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Sponges and Placozoans. Chapter 12. Origin of Metazoa. Evolution of the eukaryotic cell was followed by diversification into many lineages including: Modern protozoans Plants Fungi Animals Multicellular animals are called metazoans . Uniramia. Crustacea. Echinodermata. Chelicerata.

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origin of metazoa
Origin of Metazoa
  • Evolution of the eukaryotic cell was followed by diversification into many lineages including:
    • Modern protozoans
    • Plants
    • Fungi
    • Animals
  • Multicellular animals are called metazoans.
dendrogram of major phyla



























Dendrogram of Major Phyla
  • Choanoflagellates are solitary or colonial protozoans with a flagellum surrounded by a collar of microvilli.
  • Choanoflagellates resemble sponge feeding cells (choanocytes).
  • Scientists are studying colony formation and cell-to-cell communication in choanoflagellates in search of clues to the evolution of multicellularity.
colonial flagellate hypothesis
Colonial Flagellate Hypothesis
  • Colonial Flagellate Hypothesis – metazoans descended from ancestors characterized by a hollow, spherical colony of flagellated cells.
    • Individual cells became specialized for different functions.
    • Radially symmetrical, similar to a blastula.
    • First proposed by Haeckel in 1874
phylum porifera
Phylum Porifera
  • Sponges, Phylum Porifera, are multicellular heterotrophs.
  • They are asymmetrical.
  • They lack true tissues and organs.
  • Molecular evidence suggests they do share a common ancestor with other animals.
    • Kingdom Animalia is monophyletic.
phylum porifera8
Phylum Porifera
  • Sponges are sessile animals that have a porous body and choanocytes.
    • Supported by a skeleton of tiny needlelike spicules and protein.
  • They live in both fresh and marine waters.
phylum porifera9
Phylum Porifera
  • Sponges range in size and shape.
    • Up to 2 meters in diameter!
    • Encrusting, boring, finger, tube or vase shaped.
  • Many organisms, including crabs, nudibranchs, mites, bryozoans, and fish live as commensals or parasites in sponges.
skeletal framework
Skeletal Framework
  • The skeletal framework of a sponge may be fibrous or rigid.
  • The fibrous part comes from collagen fibrils in the intercellular matrix.
    • Spongin
  • Rigid skeletons consist of needlelike spicules.
    • Calcareous
    • Siliceous
  • Composition and shape the spicules forms the basis of sponge classification.
suspension feeders
Suspension Feeders
  • Sponges are suspension feeders capturing food particles suspended in the water that passes through their body.
suspension feeders13
Suspension Feeders
  • Water flows in through incurrent pores called dermal ostia.
  • It flows past the choanocytes where food particles are collected on the choanocyte collar.
suspension feeders14
Suspension Feeders
  • Choanocytes take in small particles by phagocytosis. Protein molecules are taken in by pinocytosis.
  • Sponges can also absorb nutrients dissolved in the water.
canal systems
Canal Systems
  • Asconoid – the simplest canal system.
    • Choanocytes line the spongocoel.
    • Water enters through the ostia and exit through the large osculum.
    • Usually tube shaped.
    • Found only in the Class Calcarea.
canal systems16
Canal Systems
  • Syconoid – tubular body and singular osculum like asconoids.
  • The walls of the sponge are folded to form choanocyte lined canals.
    • Increased area for feeding.
  • Class Calcarea.
canal systems17
Canal Systems
  • Leuconoids – most complex, permits an increase in sponge size.
  • Choanocytes line the walls of small chambers where they can filter all the water that flows through.
  • Most sponges.
types of cells
Types of Cells
  • Absence of tissues & organs means that fundamental processes occur on the cellular level.
  • Respiration and excretion occur by diffusion in each cell.
  • Mesohyl is the gelatinous matrix containing skeletal elements & amoeboid cells.
types of cells19
Types of Cells
  • Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food.
types of cells20
Types of Cells
  • The choanocytes pass food particles to archaeocyte cells for digestion.
  • Digestion occurs entirely within cells, there is no gut.
  • Other cell types secrete spicules (sclerocytes), spongin (spongocytes), & collegen (collenocytes).
types of cells21
Types of Cells
  • Pinacocytes are thin, flat, epithelial-type cells that cover the exterior and some interior surfaces of the sponge.
    • Almost a true tissue.
  • Sponges have remarkable regeneration capabilities.
  • Regeneration following fragmentation is a form of asexual reproduction.
  • External buds can break off to form new sponges.
  • Internal buds (gemmules) in freshwater sponges can remain dormant in times of drought.
  • Most sponges are hermaphrodites meaning that each individual functions as both male and female.
    • Monoecious
    • Gametes are derived from choanocytes or sometimes archaeocytes.
  • Most sponges are viviparous.
  • After fertilization, the zygote is retained and is nourished by the parent. Ciliated larvae are later released.
  • Some are oviparous releasing gametes into the water.
  • Sponges in the class Calcarea and a few Demospongiae have an unusual developmental pattern where the embryo turns inside out.
    • Flagellated cells become choanocytes & archaeocytes.
    • Larger cells become pinacocytes.
class calcarea
Class Calcarea
  • Calcareous sponges (Class Calcarea) have spicules composed of calcium carbonate.
  • Small, usually vase shaped.
  • Asconoid, syconoid, or leuconoid in structure.
class hexactinellida
Class Hexactinellida
  • Glass sponges (Class Hexactinellida) are mostly deep sea forms.
    • Spicules are six-rayed and made of silica.
  • Hexactinellids lack a pinacoderm or gelatinous mesohyll.
  • Chambers appear to correspond to both syconoid and leuconoid types.
class hexactinellida28
Class Hexactinellida
  • Some advocate placing hexactinellids in a subphylum separate from other sponges.
  • Trabecular reticulum made of a fusion of archaeocyte pseudopodia - forms the chambers opening to spongocoel.
    • Trabecular reticulum is largest continuous syncytial tissue known in Metazoa.
  • Choanoblasts are associated with flagellated chambers.
  • Collar bodies do not participate in phagocytosis – this is the function of the primary and secondary reticula.
class demospongiae
Class Demospongiae
  • Class Demospongiae contains most of the sponge species.
  • Spicules are siliceous, but not six-rayed.
  • Spicules may be bound together by spongin, or absent.
  • All leuconoid, mostly marine.
class homoscleromorpha
Class Homoscleromorpha
  • A fourth class, Homoscleromorpha, was formed to contain sponges without a skeleton or with siliceous spicules without an axial filament
phylogeny and adaptive diversification
Phylogeny and Adaptive Diversification
  • Sponges appeared before the Cambrian.
    • Glass sponges expanded in the Devonian.
  • One theory - sponges arose from choanoflagellates.
    • However, some corals and echinoderms also have collar cells, and sponges acquire them late in development.
phylogeny and adaptive diversification33
Phylogeny and Adaptive Diversification
  • Molecular rRNA evidence suggests a common ancestor for choanoflagellates and metazoans.
    • Sponges and Eumetazoa are sister groups with Porifera splitting off before radiates and placozoans.
phylum placozoa
Phylum Placozoa
  • Trichoplax adhaerens is the sole species of phylum Placozoa (marine).
    • No symmetry
    • No muscular or nervous organs
  • Placozoans glide over food, secrete digestive enzymes, and absorb nutrients.
phylum placozoa35
Phylum Placozoa
  • Cell layers
    • Dorsal epithelium
    • Thick ventral epithelium of monociliated cells and nonciliated gland cells.
    • Space between the epithelia contain fibrous “cells” within a contractile syncytium.
  • Grell considers it diploblastic.
    • Dorsal epithelium represents ectoderm and ventral epithelium represents endoderm.