1. 12-1 CHAPTER 12 Sponges and Placozoans

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3. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-3 Origins of Multicellularity Advantages Nature’s experiments with larger organisms without cellular differentiation are limited Nature’s experiments with larger organisms without cellular differentiation are limited Increasing the size of a cell causes problems of exchange Increasing the size of a cell causes problems of exchange Multicellularity avoids surface-to-mass problems Multicellularity avoids surface-to-mass problems Cell assemblages in sponges are distinct from other metazoans Cell assemblages in sponges are distinct from other metazoans Molecular evidence demonstrates that sponges are phylogenetically grouped Molecular evidence demonstrates that sponges are phylogenetically grouped

4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-4 Origin of Metazoa Evolution of the Metazoa Evolution of the Metazoa Evolution of eukaryotic cell followed by diversification Evolution of eukaryotic cell followed by diversification Modern descendants Modern descendants Protozoa, plus multicellular animals Protozoa, plus multicellular animals Multicellular animals Multicellular animals Referred to collectively as metazoans Referred to collectively as metazoans Metazoans placed in Opisthokont clade Metazoans placed in Opisthokont clade

5. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-5 Origin of Metazoa Choanoflagellates Choanoflagellates Solitary or colonial aquatic eukaryotes Solitary or colonial aquatic eukaryotes Each cell (choanocyte) has a flagellum surrounded by collar of microvilli Each cell (choanocyte) has a flagellum surrounded by collar of microvilli Beating the flagellum draws water into collar Beating the flagellum draws water into collar Microvilli collect mostly bacteria Microvilli collect mostly bacteria Most are sessile Most are sessile One species attaches to floating diatom colonies One species attaches to floating diatom colonies Strongly resemble sponge feeding cells Strongly resemble sponge feeding cells Much debate whether sponge choancytes are ancestral to choanoflagellates Much debate whether sponge choancytes are ancestral to choanoflagellates One approach to metazoan origins suggests transitional forms between protozoan ancestors and simple metazoans One approach to metazoan origins suggests transitional forms between protozoan ancestors and simple metazoans

6. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-6 Origin of Metazoa Theories of Unicellular Origin of Metazoans Some propose Some propose Metazoans arose from syncytial (multinucleate) ciliated forms Metazoans arose from syncytial (multinucleate) ciliated forms Cell boundaries evolved later Cell boundaries evolved later Body form resembled modern ciliates with tendency toward bilateral symmetry Body form resembled modern ciliates with tendency toward bilateral symmetry Would resemble flatworms, but their embryology fails to show cellularization, and flatworms have flagellated sperm Would resemble flatworms, but their embryology fails to show cellularization, and flatworms have flagellated sperm This would mean that radial cnidarians had a bilateral ancestor This would mean that radial cnidarians had a bilateral ancestor

7. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-7 Origin of Metazoa Others suggest Others suggest Metazoans arose from a colonial flagellated form Metazoans arose from a colonial flagellated form Cells gradually became specialized Cells gradually became specialized First proposed by Haeckel (1874) First proposed by Haeckel (1874) As cells in a colony became more specialized As cells in a colony became more specialized Colony became dependent on them Colony became dependent on them Colonial ancestral form at first Colonial ancestral form at first Radially symmetrical Radially symmetrical Reminiscent of a blastula stage of development Reminiscent of a blastula stage of development Hypothetical ancestor was called a blastea Hypothetical ancestor was called a blastea Some believe ancestral forms similar to a gastrula existed Some believe ancestral forms similar to a gastrula existed Gastraea Gastraea Bilateral symmetry evolved when the planula larvae adapted to crawling Bilateral symmetry evolved when the planula larvae adapted to crawling

8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-8 Molecular Evidence By comparing the genomes or proteomes of simple metazoans like sponges with more complex taxa, scientists can discover what cell transmitters or morphogens the first metazoans possessed By comparing the genomes or proteomes of simple metazoans like sponges with more complex taxa, scientists can discover what cell transmitters or morphogens the first metazoans possessed Recent research indicates Recent research indicates Sponge genome contains elements that code for regulatory pathways of more complex metazoans Sponge genome contains elements that code for regulatory pathways of more complex metazoans Includes proteins involved in spatial patterning Includes proteins involved in spatial patterning Some hypothesize modern sponges are less morphologically complex than their ancestors Some hypothesize modern sponges are less morphologically complex than their ancestors Origin of Metazoa

9. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-9 Phylum Porifera General Features Sessile sponges are filter feeders Sessile sponges are filter feeders Porifera means “pore ‑ bearing” Porifera means “pore ‑ bearing” Sac-like bodies perforated by many pores Sac-like bodies perforated by many pores Use flagellated “collar cells”, or choanocytes, to move water Use flagellated “collar cells”, or choanocytes, to move water Body is efficient aquatic filter Body is efficient aquatic filter Approximately 15,000 species of sponges Approximately 15,000 species of sponges Most are marine Most are marine Few live in brackish water, 150 in fresh water Few live in brackish water, 150 in fresh water

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12. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-12 Phylum Porifera Marine sponges found in all seas at all depths and vary greatly in size Marine sponges found in all seas at all depths and vary greatly in size Many species are brightly colored because of pigments in dermal cells Many species are brightly colored because of pigments in dermal cells Embryos are free-swimming, adult sponges always attached Embryos are free-swimming, adult sponges always attached Some appear radially symmetrical but many are irregular in shape Some appear radially symmetrical but many are irregular in shape Some stand erect, some are branched, and some are encrusting Some stand erect, some are branched, and some are encrusting

13. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-13 Phylum Porifera Growth patterns often depend on characteristics of the environment Growth patterns often depend on characteristics of the environment Many live as commensals or parasites in or on sponges Many live as commensals or parasites in or on sponges Also grow on a variety of other living organisms Also grow on a variety of other living organisms Few predators Few predators Sponges may have an elaborate skeletal structure and often have a noxious odor Sponges may have an elaborate skeletal structure and often have a noxious odor

14. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-14 Phylum Porifera Skeletal structure of a sponge can be fibrous and/or rigid Skeletal structure of a sponge can be fibrous and/or rigid If present, rigid skeleton consists of calcareous or siliceous spicules If present, rigid skeleton consists of calcareous or siliceous spicules Fibrous portion Fibrous portion Collagen fibrils in intercellular matrix Collagen fibrils in intercellular matrix Several types of one form of collagen, spongin, exists Several types of one form of collagen, spongin, exists Composition and shape the spicules Composition and shape the spicules Forms the basis of sponge classification Forms the basis of sponge classification

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16. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-16 Phylum Porifera Fossil record of sponges dates back to the early Cambrian Fossil record of sponges dates back to the early Cambrian Living sponges traditionally assigned to 3 classes: Calcarea, Hexactinellida, and Demospongiae Living sponges traditionally assigned to 3 classes: Calcarea, Hexactinellida, and Demospongiae Members of Calcarea typically have calcium carbonate spicules with one, three, or four rays Members of Calcarea typically have calcium carbonate spicules with one, three, or four rays Hexactinellids are glass sponges with six-rayed siliceous spicules Hexactinellids are glass sponges with six-rayed siliceous spicules Members of Demospongiae have siliceous spicules, spongin, or both Members of Demospongiae have siliceous spicules, spongin, or both A fourth class, Sclerospongiae, was formed to contain sponges with massive calcareous skeletons and siliceous spicules A fourth class, Sclerospongiae, was formed to contain sponges with massive calcareous skeletons and siliceous spicules

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18. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-18 Phylum Porifera Form and Function Body openings consist of small incurrent pores or dermal ostia Body openings consist of small incurrent pores or dermal ostia Incurrent pores: Average diameter of 50 μm Incurrent pores: Average diameter of 50 μm Inside the body Inside the body Water is directed past the choanocytes where food particles are collected Water is directed past the choanocytes where food particles are collected Choanocytes (flagellated collar cells) line some of the canals Choanocytes (flagellated collar cells) line some of the canals Keep the current flowing by beating of flagella Keep the current flowing by beating of flagella Trap and phagocytize food particles passing by Trap and phagocytize food particles passing by

19. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-19 Phylum Porifera Sponges non-selectively consume food particles sized between 0.1 μm and 50 μm Sponges non-selectively consume food particles sized between 0.1 μm and 50 μm The smallest particles are taken into choanocytes by phagocytosis The smallest particles are taken into choanocytes by phagocytosis Protein molecules may be taken in by pinocytosis Protein molecules may be taken in by pinocytosis Two other cell types, pinacocytes and archaeocytes, play a role in sponge feeding Two other cell types, pinacocytes and archaeocytes, play a role in sponge feeding

20. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-20 Phylum Porifera Types of Canal Systems Types of Canal Systems Asconoids: Flagellated Spongocoels Asconoids: Flagellated Spongocoels Simplest body form Simplest body form Small and tube-shaped Small and tube-shaped Water enters a large cavity, the spongocoel Water enters a large cavity, the spongocoel Lined with choanocytes Lined with choanocytes Choanocyte flagella pull water through Choanocyte flagella pull water through All Calcarea are asconoids All Calcarea are asconoids Leucosolenia and Clathrina are examples. Leucosolenia and Clathrina are examples.

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23. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-23 Phylum Porifera Syconoids: Flagellated Canals Syconoids: Flagellated Canals Resemble asconoids but larger with a thicker body wall Resemble asconoids but larger with a thicker body wall Wall contains choanocyte-lined radial canals that empty into spongocoel Wall contains choanocyte-lined radial canals that empty into spongocoel Water enters radial canals through tiny openings called prosopyles Water enters radial canals through tiny openings called prosopyles Spongocoel is lined with epithelial cells rather than choanocytes Spongocoel is lined with epithelial cells rather than choanocytes Food is digested by choanocytes Food is digested by choanocytes

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25. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-25 Phylum Porifera Flagella draw water through internal pores called apopyles into the spongocoel and out the osculum Flagella draw water through internal pores called apopyles into the spongocoel and out the osculum Sycons pass through an asconoid stage in development but do not form highly branched colonies Sycons pass through an asconoid stage in development but do not form highly branched colonies Flagellated canals form by evagination the body wall Flagellated canals form by evagination the body wall Developmental evidence of being derived from asconoid ancestors Developmental evidence of being derived from asconoid ancestors Classes Calcarea and Hexactinellida have syconoid species (ex: Sycon) Classes Calcarea and Hexactinellida have syconoid species (ex: Sycon)

26. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-26 Phylum Porifera Leuconoids: Flagellated Chambers Leuconoids: Flagellated Chambers Most complex and are larger with many oscula Most complex and are larger with many oscula Clusters of flagellated chambers are filled from incurrent canals, and discharge to excurrent canals Clusters of flagellated chambers are filled from incurrent canals, and discharge to excurrent canals Most sponges are leuconoid Most sponges are leuconoid The leuconoid system The leuconoid system Evolved independently many times in sponges Evolved independently many times in sponges System increases flagellated surfaces compared to volume System increases flagellated surfaces compared to volume More collar cells can meet food demands More collar cells can meet food demands Large sponges filter 1500 liters of water per day Large sponges filter 1500 liters of water per day

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28. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-28 Phylum Porifera Types of Cells Types of Cells Sponge cells are arranged in a gelatinous matrix, mesohyl Sponge cells are arranged in a gelatinous matrix, mesohyl Connective “tissue” of sponges Connective “tissue” of sponges Absence of true tissues or organs requires that all fundamental processes occur at the level of individual cells Absence of true tissues or organs requires that all fundamental processes occur at the level of individual cells Only visible activities of sponges are Only visible activities of sponges are Slight alterations in shape, local contraction, propagating contractions, and closing and opening of incurrent and excurrent pores Slight alterations in shape, local contraction, propagating contractions, and closing and opening of incurrent and excurrent pores Movements occur very slowly Movements occur very slowly Still interesting in that they suggest a whole body response in organisms lacking organization above the cellular level Still interesting in that they suggest a whole body response in organisms lacking organization above the cellular level Apparently excitation spreads from cell to cell by an unknown mechanism Apparently excitation spreads from cell to cell by an unknown mechanism

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30. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-30 Phylum Porifera Choanocytes Choanocytes Oval cells with one end embedded in mesohyl Oval cells with one end embedded in mesohyl Exposed end has one flagellum surrounded by a collar Exposed end has one flagellum surrounded by a collar Collar consists of adjacent microvilli Collar consists of adjacent microvilli Forms a fine filtering device to strain food Forms a fine filtering device to strain food Particles too large to enter collar are trapped in mucous Particles too large to enter collar are trapped in mucous Moved to the choanocyte and phagocytized Moved to the choanocyte and phagocytized Food engulfed by choanocytes is passed to archaeocytes for digestion Food engulfed by choanocytes is passed to archaeocytes for digestion

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32. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-32 Phylum Porifera Archaeocytes Archaeocytes Move about in the mesohyl Move about in the mesohyl Phagocytize particles in the pinacoderm Phagocytize particles in the pinacoderm Can differentiate into any other type of cell Can differentiate into any other type of cell Sclerocytes secrete spicules Sclerocytes secrete spicules Spongocytes secrete spongin Spongocytes secrete spongin Collencytes secrete fibrillar collagen Collencytes secrete fibrillar collagen Lophocytes secrete collagen Lophocytes secrete collagen

33. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-33 Phylum Porifera Pinacocytes Pinacocytes Form pinacoderm Form pinacoderm Flat epithelial-like cells Flat epithelial-like cells Somewhat contractile Somewhat contractile Some are myocytes that help regulate flow of water Some are myocytes that help regulate flow of water

34. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-34 Phylum Porifera Cell Independence: Regeneration and Somatic Embryogenesis Cell Independence: Regeneration and Somatic Embryogenesis Sponges have a great ability to regenerate lost parts and repair injuries Sponges have a great ability to regenerate lost parts and repair injuries Complete reorganization of the structure and function of participating cells or bits of tissue occurs in somatic embryogenesis Complete reorganization of the structure and function of participating cells or bits of tissue occurs in somatic embryogenesis Process of reorganization differs in sponges of differing complexity Process of reorganization differs in sponges of differing complexity Regeneration following fragmentation is one means of asexual reproduction Regeneration following fragmentation is one means of asexual reproduction

35. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-35 Phylum Porifera Asexual reproduction can occur by bud formation Asexual reproduction can occur by bud formation External buds External buds Small individuals that break off after attaining a certain size Small individuals that break off after attaining a certain size Internal buds or gemmules Internal buds or gemmules Formed by archaeocytes that collect in mesohyl Formed by archaeocytes that collect in mesohyl Coated with tough spongin and spicules Coated with tough spongin and spicules Survive harsh environmental conditions Survive harsh environmental conditions

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37. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-37 Phylum Porifera Sexual Reproduction Sexual Reproduction Most are monoecious Most are monoecious Sperm sometimes arise from transformed choanocytes Sperm sometimes arise from transformed choanocytes In some Demospongiae and Calcarea In some Demospongiae and Calcarea Gametes develop from choanocytes Gametes develop from choanocytes Some derive gametes from archaeocytes Some derive gametes from archaeocytes Sponges provide nourishment to zygote until it is released as a ciliated larva Sponges provide nourishment to zygote until it is released as a ciliated larva

38. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-38 Phylum Porifera In some, one sponge releases sperm which enter the pores of another sponge In some, one sponge releases sperm which enter the pores of another sponge Choanocytes phagocytize the sperm Choanocytes phagocytize the sperm Transfer sperm to carrier cells Transfer sperm to carrier cells Transport sperm through mesohyl to oocytes Transport sperm through mesohyl to oocytes Some sponges release both sperm and oocytes into water Some sponges release both sperm and oocytes into water The free-swimming larva of sponges is a solid parenchymula The free-swimming larva of sponges is a solid parenchymula

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40. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-40 Phylum Porifera Calcarea and some Demospongiae Calcarea and some Demospongiae Hollow stomoblastula develops with flagellated cells oriented toward the interior Hollow stomoblastula develops with flagellated cells oriented toward the interior Blastula then turns inside out (inversion) Blastula then turns inside out (inversion) Flagellated cells or micromeres of larva located at anterior end Flagellated cells or micromeres of larva located at anterior end Larger non-flagellated macromeres located at posterior end Larger non-flagellated macromeres located at posterior end Macromeres overgrow the micromeres during metamorphosis during settlement Macromeres overgrow the micromeres during metamorphosis during settlement Flagellated micromeres become choanocytes, archaeocytes and collencytes Flagellated micromeres become choanocytes, archaeocytes and collencytes Nonflagellated cells give rise to pinacoderm and sclerocytes Nonflagellated cells give rise to pinacoderm and sclerocytes

41. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-41 Phylum Porifera Class Calcarea (Calcispongiae) Calcareous sponges with spicules of calcium carbonate Calcareous sponges with spicules of calcium carbonate Spicules are straight or have three or four rays Spicules are straight or have three or four rays Most are small with tubular or vase shapes Most are small with tubular or vase shapes Many are drab in color, but some are bright yellow, green, red, or lavender Many are drab in color, but some are bright yellow, green, red, or lavender Leucosolenia and Sycon are marine shallow- water Leucosolenia and Sycon are marine shallow- water Asconoid, syconoid and leuconoid body forms Asconoid, syconoid and leuconoid body forms

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43. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-43 Phylum Porifera Class Hexactinellida (Hyalospongiae) Glass sponges with six-rayed spicules of silica Glass sponges with six-rayed spicules of silica Nearly all are deep-sea forms Nearly all are deep-sea forms Most are radially symmetrical Most are radially symmetrical Stalks of root spicules attach them to substrate Stalks of root spicules attach them to substrate Spicular system forms a network Spicular system forms a network Trabecular reticulum made of a fusion of archaeocyte pseudopodia forms the chambers opening to spongocoel 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 Trabecular reticulum is largest continuous syncytial tissue known in Metazoa

44. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-44 Phylum Porifera Choanoblasts are associated with flagellated chambers Choanoblasts are associated with flagellated chambers Layers of the trabecular reticulum separate into a primary reticulum and a secondary reticulum Layers of the trabecular reticulum separate into a primary reticulum and a secondary reticulum Choanoblasts are located on the primary reticulum Choanoblasts are located on the primary reticulum Each has one or more process extending to collar bodies. Each has one or more process extending to collar bodies. Hexactinellids Hexactinellids Lack a pinacoderm or gelatinous mesohyll Lack a pinacoderm or gelatinous mesohyll Myocytes are absent Myocytes are absent

45. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-45 Phylum Porifera Chambers appear to correspond to both syconoid and leuconoid types Chambers appear to correspond to both syconoid and leuconoid types Adapted to a deep-water habitat with a large and easy flow of water Adapted to a deep-water habitat with a large and easy flow of water Some advocate placing hexactinellids in a subphylum separate from other sponges Some advocate placing hexactinellids in a subphylum separate from other sponges Collar bodies do not participate in phagocytosis Collar bodies do not participate in phagocytosis Function of the primary and secondary reticula Function of the primary and secondary reticula

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47. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-47 Phylum Porifera Class Demospongiae Contains 95% of living sponge species Contains 95% of living sponge species Spicules are siliceous but not six rayed Spicules are siliceous but not six rayed Absent or bound together by spongin Absent or bound together by spongin Leuconoid body form Leuconoid body form All marine except for Spongillidae, the freshwater sponges All marine except for Spongillidae, the freshwater sponges Freshwater sponges Freshwater sponges Widely distributed in well-oxygenated ponds and springs Widely distributed in well-oxygenated ponds and springs Flourish in summer and die in late autumn Flourish in summer and die in late autumn Leave behind gemmules Leave behind gemmules Reproduce sexually, but existing genotypes may also reappear annually from gemmules Reproduce sexually, but existing genotypes may also reappear annually from gemmules

48. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-48 Phylum Porifera Marine demosponges Marine demosponges Highly varied in color and shape Highly varied in color and shape Bath sponges Bath sponges Lacks siliceous spicules Lacks siliceous spicules Have spongin skeletons Have spongin skeletons

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50. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-50 Phylum Porifera Phylogeny and Adaptive Diversification Phylogeny and Adaptive Diversification Sponges appeared before the Cambrian Sponges appeared before the Cambrian Glass sponges expanded in the Devonian Glass sponges expanded in the Devonian One theory One theory Sponges arose from choanoflagellates Sponges arose from choanoflagellates However, some corals and echinoderms also have collar cells, and sponges acquire them late in development However, some corals and echinoderms also have collar cells, and sponges acquire them late in development Molecular rRNA evidence suggests Molecular rRNA evidence suggests Common ancestor for choanoflagellates and metazoans Common ancestor for choanoflagellates and metazoans Sponges and Eumetazoa are sister groups with Porifera splitting off before radiates and placozoans Sponges and Eumetazoa are sister groups with Porifera splitting off before radiates and placozoans

51. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-51 Phylum Porifera Classes of sponges Classes of sponges Distinguished on basis of spicule form and chemical composition Distinguished on basis of spicule form and chemical composition Phylogenetic studies indicate Phylogenetic studies indicate Sponges with calcareous spicules in class Calcarea belong in a separate clade than those with spicules made of silica Sponges with calcareous spicules in class Calcarea belong in a separate clade than those with spicules made of silica

52. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-52 Phylum Porifera Adaptive Diversification Adaptive Diversification Poriferans are a highly successful group Poriferans are a highly successful group Diversification centers on their unique water- current system and its degree of complexity Diversification centers on their unique water- current system and its degree of complexity New feeding mode has evolved for a family of sponges found in deepwater caves New feeding mode has evolved for a family of sponges found in deepwater caves Tiny hook-like spicules cover body Tiny hook-like spicules cover body Spicule layer entangles crustaceans Spicule layer entangles crustaceans Filaments of the sponge body grow over prey Filaments of the sponge body grow over prey Carnivores, not suspension feeders Carnivores, not suspension feeders Contain siliceous spicules, but lack choanocytes and internal canals Contain siliceous spicules, but lack choanocytes and internal canals Illustrates the non-directional nature of evolution Illustrates the non-directional nature of evolution

53. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-53 Phylum Porifera Classification Classification Class Calcarea Class Calcarea Class Hexactinellida Class Hexactinellida Class Demospongiae Class Demospongiae

54. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-54 Phylum Placozoa Trichoplax adhaerens Trichoplax adhaerens Sole species of phylum Placozoa (marine) Sole species of phylum Placozoa (marine) K. G. Grell (1971) described the phylum K. G. Grell (1971) described the phylum No symmetry No symmetry No muscular or nervous organs No muscular or nervous organs Cell layers Cell layers Dorsal epithelium Dorsal epithelium Thick ventral epithelium of monociliated cells and nonciliated gland cells Thick ventral epithelium of monociliated cells and nonciliated gland cells Space between the epithelia contain fibrous “cells” within a contractile syncytium Space between the epithelia contain fibrous “cells” within a contractile syncytium

55. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-55 Phylum Placozoa Glides over food, secretes digestive enzymes, and absorb nutrients Glides over food, secretes digestive enzymes, and absorb nutrients Grell considers it diploblastic Grell considers it diploblastic Dorsal epithelium represents ectoderm and ventral epithelium represents endoderm Dorsal epithelium represents ectoderm and ventral epithelium represents endoderm

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