Unsegmented Worm

Unsegmented Worms

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Phylum Platyhelminthes

  • Called flatworms because bodies are flattened dorso-ventrally

  • Acoelomate – solid bodies without a lined body cavity
  • Have 3 body layers — outer ectoderm, middle mesoderm, & inner endoderm
  • Bilaterally symmetrical
  • Show cephalization (concentration of sensory organs at anterior or head end)
  • Body cells exchange oxygen & carbon dioxide directly with environment by diffusion
  • Single opening into gastrovascular cavity; two-way digestive tract
  • Some are parasites & others are free-living
  • Parasitic worms have thick cell layer called tegument covered with a nonliving cuticle covering their bodies as protection inside hosts
  • Includes 3 classes — Turbellaria (planarians), Trematoda (parasitic flukes), & Cestoda (parasitic tapeworms

Class Turbellaria

  • Most are marine but includes freshwater planarian (Dugesia)


  • Spade-shaped at the anterior end & have two, light-sensitive eyespots
  • Can sense light, touch, taste, & small
  • Have 2 clusters of nerve cells or ganglia to form a simple brain
  • Nervous system composed of a nerve net
  • Capable of simple learning
  • Move by tiny hairs or cilia over a mucus layer that they secrete
  • Feed by scavenging or protozoans
  • Have a single opening or mouth located at the end of a muscular tube called the pharynx which can be extended when feeding
  • Flame cells help remove wastes to excretory pores

  • Hermaphrodites that cross-fertilize eggs that are then deposited into a capsule until hatching in 2-3 weeks
  • Reproduce asexually by fragmentation

Class Trematoda

  • Includes parasitic flukes
  • About 1 cm long & oval shaped

  • Require a host to live
  • Have both oral & ventral suckers to cling to host & suck blood, cells, & body fluids
  • Oral sucker around mouth at anterior end sucks blood
  • May be endoparasites (live inside a host) or ectoparasites (live on the outside of host
  • Covered in tough, unciliated tegument
  • Nervous & excretory systems like turbellarians
  • Hermaphrodites
  • Have a long, coiled uterus that stores & releases 10,000+ eggs
  • Eggs released through genital pore & develop into larva
  • Show complex life cycles
  • Life cycle of sheep liver fluke:
    * Adult liver flukes live in sheep liver & gall bladder where they mate & form eggs
    * Eggs enter intestines, pass out with feces, & hatch in water
    * Larva enter snails, asexually multiply, then leave snail & form cysts
    * Cysts (dormant larva with hard, protective covering) clings to grass
    * Sheep ingest cysts when they eat grass
    * Cysts hatch in digestive tract & bore through intestines into bloodstream
    * Mature & reproduce in the liver

  • Schistosomiasis (disease caused by parasitic blood flukes) infects people in Asia, Africa, & South America causing intestinal bleeding & tissue decay that can result in death

Class Cestoda

  • Includes tapeworms
  • Adapted for parasitic life
  • Tough outer tegument prevents being digested by host
  • Anterior end called scolex contains hooks & suckers for attachment to intestine of host

  • Long, ribbon-like bodies up to 12 m in length
  • Nervous system extends length of body but lacks sense organs
  • Lacks mouth & digestive tract but absorbs digested nutrients from host
  • Grows by making body segments called proglottids
  • Each proglottid produces eggs & sperm that cross-fertilize with other segments & also self-fertilize (hermaphrodites)
  • Oldest, mature proglottids containing eggs at posterior end break off & pass out with feces
  • Life cycle of beef tapeworm:
    * Cattle eat grass with proglottids containing fertilized eggs
    * Eggs hatch into larva & bore through cow’s intestine into bloodstream
    * Larva burrow into cow’s muscle & form cysts
    * Humans eat beef (muscle) & cysts travels to intestines
    * Cyst breaks open & adult beef tapeworm forms


Phylum Nematoda

  • Called roundworms
  • Includes Ascaris, hookworms, Trichinella, & pinworms
  • Pseudocoelomates have fluid-filled body cavity partially lined with mesoderm
  • Pseudocoelom contains the body organs & provides hydrostatic skeletal support for muscles
  • Have long slender bodies that taper at both ends

  • Covered with flexible cuticle
  • Digestive tract with anterior mouth & posterior anus; called one-way digestive tract
  • Separate sexes in most species
  • Most are free living
  • Some are parasites on plants & animals
  • Ascaris is a parasitic roundworm living in the intestines of pigs, horses, & humans
  • Ascaris life cycle:
    * Enter body in contaminated food or water & hatch in intestines
    * Larva bore into bloodstream & carried to lungs & throat
    * Larva coughed up, swallowed, & return to intestines to mature & mate
    * Block the intestine causing death

  • Hookworm eggs hatch in moist soil & larva bore through bare feet of new host 
  • Trichinella are human parasites caused by eating undercooked pork containing the cysts
    * Cause disease called trichinosis
    * Cysts cause muscle pain & stiffness


Phylum Rotifera

  • Known as rotifers or wheel animals
  • Transparent, free-swimming microscopic animal
  • Freshwater & marine
  • Have a ring of cilia around mouth that rotates like a wheel to bring in food
  • Feed on unicellular algae, bacteria, & protozoa
  • Have a muscular organ called the mastax behind the pharynx to chop food
  • Nervous system composed of anterior ganglia & 2 long nerve cords
  • Show cephalization (head end)
  • Have 2 anterior, light-sensitive eyespots

Sponge & Cnidarian Study Guide

Study guide for Sponge, Cnidarians, & Ctenophores

·         Know relatives of the jellyfish
·         How are sponges different from other animals
·         Know characteristics of all invertebrates
·         Know characteristics of sponges
·         What is the function of collar cells in sponges
·         What are spicules
·         Know characteristics of adult sponges
·         Be able to explain skeletal support of sponges
·         How do sponges obtain their food
·         What helps draw water into a sponge
·         What is the function of amebocytes in sponges
·         How does excess water leave a sponge
·         What is the purpose of gemmules in sponges
·         What is a hermaphrodite
·         How can sponges reproduce
·         Know animals that capture prey by using nematocysts
·         What are the 2 distinct life stages of cnidarians
·         Describe nematocysts
·         What organisms have tentacles with stinging cells
·         Know examples of cnidarians
·         Describe the life of a planula larva
·         Know the life stage that is dominant in sea anemones
·         What organisms would be anthozoans
·         Know the dominant life stage of jellyfish
·         Know the main characteristics of ctenophores

Sponges & Cnidarian

Sponges, Cnidarians, & Ctenophores

Phylum Porifera

  • Includes marine & freshwater sponges
  • Found in the kingdom Animalia & subkingdom Parazoa
  • Sessile as adults
  • Simplest of all animals

  • Contain specialized cells, but no tissue
  • Asymmetrical
  • Bodies filled with holes or pores for water circulation
  • Marine sponges are larger & more colorful than freshwater sponges
  • Range in size from 2 centimeters to 2 meters
  • Osculum is single, large body opening at the top for water & wastes to leave
  • Spongocoel is the body cavity of sponges
  • Have only 2 cell layers (ectoderm & endoderm) separated by jellylike material
  • Flagellated cells called choanocytes or collar cells line their internal body cavity
  • Flagella of choanocytes beat & pull in water containing food which the collar traps


  • Spongin is a network of flexible, protein fibers making up the sponge’s skeleton
  • Spicules are tiny, hard particles shaped like spikes or stars in the  skeleton of some sponges
  • Spicules are made of calcium carbonate or silica


  • Sponges are filter feeders that remove plankton (food) from the water that is brought in through pores lined with collar cells
  •  Flagella pull in bacteria, protozoans, & algae that sticks to collar of choanocytes where it is digested
  • Amebocytes are specialized cells in sponges that can roam to pick up food from choanocytes & distribute it to all other parts of the sponge
  • Amebocytes also transport carbon dioxide & wastes away from sponge cells
  • Excess water & food leaves through the excurrent osculum


  • Sponges can reproduce asexually by external buds that break off & form new sponges or stay attached to form sponge colonies
  • Gemmules are specialized, internal buds formed by sponges during cold or dry weather that can survive harsh conditions
  • Gemmules consist of a food-filled ball of amebocytes surrounded by a protective coat with spicules & released when adult sponge dies
  • Gemmules break open when conditions improve & the cells form new sponges

  • Sponge can also asexually regenerate missing parts or a new sponge from a small piece of sponge
  • Sponges are hermaphrodites (produce both eggs & sperm), but they exchange sperm & cross-fertilize eggs during sexual reproduction
  • Planula is the flagellated, free-swimming larva that forms from the zygote
  • Planula larva eventually settles to the bottom & attaches to develop into an adult, sessile sponge

Classes of Sponges

  • Calcarea are chalky sponges with calcium carbonate spicules
  • Hexactinella includes glass sponges & the Venus flower basket with silica spicules
  • Demospongiae include horny & bath sponges with only spongin or spongin & silica spicules
  • Sclerospongiae are coral sponges & have spongin & silica and calcium carbonate spicules

Phylum Cnidaria

  • Includes marine organisms such as jelllyfish, Portuguese man-of-war, coral, sea anemone, & sea fans
  • Hydra is a freshwater cnidarian

  • All carnivorous
  • Have 2 cell layers (epidermis -outer & gastrodermis-inner) with a hollow body called gastrovascular cavity
  • Contain a jelly-like layer between epidermis 7 gastrodermis  called mesoglea
  • Single opening (mouth/anus) to gastrovascular cavity where food & water enter & wastes leave; called two-way digestive system
  • Have tentacles around mouth to pull in water & capture food

  • Have a simple nerve net with to help with movement & senses
  • Sessile members include corals, sea anemones, & sea fans
  • Have radial symmetry as adults

  • Contain stinging cells called cnidocytes in their tentacles that contain coiled stingers called nematocysts that can shoot out & paralyze prey 

Body Forms

  • Have 2 basic body forms —polyp & medusa






  • Polyp forms are usually sessile with upright tentacles arranged around the mouth at the top and with a thin layer of mesoglea
  • Polyps are the asexual stage
  • Corals, hydra, & sea anemones exist in the polyp form as adults 


  • Medusa forms are usually free-swimming, bell-shaped animals with tentacles that hang down around the mouth and with a thick layer of mesoglea for support
  • Medusa are the sexual stage
  • Jellyfish & Portuguese man-of-war are medusa form as adults
  • Some cnidarians are dimorphic or go through both polyp & medusa stages in their life cycle

Life cycle of a jellyfish

  • Some are solitary (Hydra) others are colonial (corals)
  • Three classes include Hydrozoa (hydra), Scyphozoa (jellyfish), & Anthozoa (sea anemones & corals)


  • Includes freshwater, sessile hydra (exists only as polyps) 
  • Portuguese man-of-war (exists as colony of polyps & medusa)
  • Group of cells called basal disk produces sticky secretion for attachment & can secrete gas bubbles to unattach & let hydra float
  • Hydra also move by somersaulting (tentacles bend over to bottom as basal disk pulls free)
  • Tentacles pull food into gastrovascular cavity where enzymes digest it
  • Reproduce asexually by budding during warm weather & sexually in the fall
  • Hermaphrodites that release sperm into water to fertilize eggs of another hydra



  • Includes bell-shaped jellyfish
  • Medusa stage is dominant in the life cycle
  • Tentacles may be meters in length & carry poisons that cause severe pain or death
  • Have both asexual polyps & sexual medusa stages in their life cycles
  • Adult medusa stage releases eggs & sperm into water
  • Fertilization produces ciliated planula larva that settles to the bottom, attaches, & forms tentacles 
  • New medusa bud off of reproductive polyps & form adult jellyfish

jellyfish life cycle photo


  • Include corals in a limestone case & sea anemones
  • Called “flower animals”
  • All marine
  • Sea anemone is a sessile, polyp-form that uses its tentacles to paralyze fish
  • Some anemones in the Pacific Ocean live symbiotically with the clownfish sharing food & protecting each other


  • Corals are small, colonial polyps living in limestone cases
  • Coral reefs form as polyps die & provide a home and protection for other marine animals
  • Reefs form in warm, shallow water & only the top layer has living polyps
  • Algae may live symbiotically with coral supplying them with oxygen

Phylum Ctenophora


  • All marine
  • Includes comb jellies

  • Have eight rows of fused cilia called “comb rows”
  • Largest animal to move by cilia
  • Move by beating cilia
  • Lack cnidocytes but have cells sticky cells called colloblasts that bind to prey
  • Colloblasts located on two ribbon-like tentacles
  • Have sensory structure called apical organ to detect direction in the water
  • Most are hermaphrodites (make eggs & sperm)
  • Produce light by bioluminescence