water and some are parasites. In a specific habitat, there are often more nematodes, in species and numbers, than any other animals. A nematode researcher once said that if all the earth’s components except nematodes could be removed, the outline of the earth and its major structures would still be recognizable from the film of nematodes remaining.

Phylum Nematomorpha (Horsehair Worms)
There are two species of horsehair worms in Illinois. They are found in water or damp places on land. The adults are free-living, but the larvae are parasitic on arthropods. They can tie their long, slender body into knots.

Phylum Acanthocephala (Spiny-headed or Thorny-headed Worms)
These parasitic worms are often known as spiny-headed worms or thorny-headed worms. Twenty-seven species are known from Illinois. They have a spiny proboscis that is used to anchor them to the gut of their host animal. The life cycle is complex and involves at least two hosts.

Phylum Gastrotricha (Gastrotrichs)
The gastrotrichs are aquatic worms sometimes known as “hairybacks.” They are found on the bottom of water bodies or in areas of damp soil on land. These microscopic animals live for only a few days and can rapidly reproduce by producing unfertilized eggs. They eat dead organic material. About 60 species are known from Illinois.

Phylum Rotifera (Rotifers)
The 150-175 rotifer species in Illinois are part of a group of microscopic to extremely small animals that provide an important food source for aquatic animals and are important in building soil by breaking down organic matter. The body has a general, cylindrical shape with a crown of cilia on the head.

Phylum Entoprocta (Goblet Worms)
One species of goblet worm is known from Illinois. These tiny aquatic animals are colonial, and their body is shaped like a goblet on a long stalk. The rim of the “goblet” or “crown” has a ring of solid cilia that beat to create a current to draw water and the food particles it carries into the crown, where the mouth and anus are both located.

Phylum Annelida (Segmented Worms)
The segmented worms include suction-feeding worms, crayfish worms, earthworms and leeches. The body is long and composed of multiple segments. Most of the segments contain the same set of organs, but they also share organs throughout the body. They are covered by a cuticle that does not molt. Some segmented worms are predators or parasites. Others feed on dead organic matter. In Illinois, 87 species of segmented worms are known. Segmented worms in Illinois live in water and in moist places on land.

Phylum Arthropoda (Arthropods)
Arthropods have an external skeleton, a segmented body and jointed appendages. They are covered with a cuticle made of chitin. The cuticle is hard and must be molted, or shed, for the animal to be able to grow. Arthropods in Illinois include scorpions, spiders, pseudoscorpions, daddy long-legs, mites, ticks, millipedes, centipedes, pauropods, symphylans, insects and crustaceans. There are nearly 28,000 types of arthropods known from Illinois. They are represented in all Illinois habitats.

Phylum Tardigrada (Tardigrades)
Tardigrades, also known as water bears or mossy bears, are microscopic, aquatic animals with eight legs. They can survive extreme conditions of heat, cold, pressure, dehydration, radiation and other stressful environmental conditions. They eat plants, algae and other small invertebrates. Thirteen species of tardigrades have been found in Illinois.
Phylum Ectoprocta (Moss Animals)
Like the goblet worms, the moss animals have a body shaped like a goblet or crown on a long stalk. However, one of the differences is the cilia around the crown are hollow instead of solid. The cilia are used to set up a current to draw water to the animal where it is filtered for food particles. Nine species of these tiny aquatic animals are known from Illinois.

Phylum Mollusca (Mollusks)
Mollusks are a very diverse group of animals that have in common a mantle with a cavity used for breathing and expelling wastes, a radula for feeding (except in the two-shelled organisms) and a similar nervous system. The mantle secretes the shell in those mollusks that have one. Snails, slugs, clams and mussels are mollusks in Illinois of which 274 species are known. They live in water or in moist terrestrial habitats. The feeding habits vary with species: some eat algae and aquatic plants; some are filter-feeders; some eat decaying plants; and some are predators.