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