Category: Invertebrate Unit

Unsegmented Worms

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Phylum Platyhelminthes
Characteristics

• 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)

Planarians

• 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

BEEF TAPEWORM LIFE CYCLE

Phylum Nematoda
Characteristics

• 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

 CYSTS IN CONTAMINATED PORK

Phylum Rotifera
Characteristics

• 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

Sponges, Cnidarians, & Ctenophores

Phylum Porifera
Characteristics

• 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

Feeding

• 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

Reproduction

• 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
Characteristics

• 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 

 

MEDUSA

POLYP

 

• 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 

CORAL POLYPS

• 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

JELLYFISH LIFE CYCLE

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

Hydrozoa

• 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

HYDRA

Scyphozoa

• 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

Anthozoa

• 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

Characteristics

• 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