Chapter 33: Invertebrates

By Erica Cron


Phylogenetic Tree

Key Concepts
  1. Sponges are basal animals that lack true tissue.
  2. Cnidarians are an ancient phylum of eumetazoans.
  3. Lophotrochozoans, a clafe identified by molecular data, have the widest range of animal body forms.
  4. Ecdysozoans are the most species-rich animal group.
  5. Echinoderms and chordates are deuterostomes.

Hey, what are invertebrates anyways? They are animals without a back bone! Overview:
  • They make up 95% of known animal species
  • They are in almost every habitat on Earth

Here is a link to get a closer look at each phylum:

33.1- Sponges are basal animals that lack true tissues
Sponges are animals in the phyla Calcarea and Silicea. They are one of the simplest animals. They come in all different sizes and in both fresh and salt waters. Sponges are suspension feeders. They bring in water through the spongocoel and let out water through the osculum. They are basal animal
s, which means they represent the early animals that originated from the root of the phylogenetic tree. Sponges don’t have true tissues. This means they don’t have tissues that work together as a functional unit. They just have several different cell types. An example of this are the choanocytes. Sponge
bodies have two layers that are separated by the mesohyl. Ameobocytes wander through the mesohyl. Sponges are hermaphrodites.

Suspension feeders- capture food particles in the water that pass through the body
Spongocoel- water passes through pores
to enter the spongocoel, a cavity
Osculum- opening for water to flow out
Choanocytes- collar cells; membranous collar around the b
ase of the flagellum
Mesohyl- the gelatinous region that separates the two layers of sponge bodies
Ameobocytes- take up food, digest food, carry nutrients, create tough skeletal fibers
Hermaphrodites- organisms that can act as both a male or a female in sexual reproduction

33.2 Cnidarians are an ancient phylum of eumetazoans.

One of the oldest lineages of the Eumetazoa clade (animals with true tissues) is the phylum Cnidaria. Their basic body plan is a cnidarian sac with a gastrovascular cavity. There are two different forms of this body plan. There is the sessile polyp and the motile medusa. Cnidarians are carnivores that use tentacles prepared for battle with cnidocytes to catch prey. These cnidocytes have cnidae, which are little organelles that can shoot outward. Specialized cnidae are called nematocysts. Cnidaria is divided into four main classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa.
Gastrovascular cavity- central digestive compartment; opening is the anus and the mouth
Polyp- shaped more like a cylinder
Medusa- shape is flatter and mouth-down
Cnidocytes- cells that only cnidarians have to defend their selves and capture prey
Nematocysts- have a single thread that can puncture the body or prey


I discovered that for the sections in the rest of this chapter it would be easier to list stuff since its telling about each set of animals.

33.3 Lophotrochozoans, a clade identified by molecular data, have the widest range of animal body forms.

Most animal species belong to the clade Bilateria, which are animals that have bilateral symmetry and triploblastic development. Bilateria is divided into three main clades: Lophotochozoa, Ecdysozoa, and Deuterostomia. In this part I’ll tell you about Lophotochozoa. The name comes from the structure of some lophotrochozoans, lophophore. This is a crown of harilike tentacles that they use for feeding. Or they could have trochophore larva, when they feed on plankton. Some have neither. The six phylas we will be talking about are: flatworms, rotifers, ectoprocts, brachiopods, mulluscs, and annelids.

· Marine, freshwater, damp habitats
· Many are parasitic
· No specialized organs for gas exchange
· Simple excretory apparatus functions; protonephridia
· Lack a circulatory system
· Four classes of flatworms: Turbellaria, Monogenea, Trematoda, and Cestoda

· Mostly marine
· Planarians
· Prey on smaller animals/ dead animals
· Use cilia or muscles to swim
· Complex nervous system
· Asexual reproduction or sexual reproduction

· Parasites
· Trematodes Alternate between sexual and asexual stages
· Trematodes need an immediate host to develop larvae; wide range of hosts
· Monogeneans are external parasites of fish
· Monogeneans have a simple life: Infect da fish

· Usually live inside vertebrates
· Anterior end has suckers and hooks; no mouth
· No gastrocvascular cavity
· Long ribbons of units (prolottids) that are sacs of sex organs. Tapeworms release these and they are excreted through feces and infect what they land in (food or water).

· Tiny animal in freshwater, marine, and damp soil habitats
· Alimentary canal
· Pharynx bear jaws called trophy grinds up food
· Parthenogenesis

cts and Brachiopods)-
· Have a crown of ciliated tentacles
· True coelom lined by mesoderm
Ectoprocts resemble moss

· Ectoprocts have exoskeletons
· Ectoprocts live in all types of water
· Brachiopods look like clams (dorsal and ventral)
· Brachiopods are attached to seafloors by their stalk

· Soft bodied animals in a hard protective shell
· 3 main body parts:foot, visceral mass, and a mantle
· Many have a mantle cavity
· Feed by using a radula
· Most have separate sexes
· Four of the body variations: Chitons, Gastropods, Bivalves, and Cephalopods

· Oval shaped bodytapeworm-80013858-l.jpg
· Shell divided into 8 dorsal plates
· Hang around rocks on a low tide shore

· Snails and slugs
· Marine and freshwater
· Torsion
· Usually have a single spiraled shell
· Lining of mantle acts as lung

· Clams, oysters, mussels, scallops
· Shell divided into two halves
· Suspension feeders
· Gills used for gas exchange and feeding

· Squids, octopuses, chambered nautiluses
· Active predators; use tentacles to grab prey
· Beak like jaws
· Shell can be outside, inside, or missing
· Complex organs and developed brain

· Have ammonites

  • Body made of “little rings”
  • Worms that live in the sea
  • 3 classes: Oligochaeta, Polychaeta, and Hirudinea

· Earthworms and other aquatic species
· Help soild by excreting undigested material and mucus
· Hermaphrodites

· Each segment has ridge-like structures called parapodia
· Parapodia function as gills, in locomotion, and are rich in blood vessels
· Usually in freshwater
· Most suck blood from other invertebrates

Protonephridia- Networks of tubes that pull fluid through open ducts to the outside
Planarians- prey on smaller animals; move by cilia
Alimentary Canal- digestive tube with separate mouth and anus
Parthenogenesis- when females produ
ce more females
Ectoprocts- animals that look like clumps of moss
Exoskeleton- a hard external skeleton studded with pores
Foot- used for movement
Visceral Mass- hold most internal organs
Mantle- tissue that covers visceral mass and creates a shell
Mantle Cavity- houses the gills, anus, and excretory pores
Radula- organ to scrape up food
Ammonites- shelled cephalopods

33.4 Ecdysozoans are the most species-rich animal group
  • Ecdysozoa shed the cuticle as they grow
  • Molting
  • 2 largest phyla: Nematodes and Arthropodsnematode.jpg

  • Aquatic habitats, soil, tissues of plants, body fluids and tissues of animals
  • Wall muscles are longitudinal
  • Reproduce sexually
  • Play an important role in decomposition and nutrient recycling
  • Parasitic ones have a toolkit that allows them to avoid immune systems
  • Some are like animals that are viruses

  • There is about a billion billion on Earth
  • Most are insects
  • Most successful animal phyla
  • Segmented bodies, exoskeleton, and jointed appendages= success
  • Existing Hox genes that cause the segmentation
  • Appendages become modified over time
  • Have an open circulatory syst em
  • Specialized gas exchange organs to breathe through exoskeleton
  • 4 major lineages: cheliceriforms, myriapods, hexapods, and crustaceans

  • Sea spiders, horseshoe crabs, scorpions, ticks, mites, spiders
  • Clawlike feeding appendages
  • Simple eyes (one lens)
  • Earliest were eurypterids
  • Modern ones are arachnids
  • Gas exchange is carried out by book lungs

  • Centipedes and millipedes
  • Have mandibles
  • Millipedes have two pairs of legs per segment
  • Millipedes eat decaying leaves and plant matter
  • Centipides have one pair of legs per segment
  • Centipedes have poison claws on their foremost trunk


  • Flight= key to success
  • Insects undergo metamorphosis
  • incomplete metamorphosis
  • complete metamorphosis


  • Live in water for the most part
  • special appendages
  • small ones, gas through cuticle
  • large ones, gas through gills
  • largest group is isopods
  • relatively large are called decapods
  • small crustaceans are called copepods

Arthrpodos- animals with hard exoskeleton, segmented bodies, and jointed appendages
Open Circulatory System- fluid is moved through heart through short arteries and then into spaces around tissues and organs (sinuses)
Chelicerae- serve as pinchers or fangs
Eurypterids- water scorpions
Book Lungs- stacked platelike structures in a chamber
Mandibles- three pairs of appendages modified as mouthparts; jaw-like
Incomplete Metamorphosis- the young resemble the adults still
Complete Metamorphosis- the young look like a totally different organism than the adult
Isopods- terrestrial, freshwawter, marine species; pill bugs, woodlice,
Decapods- Cuticle hardened by calcium carbonate; lobsters, crayfish, shrimp, crabs
Copepods- planktonic crustaceans; food for bigger animals

33.5 Echinoderms and chordates are deuterostomes
  • Deuterostomia is DNA similarities, not developmental

  • slow-moving marine animals
  • water vascular system
  • tube feet
  • usually five spokes from a center base (star fish)
  • 6 classes: sea stars (asteroidea), brittle stars (ophiuroidea), sea urchins and sand dollars (echinoidea), sea lilies and feather stars (crinoidea), sea cucumbers (holothuroidea), and sea daisies (concentricycloidea)

  • star shaped
  • multiple arms
  • mouth towards substrate

  • noticeable central disk
  • long, flexible arms
  • digestive system is not complete

  • spherical or disk shaped
  • no arms
  • 5 rows of tube feet
  • mouth has jaw-like structure

  • feathered arms
    Try to match each class of phylum echinodermata to a picture above!
  • they support the upward mouth
  • suspension feeders

  • cucumber shaped body
  • 5 rows of tube feet
  • feet around mouth are used as tentacles
  • no spine
  • armless
  • disk-shaped body
  • digestive system is not complete
  • live on wood underwater

Additional websites on invertebrates that may be helpful:
National Geographic
Animal Planet
Photos of Invertebrates

Here are a couple of recent articles about invertebrates that I found kind of interesting:
Ants Team up to Stay Dry
Tired Bees Make Poor Dancers
Radiation Release Will Hit Marine Life

How much did you learn? Here are some quizes:

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