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Sponges and Placozoans
Origin of Metazoa
Evolution of the eukaryotic cell was followed by diversification into many lineages including:Modern protozoansPlantsFungiAnimals
Multicellular animals are called metazoans.
PoriferaPorifera
ActinopodaActinopoda
CnidariaCnidaria
ApicomplexaApicomplexa
CiliophoraCiliophora
MolluscaMolluscaAnnelidaAnnelida
EuglenozoaEuglenozoa
BryozoaBryozoa
PlatyhelminthesPlatyhelminthes
RhizopodaRhizopoda
RotiferaRotifera
GranuloreticulosaGranuloreticulosa
NematodaNematodaPriapulidaPriapulidaKinorhynchaKinorhynchaLoriciferaLoricifera
PhoronidaPhoronidaBrachiopodaBrachiopoda
EchinodermataEchinodermata
HemichordataHemichordata
ChelicerataChelicerata
UniramiaUniramia CrustaceaCrustacea
ProtochordataProtochordata
VertebrataVertebrata
Dendrogram of Major Phyla
Choanoflagellates
Choanoflagellates are solitary or colonial protozoans with a flagellum surrounded by a collar of microvilli.
Choanoflagellates
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.
Resulted to two origins of multicellularity: Syncitial ciliated and Colonial flagellated
Syncitial Ciliate Hypothesis
Syncitial ciliate hypothesis – metazoans arose from an ancestor shared with single celled ciliates.Recall multiple nuclei in reproducing ciliates.Later, each nucleus becomes partitioned.Trend toward bilateral symmetry as in
flatworms.
Syncitial Ciliate Hypothesis
Problems: In flatworm embryology nothing like
cellularization occurs.Does not explain flagellated sperm in
metazoans. Implies that radial symmetry is derived.histoincompatibility
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
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 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 Porifera
Sponges range in size and shape. Up to 2 meters in diameter! Encrusting, boring, finger, tube or vase
shaped.
Neighbors
Many organisms, including crabs, nudibranchs, mites, bryozoans, and fish live as commensals or parasites in sponges.
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
Suspension Feeders
Sponges are suspension feeders capturing food particles suspended in the water that passes through their body.
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 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
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 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 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
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 Cells
Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food.
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 Cells
Pinacocytes are thin, flat, epithelial-type cells that cover the exterior and some interior surfaces of the sponge.Almost a true
tissue.
Reproduction
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.
Reproduction
Most sponges are hermaphrodites meaning that each individual functions as both male and female.MonoeciousGametes are derived from choanocytes or
sometimes archaeocytes.
Reproduction
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.
Reproduction
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
Calcareous sponges (Class Calcarea) have spicules composed of calcium carbonate.
Small, usually vase shaped. Asconoid, syconoid, or leuconoid in structure.
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 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 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.
Cladogram of Sponge Classes
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 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
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 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.
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