Origins of Multicellular
Animals
Three Hypotheses
• Syncytial ciliate hypothesis
– Ancestor is single celled ciliate with multiple
nuclei
• Colonial flagellate hypothesis
– Ancestor is colonial flagellate like Volvox
• Polyphyletic hypothesis
– There may be multiple ancestors
THE PHYLUM
PORIFERA
INTRODUCTION TO PORIFERA
• unusual animals
• originally thought they were plants
• Are primarily marine, mostly in
shallower waters
• Are sessile and attached to substrate or
objects- occasionally on other animals
such as crabs
• Are the most primitive metazoans and
have neither true tissues or organs
Morphology of Sponges
Sponge Cell Types
• Pinacocytes- outer cells; equivalent of epiderm
Sponge Anatomy
Porocyte
Amoebocyte
Pinacocyte
Choanocyte
Pechenik, 1996
Sponge Cell Types
• Pinacocytes- outer cells covering sponge; equivalent
of epidermis
• Choanocytes– similar to choanoflagellates
– collared cells with flagella - create water current and collect
food matter.
Sponge Anatomy
Porocyte
Amoebocyte
Pinacocyte
Choanocyte
Pechenik, 1996
Sponge Cell Types
• Pinacocytes- outer cells covering sponge; equivalent
of epiderm
• Choanocytes– similar to choanoflagellates
– collared cells with flagella - create water current and collect
food matter
• Amoebocytes– amoeba-like cells
– store, digest and transport food, excrete wastes, secrete
skeleton
– give rise to buds in asexual reproduction
Mesophyl
(=Mesenchyma)
• Beneath the pinacocytes - a gelatinous
protein layer
• it contains the skeletal material (ie.
spongin and spicules) and amoebocytes
Sponge Anatomy
Porocyte
Amoebocyte
Pinacocyte
Choanocyte
Pechenik, 1996
Types of Spicules
4 general types
• Monaxon- needle-like or rod-like; straight or
curved
Types of Spicules
4 general types
• Monaxon- needle-like or rod-like; straight or
curved
• Tetraxon- has 4 prongs
Types of Spicules
4 general types
• Monaxon- needle-like or rod-like; straight or
curved
• Tetraxon- has 4 prongs
• Triaxon or Hexaxon- 3 or 6 rayed
• Polyaxon- multiple short rods radiating from a
common center; burr shaped, star shaped or
like a child's jack.
– Some species have a mixture of types
Spongin
Give phylum its common name
• Some species have no spicules, but do
have spongin
• spongin is a type of hardened secreted
protein
• Some species have both spicules and
spongin
Three Basic Sponge Types
• Asconoid
• Syconoid
• Leuconoid
Asconoid Sponges
• most primitive and simplistic in structure
• have radial symmetry
• are tube shaped
Asconoid Sponge
two basic openings
• Ostia– incurrent pores that open into a central cavity
called the spongocoel
– it is lined with choanocytes or collar cells
• Osculum
– the opening of the spongocoel to the outside
– water leaves the sponge
Asconoid Sponge Design
• Imposes definite size limits to sponges
due to the problem of water flow
• The spongocoel contains such a large
volume of water that it is hard to push it
out rapidly
Syconoid Sponges
• next level of complexity
– walls are invaginated
– allowing for greater surface area over
which water can pass
• typically vase shaped like the asconoid
sponges
• radial symmetry
Syconoid Structure
• helps to rectify some of the water
movement problem
– increasing the surface area
• so there are more choanocytes to water volume
– decreasing the spongocoel volume
• these sponges able to get bigger than
asconoid
Leuconoid Sponges
• highest level of complexity in sponges
• lost radial symmetry and are very
irregular in shape and may attain large
sizes
• invaginated canals are even further
invaginated and folded to from small
flagellated chambers
Leuconoid Sponge Design
• further increase in surface area makes these
sponges highly efficient in moving and
filtering water
• spongocoel is gone except for canals that
lead to the osculum- or there may be a series
of excurrent openings
• the largest sponges; most hydrologically
efficient
Sponge Reproduction
• Sexual
• Asexual
Sexual Reproduction in
Sponges
• gametes formed by amoebocytes
• there are both hermaphroditic and dioecious
species
– most hermaphroditic species produce eggs and sperm
at different times so they do not self fertilize
• sperm is released into environment via osculum
and is brought in by another sponge via ostia
• fertilization takes place in parent sponge
• zygote is expelled - it drops to bottom and begins
to develop
Asexual Reproduction in
Sponges
• two types:
• Budding- fragmentation of body wall, buds
appear as outgrowth on sides of sponge
• when they reach a certain size they drop off and settle to
bottom to form a new sponge
• Gemmules- occurs only in freshwater
sponges
• gemmules are groups of food laden amoebocytes that
deposit a hard covering of spicules around them
• formation is triggered by environmental conditions such
as decreased temperatures
• they allow the sponge to pass the winter or periods of
drought
• after which the outer covering breaks open and a new
sponge develops
Osmoregulation and Excretion
in Sponges
• no special organs
• main waste is ammonia
– it is removed by water currents within the
sponge
HIGHER CLASSIFICATION
OF SPONGES
4 classes of sponges
Class Calcarea
• spicules composed of calcium
carbonate
• spicules are monaxons or tri or
quadraxons
• all three types of sponges exhibited
• All less than 10 cm high
– ex. Leucosolenia and Grantia
• found in shallow coastal waters
• all are marine
Class Hexactinellida
(glass sponges)
• Spicules of Silica and fused to form a lattice like
skeleton
• cup or vase shaped with well developed
spongocoel
• most beautiful example is Euplectella - venus
flower basket
• chiefly live in 500-1000 meter depth
• are syconoid sponges
• all are marine
• may have commensal relationship with shrimp where a male and female live inside the sponge;
get trapped inside when they out-grow the pores
of sponge
Class Demospongiae
• Largest class - 95% of sponges in this class
• spicules are silicious if present otherwise
skeleton is made of spongin
• variously shaped some are huge
• all are leuconoid
• all but one family is marine- Spongillidae- is
freshwater about 150 freshwater species
• this is the group from which we get our
commercial sponges
Class Sclerospongiae
• proposed in 1970 to include 6 species from
Jamaica
• have silicious spicules and spongin
• also have an outer covering composed of
calcium carbonate
• are leuconoid sponges