Invertebrate Notes


Notes on Invertebrate Animals


1. Porifera-sponges

2. Cnidaria

a. sea anemones

b. hydra

c. corals

d. jelly‑fish

3. Platyhelminthes-flatworms

a. flukes

b. tapeworms

4. Nematoda-roundworms

a. Trichinella

b. Ascaris

c. hookworms

d. pinworms

5. Rotifera–rotifers

6. Annelida-segmented worms

a. earthworm

b. leeches

7.  Mollusca-clams, oysters, snails, and octopus

8. Arthropoda

     subphylum: Trilobita–trilobites (extinct)

     subphylum: Chelicerata-horseshoe crabs, spiders, scorpions, mites, & ticks

subphylum: Mandibulata–crustaceans, insects, millipedes, centipedes

9. Echinodermata: starfish, sea cucumbers, sea lilies

About 97% of all animals are invertebrates.  Invertebrates are animals which do not have a backbone.  In this unit we cover nine phyla of invertebrates:  Porifera, Cnidaria, Platyhelminthes, Nematoda, Rotifera, Mollusca, Annelida, Arthropoda, & Echinodermata.


The phylum Porifera are sponges.  There are about 800 different species of sponges, and 88% are marine.  “Marine” means that they live in salt water, such as an ocean or a sea.  Freshwater sponges are smaller and less brightly colored than marine 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 pore 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.  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.

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. This is how sponges form colonies. 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 which is release 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 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.  Cnidarians have a hollow gastrovascular cavity on the inside lined with gastrodermisEpidermis covers the outside and a jellylike material called mesoglea is between the layers.  Mesoglea is thin in polyp forms but thick in medusa forms. 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.  The 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.

Platyhelminthes (flatworms)

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.  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.  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 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.

Nematoda (roundworms)

The phylum Nematoda are the roundworms.  Roundworms are cylindrical in shape and vary in length from being microscopic to  20 inches long.  Roundworms are pseudocoelomate having a body cavity that is not completely lined. The body cavity or pseudocoel 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.  However, 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.  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 undercooked pork.  Trichinella gets into muscles and leaves calcium deposits which effect muscle contraction.  Trichinosis can affect the heart.  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.


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. Having separate sexes, they reproduce sexually.  Some species contain only females and reproduce by parthenogenesis (unfertilized eggs developing into females).


The phylum Mollusca contains snails and slugs, bivalves, octopus, squid, and the chambered nautilus. Many members of this phylum have durable limestone shells and are found in all habitats. Members of this group are economically important as sources of human food , pearl and shell production, crop & flower damage, destruction to submerged wooden structures, and intermediate hosts for some parasitic diseases. The giant squid and giant clam are the two largest invertebrates.  Mollusks have bilateral symmetry and a visceral mass containing their body organs. Mollusks also have a muscular foot for movement which can be modified into arms or tentacles in some species.  Mollusks breathe through gills or lungs located below a protective layer called the mantle.  The mantle forms the shell in some species and also protects the body organs. All mollusks except bivavles contain a rasping, tongue-like radula for scraping food.  The circulatory system consists of a three-chambered heart  and open-flowing system except for octopus & squids which have a closed circulatory system. Reproduction is sexual even in hermaphroditic forms.  Mollusks go through a free swimming larval stage called the trochophore.

The class of mollusks called gastropods have a foot on their belly.  An example of a gastropod is the snail.  When a snail lacks a shell it is called a slug.  Snails and slugs walk on their belly.  Most snails are marine, but some do live on land.  Marine snails have gills.  Land snails are called pulmonate snails and have an air hole for breathing.  Snails can be very large.  The helmet snail can be as big as 15 pounds.

The class of mollusk called Bivalvia includes clams, oysters, mussels , and scallops. These mollusks have two shells hinged together by a ligament.  Strong adductor muscles open and close the shells. Incurrent and excurrent siphons circulate water containg food and oxygen through the bivalve.  Gills extract the oxygen from the water,  and they move by jet propulsion.  Their muscular foot can be extended from the shell for movement or anchoring.

The class of mollusks called cephalopods have a foot on their head.  Examples of cephalopods are octopus, squid and nautilus.  Most cephalopods have beaks, tentacles and jaws and are active predators. Their musclar foot has been modified into arms or tentacles. They lack external shells except for the natilus.  These are the most intelligent of all invertebrates.  They used their siphons to move by jet propulsion.  Octopus have their shell inside of their body.  Octopus secrete an inky substance which they spit out to help them escape from predators.  The giant squid is the largest cephalopod.  It can be up to 60 meters in length and has been known to eat whales.

 Annelids (segmented worms)

The phylum Annelida are the segmented worms and are abundant in all habitats. External segments  are characterized by ringlike structures along the body, and corresponding internal segments are called septaSegmentation gives worms more flexiblity in movement. If one segment is damaged, it isn=t usually fatal to the animal because their organs are duplicated in other segments.  Annelids have a Atube within a tube@ body plan known as a coelom which is fully lined and contains the body organs.  The coelom runs from the mouth to the anus. Annelids have bilateral symmetry, and a well-developed brain and diverse sense organs showing cephalization. Coelomic fluid serves as a  hydrostatic skeleton.

Earthworms belong to this phylum.  Each segment of the earthworm has setae or external bristles made of chitin.  These bristles allow the earthworm to move and to burrow into soil.   Earthworms have a head and a central nervous system.  Earthworms respire through their moist skin as they dig through the soil and help loosen it. They have a closed circulatory system in which blood is pumped by five pairs of hearts.  Most earthworms feed on decomposing vegetation causing it to decompose faster. A  pharynx sucks in the organic debris which the muscular gizzard grinds. Earthworms bring the nutrients from the subsoil to the top soil, thereby helping plants to grow.  Undigested materials or castings are deposited outside burrows.

Leeches are also in the phylum Annelida.  Most leeches live in water and have suckers at both ends of their bodies. The tail suckers are used to latch on to a host, while the head suckers are used to suck blood from the host.  Most leeches are predators or scavengers, but some suck blood.  Because of this, blood sucking leeches are collected for anticoagulant. Leeches bodies are flattened dorsoventrally and lack setae except for one species.  Like earthworms, leeches are hermaphrodites that exchange sperm with other members of their species.

Polychaetes are marine annelids that have their setae modified into paddle-like structures called parapodia.  Parapodia improvement movement and give more area for gas exchange. Polychaetes often live commensally with sponges, mollusks, echinoderms, and crustaceans. Sexes are separate with external fertilization.


The members of the phylum Arthropoda all have jointed appendages.  In fact, the word “arthropod” means jointed leg.  There are more species of arthropods than any other phylum. Arthropods have these characteristics:

a. hard exoskeleton which is usually composed of substance called  chitin

b. go through periodic ecdysis as they shed or molt their exoskeleton

c. they have specialized body segments (head,  thorax, cephalothorax, & abdomen)

d. jointed appendages such as legs, antenna, and mouthparts.

e. open circulatory system

The phylum Arthropoda is divided according to their type of appendages.  The subphylum Chelicerata possess chelicerae or fangs and no antenna, while the subphylum Mandibulata have antenna and mandibles or jaws.  Crustaceans have pincers called chelipeds.  The subphylum Trilobita are an extinct group with a head and trunk with a pair of legs on each segment.

Terrestrial arthropods like insects, millipedes, & centipedes have a system of hollow air tubes called trachae as their respiratory system. Aquatic chelicerates like the horseshoe crab have book gills, while terrestrial chelicerates such as spiders, ticks, mites, & scorpions  use  book lungs.    Book lungs have numerous blood vessel lined surfaces which look like the pages in a book & get oxygen from air.  Crustaceans respire through gills. Gills are folded tissue which are lined with blood vessels which  remove oxygen from water.

Terrestrial mandibulates are uniraimous with one-branched appendages, but aquatic mandibulates like crustaceans are biramous or two-branched.   Arthropods have a brain and nervous system and possess a variety of sensory receptors such as simple eyes called ocelli or compound eyes, typmpanic membranes for hearing, and antenna that can smell and taste.  Excretory structures in arthropods vary, but terrestrial arthropods have Malpighian tubules to filter nitrogenous wastes.

The subphylum Chelicerata (ki-LISS-uh-ruh) include the class Xiphosura or horseshoe crabs which have a cephalothorax and abdomen, live in marine environments breathing through book gills, lack antenna, but have chelicera & 4 pairs of walking legs.  The class Arachnida containing spiders, scorpions, mites, and ticks are also chelicerates that lack antenna, have chelicera (fangs) and 4 pairs of legs, but they live in terrestrial habitats and breathe through book lungs or trachae Chelicerates also have appendages on their head called pedipalps that are sensory and can help move food into their mouth.   Unlike most arthropods,  spiders do not see well; however, they are good at detecting movement.  Spiders have glands called spinnerets on the posterior end of their abdomen that produce silk to make webs.  When prey get caught in a spider’s web, it is the movement which alerts the spider to the captured prey.  Most spiders also have hairs on their body to assist them in feeling movement.  Spiders  poison their prey once they are caught in their webs. Spiders are very beneficial because they catch and eat insects.  Two spiders which are dangerous are the black widow and the brown recluse.  Both of these spiders have distinct markings on the underside of their abdomen..  Spiders differ from insects in having eight, not six legs,   having simple eyes  and not compound eyes, and having only 2 body regions (cephalothorax & abdomen) instead of 3 regions ( head, thorax, & abdomen).

The subphylum Mandibulata contains the class Crustacea.  Most crustaceans live in the water and include crabs, shrimp, lobster, crayfish, & barnacles. Terrestrial crustaceans include pillbugs and sowbugs.  Crustaceans have a pair of antenna to smell and detect chemicals and a shorter pair of antennules used for balance. They have 2 body regions (cephalothorax and abdomen), and their mouthparts include mandibles, maxilla, and maxillipeds.  They also have pincers called chelipeds to help them  catch food.  Aquatic crustaceans  have a shell called a carapace that they regularly shed as they grow to produce a larger one.    Crustaceans are economically important to man as a food source.

The classes Chilopoda and Diplopoda are alo in the subphylum Mandibulata.  Chilopoda or centipedes are poisonous predators feeding on other terrestrial arthropods. Centipedes have fangs, venom glands, and a pincer on their tail. They have a single pair of legs per body segment.  Diplopoda or millipedes are vegetarians or scavengers feeding on decaying vegetation that have two pairs of legs per body segment.

The class Insecta in the subphylum Mandibulata includes all of  the insects.  This is the largest and most successful group of arthropods. Insects usually have six legs, a pair of antenna, and a pair of wings although some species may be wingless such as silverfish and termites. Flies have their second pair of wings modified into a balancing structure called halteres.  Insect’s mouths usually have four parts – the mandible or jaw, maxilla, labium or lower lip, and labrum or upper lip and are adapted for a particular food.  For example, grasshoppers  have chewing mouthparts for eating grass, mosquitos have sucking mouthparts for sucking blood, butterflies have siphoning mouthparts for getting nectar from flowers, and the house fly has spongy mouth- parts for soaking up liquid food.  Wings and legs are attached to the midsection or thorax, antenna, eyes, and mouthparts are attached to the head, and the abdomen on females may have an egg-laying tube called the ovipositor.  Insects communicate by producing sounds and by making chemicals called pheromones. Tympanic membranes on the abdomen and sensory hairs detect sound waves.  Spiracles line the sides of the insect=s abdomen and open into their breathing tubes or trachae. Insects may go through stages in their life cycle.  Butterflies, bees, flies, and beetles go through the egg, larva, pupa, and adult stages.  This is known as complete metamorphosis. Dragonflies and grasshoppers go through egg, nymph, and adult stages known as incomplete metamorphosis.  Insects such as silverfish and fleas do not go through metamorphosis.  Metamorphosis and molting are controlled by hormones.


The phylum Echinodermata include the starfish, sea urchins and sea cucumbers.  The word “echinoderm” means spiny skin.  Echinoderms are the most advanced invertebrates. All other invertebrates are protostomes in which the blastopore in their development becomes the mouth.  Echinoderms, like chordates, are deuterostomes in which the blastopore becomes the anus. Echinoderms have an endoskeleton composed of movable or fixed calcium plates called ossicles.  The members of this phylum have radial symmetry with a five part body plan. Adults have no head or brain and move be extendable tube feet.  Echinoderms also possess a water vascular system made up of a system of canals that help the organism feed and move.  Water enters through an opening called the madreporite into a short stone canal into the ring canalRadial canals connect to the ring canal and determine the five-part symmetry. This hydraulic water system is strong enough to help starfish open clam shells.  Skin gills are used for respiration and waste removal.   Echinoderms are capable of extensive regeneration whenever parts are dropped.  They can reproduce asexually by fragmentation or sexually with external fertilization.

Starfish are in the class Asteroidea and are active marine predators with 5 arms set off from a central disk and their mouth located on the underside or oral surface. Bivalve mollusks are a favorite food of the starfish, and they consume them by turning their stomach inside out and sticking it into the clam shell to digest the clam.

Sea urchins and sand dollars are in the class Echinodea and they lack distinct arms. Five rows of tube feet protrude through their skeletal.  They use the spines of their skin and tube feet to move about and graze on algae, coral, or dead fish.  Triangular teeth around the mouth scrap or crush food.

The class Crinoidea contains sea lilies and feather stars with highly branched arms around their mouth for filter feeding.  Sea liles are attached by a stalk to the substrate, but feather stars are able to detach and move about.

Brittle stars in the class Ophuroidea have slender arms attached to their central disk and can move faster than starfish. Sea cucumbers are in the class Holothuroidea and are soft, sluglike organisms with leathery outer skin. Sea cucumbers usually lie on their sides on the ocean bottom and can eject part of their intestines in order toscare away a predator.  They also move by tube feet or by wiggling their entire body. Some of these are hermaphroditic which is unusual for echinoderms.