Multicellular organisms
Tissue Levels of organization
Origins of Multicellularity
• Multicellular organisms first appeared 600 million years
ago
• Arose quickly 100 million years prior to the cambrian
period
• Two hypothesis on the origin of multicellularity
– Colonial hypothesis- cells of a dividing protist remained
together, cell invagination formed two cell layers, supported by
the colonial organization of some protozoa with radial symmetry
– Syncytial hypothesis- a large multinucleated protista developed
plasma membranes separating into multiple cells, multinucleate
bilateral ciliates support this hypothesis
• Animal kingdom is monophyletic and the most likely
protista ancestor is the Choanocyte ( collar cell )
Phylum Porifera
• Sponges- mostly marine animals consisting of loosely
organized cells
• No tissues or organs, 9000 species, Asymetrical or
radially symmetrical, Sessile
• Filter feed by a series of canals and chambers thru
body wall where water circulates
• Three Classes
– Calcarea- Calcium carbonate spicules, 3-4 rays
– Hexactinellida – Silica spicules, 6 rays, deep water sponges
– Demospongia- Brillantly colored, Siliceous spicules, needle
or 4 rayed or spongin or both, Bath sponges
CALCAREA
HEXACTINELLIDA
DEMOSPONGIA
Sponge Anatomy
• Simple but more than colonies of independent cells
• Division of labor- cells specialized for particular
functions
• Pinacocytes- flat cells line outer surface, can change
shape ( contraction ), can regulate water circulation
• Mesohyl- Jellylike layer, contains Ameboid cells which
are specialized for reproduction, secrete spicules,
Transport and store food, form contractile rings
• Choanocytes ( collar cells)- flagellated cells with a collar
like ring of microvilli forming a mesh, Line inner
chambers
CHOANOCYTES
Water Currents and Body forms
• Water currents created by Choanocytes beating
their Flagella bring food and oxygen, carry away
wastes
• Food consists of bacteria, microscopic Algae,
protists, and suspended organic matter, some
deepwater sponges capture small crustaceans
using spicule covered filaments
• Large populations of sponges help reduce turbidity
of coastal waters
• Pinacocytes in incurrent canals may phagocytize
larger food particles
• Sponges absorb dissolved nutrients from seawater
by active transport
Body forms
– Ascon- Simplest, Vase shaped, Outer opening ostia
lead to spongocoel chambers and then osculum
inside exit
– Sycon- Folded body wall, water enters thru dermal
pores to incurrent canals then to radial canals with
choanocytes, exits to spongocoel and out osculum
– Leucon- Extensively branched canal system, water
enters thru ostia to branched canals to choanocyte
lined chambers, multiple exit points osculum
Body Functions
• No nerve cells so reactions result from cells
responding to a stimulus
• Water circulation minimal at sunrise and
maximum at sunset, light inhibitits
constriction of pinacocytes
• Water circulation can cease suddenly,
choanocytes stop together
• Internal communication is present by chemical
messages
Reproduction
• Most sponges Monocious, both sexes, no self
fertilization, produce egg and sperm at different times
• Some choanocytes can loose collar and flagella, or
ameboid cells undergo meiosis and form sperm or eggs
• Choanocytes capture sperm and transport it to egg,
early development in mesohyl, Flagellated larva
blastula released, 2 days settles to bottom, turns inside
out
• Asexual reproduction, release gemmule capsules of
ameboid cells, Fragmentation of adult
GEMMULE
Cnidarians
• Radial or Biradial symmetry, no anterior or
posterior, direction based on mouth position,
oral mouth and aboral opposite
• Over 9000 species mostly marine, important
ecosystems ( coral Reefs )
• No Brain, nerve net system
• Gastrovascular Cavity
• Diploblastic tissue layers, Epidermis and
Gastrodermis, mesoglea (jelly) in between
Cnidarian Anatomy
• Ectoderm, epidermis- protection, food
gathering
• Endoderm, gastroderm- coordination,
movement, digestion, absorption and
reproduction
• Mesoglea- Jelly Layer, May be noncellular or
contain wandering mesenchyme cells.
Cnidocytes
• Found in epiderm and gastroderm, 30 types
• Used for attachment, defense and feeding
• Cnidia- fluid filled intracellular capsule attached
to a hollow tube
• Operculum- lidlike cap on cnidia
• Cnidocil- modified cilium trigger discharges (
harpoon ) using water pressure
• Nematocyst- harpoon, uses spines-barbs and
long tube to inject paralyzing toxins into prey,
ejects from cell by inverting like a sweater sleeve
CNIDOCYTES
Scyphozoa
• All marine, “True Jellyfish” because dominant
stage in life is Medusa
• Mesoglea contains ameboid cells
• Cnidocytes in gastrodermis and epidermis
• Most harmless to Humans others can cause
painful stings
• Gastrodermal cells possess cilia to circulate
seawater and digested food
Scyphozoa
• Aurelia Libiata- very common Atlantic and Pacific
• A plankton feeder, cilia on bottom carry food to
mouth
• Mouth leads to 4 gastric pouches then to radial
canals out to margin of bell
• Rhopalium (notches) along bell contain olfactory
sensory pits, statocyst and photorecptors
• Exhibits distinct phototaxis
SCYPHOZOANS
Hydrozoan
• Small common cnidarians, some freshwater
• Usually anchor to bottom substrate, Polyp
stage dominates in most
• Nematocysts only in epidermis
• Gametes epidermal released outside body
• Mesoglea acellular
• Many have colonial polyps, specialized for
feeding, budding, or defending the colony
HYDROZOAN
Cubozoan
• Medusa is cuboidal, tentacles hang from each
corner of bell
• Active swimmers and feeders
• Polyp stage very small or absent
• Warm tropical waters
• “Box Jellyfish” Austrailian waters
• Contains photoreceptors
CUBOZOAN
BOX JELLYFISH STINGS
PORTUGUESE MAN OF WAR
PORTUGUESE MAN OF WAR
PORTUGUESE MAN OF WAR STING
Anthozoa
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Include Anemones, and Stony and soft corals
Are colonial or solitary and lack Medusa
Cnidocytes lack cnidocils ( triggers )
Mouth leads to pharnyx then gastrovascular
cavity
• Mesenteries divide gastrovascular cavity into
sections
• Mesoglea contains ameboid cells
• Externally show radial symmetry, internally
biradial
Sea Anemones
• Solitary or large colorful colonies
• Attach to solid substrate, some burrow in
sand, some symbiotic relationships
• Attaches to substrate with pedal disk
• Oral disk contains mouth and tentacles
• Slitlike mouth ends have siphonoglyph
“ciliated tract” to move water into
gastrovascular cavity for hydrostatic skeleton
Sea Anemones cont.
• Locomotion by gliding on their pedal disk,
crawl on sides, walk on tentacles, some swim,
some float with bubble in pedal disk
• Feed on invertebrates and fish
ANTHOZOA
Stony and Soft Corals
• Similar to Anemones, lack siphonoglyphs
• Calcium carbonate cup exoskeleton secreted by
epithelial cells, polyps retract into cup when
threatened
• Symbiotic relationship with photosynthetic
dinoflagellate Zooxanthellae Algae
• Coral provides nitrogen and phosphorous for the
Algae and get carbon compounds in return
• Zooxanthellae promote calcium carbonate
deposition
Stony and Soft Corals
• Cover 0.17% of ocean bottom hold 25% of
oceans species
• Environmental disturbances, warm water,
stress corals causing them to loose
zooxanthellea and bleach
• Stony corals are connected to each other
under the cup, can share food
• Florida and Bahama reefs are 60% degraded
ANTHOZOA CORALS
Ctenophora
• Comb Jellies, Eight bands of cilia from oral to
aboral sides for locomotion
• Mesoglea is highly cellular, muscle cells
contained here, may be triploblastic
• Tentacles contain colloblasts adhesive cells to
capture prey, wipe tentacles across mouth
CTENOPHORA
Additional Considerations
• Sponge and Jellyfish fossils are found in the
oldest fossil deposits, the Ediacaran Formation
• Coral Reefs are one of the most endangered
habitats on earth
• Coral covers 0.17% of the ocean and yields
10% of the fish caught
• Contribute $375 Billion to the worlds economy
• Corals grow very slowly, are disturbed easily