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Simple Invertebrates. Sponges, jellyfish and coral, flatworms, roundworms, and segmented worms too!. Phylum Platyhelminthes (Flatworms). More complex than sponges and cnidarians. Organisms with long, flat body. Clearly defined head with mouth. Some have senses, a few even have eyespots.

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simple invertebrates

Simple Invertebrates

Sponges, jellyfish and coral, flatworms, roundworms, and segmented worms too!

phylum platyhelminthes flatworms
Phylum Platyhelminthes(Flatworms)
  • More complex than sponges and cnidarians.
  • Organisms with long, flat body.
  • Clearly defined head with mouth.
  • Some have senses, a few even have eyespots.
  • Some move freely.
  • Live in ponds, streams, or in a host!
  • Many are parasites






Asexually-regeneration & fission


Cellular Level-


3 tissue layers




  • Simplest of all worms
  • 1 body opening
  • Flat bodies
  • Hermaphrodites
  • Most flatworms are parasites

Reproduce asexually-


Digestive system-




  • Triangular head with eyespots that sense light
  • Parasites
  • Feed on dead plant & animal matter

headhas hooks that attach to the host

can grow up to

18 feet long!

  • Long, flat ribbons
  • Parasites
  • Live in bodies of humans & animals
liver fluke
Liver fluke:

Humans can get liver fluke by eating raw or uncooked fish that contains the parasite.


  • Parasite
  • Live in the liveror bloodof an animal host
  • Animal hosts:
    • Snails
    • Fish
    • Humans
phylum nematoda roundworms
Phylum: Nematoda(Roundworms)
  • Another step up the ladder!
  • Live on land or in the water
  • Some even have a simple nervous system.
  • Some have a digestive system that runs through the entire body -- with two openings, one for food to enter and one for wastes to leave.
  • Most are parasites.
  • Some are harmless microscopic critters that live in the soil.
  • Examples: hookworm, heartworm, pinworm, Trichinella
  • Described as a “tube within a tube”
  • First animals with a complete digestive system
    • two body openings: mouth & anus
  • Parasite
  • Lives in the muscle tissue of pigs
  • Live and reproduce in the intestine of the host.
  • Prevention?
  • The disease is called Trichinosis

Hookworm eggs are passed in the feces of an infected person.

Hookworm infection is mainly acquired by walking barefoot on contaminated soil.


  • Parasite
  • Live in small intestine where it enters the bloodstream
  • From the bloodstream it enters the lungs, coughed up, then swallowed and enters the digestive system.


  • Parasite
  • Enters the blood of a dog through a mosquito bite!
  • Worm moves to the heart where they grow and reproduce.
  • They block the valves that lead to and from the lungs.
  • Parasite
  • Adult lives in the large intestine
  • Female migrates to the anal region to lay her eggs
  • Prevention?
phylum annelid
Phylum: Annelid
  • Live in fresh & salt water, streams, and soil
  • Examples:
    • Leech
    • Earthworm
  • Most common
    • Lubricus terrestris
  • Approximately 100 segments
  • Outer layer made of mucus to help the animal glide
  • Setae- tiny bristles that help in movement
  • Closed Circulatory system- 5 pairs of tube hearts
  • Digestive system-
    • Mouth
    • Pharynx
    • Esophagus
    • Crop
    • Gizzard
    • Intestine
    • Anus
  • Nervous system-
    • Ganglion
    • Nerve cord
  • Respiratory system-
    • No organs
    • O2 enters through the skin & CO2 leaves
  • Reproductive system- sexual
    • Sperm is exchanged at the clitellum
    • Clitellum- the band on the earthworm where the sex cells are located
how do earthworms help the environment
How do earthworms help the environment?
  • After organic matter is digested, the earthworm releases waste from their bodies called castings.
  • Castings contain many nutrients that the plant can use.
  • Some people even use earthworm castings as garden fertilizer.
  • Enrich and improve soil for plants, animals & even humans.
  • Create tunnels in the soil by burrowing, which aerates the soil to allow air, water and nutrients to reach deep within the soil.
phylum porifera sponges
Phylum Porifera(Sponges)
  • Simplest animals of all (some of the oldest too).
  • All are aquatic (freshwater and marine).
  • Only two layers of cells, no tissues or organs!
  • Their skeletons are made of tiny spines called spicules.
  • Cannot move (grow attached to the bottom).
  • Mostly filter feeders.
  • Uses?
phylum cnidaria aka coelenterata jellyfish and coral
Phylum Cnidaria (aka Coelenterata): Jellyfish and Coral
  • More advanced than sponges. (for example, they have tissues, but still no organs).
  • Their body is a hollow cavity with only one opening (for food AND wastes)!
  • Entirely aquatic (freshwater and marine).
  • All have stinging cells, but not all can harm people.
  • Some are pretty nasty predators. Others filter-feed on small particles and organisms in the water.
medusae vs polyps

Ex. Jellyfish

A medusa is free-swimming.

Most are solitary.

Their opening is on the bottom of their body.


Ex. Coral

Polyps are like an upside down medusa.

Their opening is on the top of their body.

Coral polyps are tiny and live in huge colonies of thousands.

Corals build their “home” as the colony grows larger and larger.

Medusae vs. Polyps

Most jellyfish are harmless to people, although not so to small fish and other marine organims.


One teaspoon of garden soil may contain as many as 10,000 tiny nematodes (roundworms) in it!

phylum annelida segmented worms
Phylum Annelida(Segmented Worms)
  • Most complex worms of all.
  • Tube-like body divided into segments.
  • Sophisticated digestive system! (mouth, crop and gizzard, intestine, anus)
  • Many other specialized organs heart, blood vessels, nerve cords, simple brains)
  • Can move well.
  • Some aquatic (leeches, clam worms), some terrestrial (earthworms).
  • Some are parasites (leeches), others are very important decomposers (earthworms).
  • Any other uses for annelids?
so now what do you know about the simple invertebrates
So NOW what do you know about the simple invertebrates?
  • Do you know the names of the five phyla of simple invertebrates?
  • Do you know the main characteristics of each of the five phyla?
  • Can you identify examples of animals in each of the five phyla?