Nature’s experiments with larger organisms without cellular differentiation are limited. Increasing the size of a cell causes problems of exchange; multicellularity avoids surfaceto mass problems. Cell assemblages in sponges are distinct from other metazoans; molecular evidence shows common ancestry.
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Origin of Metazoa
Evolution of the Metazoa Evolution of eukaryotic cell followed by diversification Modern descendants • Protozoa, plus multicellular animals Multicellular animals • Referred to collectively as metazoans Metazoans placed inOpisthokont clade
Positions in the Animal Kingdom Multicellular organisms are divided into three grades: Mesozoa Parazoa (phylum Porifera, phylum Placozoa) Eumetazoa (all other animal phyla)
Mesozoa and Parazoa are multicellular but lack germ layers of Eumetazoa.
They have a cellular level of organization Mesozoans are entirely parasitic but have a complex reproductive cycle. Placozoans are composed of two layers of epithelia with fluid and fibrous cells between them. Sponges are more complex and organized into incipient tissues with low integration.
Origin of Metazoa Three Theories of Unicellular Origin of Metazoans Metazoans arose from syncytial (multinucleate) ciliated forms Metazoans arose from a colonial flagellated form Metazoans are polyphyletic, derived from more than one group of unicellular organisms.
Syncytial Ciliate Theory The body form resembled modern ciliates with a tendency toward bilateral symmetry. This 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.
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Colonial Flagellate Theory Haeckel first proposed this in 1874. As cells in a colony became more specialized, the colony became dependent on them. The model is freeswimming planula larvae of cnidarians; it is radially symmetrical. Bilateral symmetry would have evolved when the planula larvae adapted to crawling on the floor.
Polyphyletic Origins Theory Sponges, cnidarians, ctenophores and remaining eumetazoans each evolved independently. This would not conform to any lineage suggested above.
Molecular Evidence Small subunit ribosomal RNA and biochemical pathways support the colonial flagellate hypothesis. Metazoans appear to be monophyletic and arising from choanoflagellates. The syncitial ciliate hypothesis is excluded because Metazoa are closer to eukaryotic algae and higher plants than to ciliates.
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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 Recent research indicates
Sponge genome contains elements that code for regulatory pathways of more complex metazoans Includes proteins involved in spatial patterning
Some hypothesize modern sponges are less morphologically complex than their ancestors
Origin of Metazoa
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Origin of Metazoa Choanoflagellates • Solitary or colonial aquatic eukaryotes s Each cell (choanocyte) has a flagellum surrounded by collar of microvilli s Beating the flagellum draws water into collar s Microvilli collect mostly bacteria
s Most are sessile s One species attaches to floating diatom colonies
s Strongly resemble sponge feeding cells s Much debate whether sponge choancytes are ancestral to choanoflagellates
• One approach to metazoan origins suggests transitional forms between protozoan ancestors and simple metazoans
Phylum Placozoa Composed entirely of one marine species Trichoplax adhaerens It has no symmetry, and no muscular or nervous organs. Cell layers include: a dorsal epithelium, a thick ventral epithelium of monociliated cells and nonciliated gland cells, and a space between containing fluid and fibrous cells. Molecular evidence suggests they are a sister group of Cnidaria.
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Phylum Porifera Porifera means "porebearing"; their saclike bodies are perforated by many pores. They are sessile and depend on water currents to bring in food and oxygen and carry away wastes.
General Features Body is a mass of cells embedded in gelatinous matrix and stiffened by spicules of calcium carbonate or silica and collagen. They have no organs or tissues; cells are somewhat independent. They have no nervous or sense organs and have simplest of contractile elements. They are aside from the mainstream of animal evolution; thus they are often called Parazoa.
General Features (continued) Sponges vary from a few millimeters to 2 meters across; they vary greatly in shape and color. Most of the 5000 species are marine; about 150 are freshwater. Morphology changes with substratum, calmness of water, etc. Sponges are ancient; fossils extend to Cambrian or earlier.
Form and Function Body openings consist of small incurrent pores or ostia and a few excurrent oscula. Openings are connected by a system of canals; water passes from ostia to osculum. Choanocytes or flagellated collar cells line some of the canals.
They keep the current flowing by beating of flagella. They trap and phagocytize food particles passing by.
The framework of the sponge is composed of needle like calcareous or siliceous spicules or organic spongin fibers.
There are three types of canal systems. Asconid: Flagellated Spongocoels
Asconoids are simplest; they are small and tubeshaped. Water enters a large cavity, the spongocoel, lined with choanocytes. Choanocyte flagella pull water through. All Calcarea are asconoids: Leucosolenia and Clathrina are examples.
Three types of canal systems Syconoids: Flagellated Canals
They resemble asconoids but are bigger with a thicker body wall. The wall contains choanocytelined radial canals that empty into the spongocoel. Water entering filters through tiny openings called prosopyles. The spongocoel is lined with epithelial cells rather than choanocytes. Food is digested by choanocytes.
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Syconoids (continued) Flagella force the water through internal pores called apopyles into the spongocoel and out the osculum. They pass through an asconoid stage in development but do not form highly branched colonies. The flagellated canals form by evagination of the body wall; this is developmental evidence of being derived from asconoid ancestors. Classes Calcarea and Hexactinellida have species that are syconoid; the genus Sycon (Grantia) is an example.
Three types of canal systems Leuconoids: Flagellated Chambers
These are most complex and are larger with many oscula. Clusters of flagellated chambers are filled from incurrent canals, and discharge to excurrent canals. Most sponges are leuconoid; it is seen in most Calcarea and in all other classes. The leuconoid system has evolved independently many times in sponges. This system increases flagellated surfaces compared to volume; more collar cells can meet food demands.
Types of Cells Sponge cells are arranged in a gelatinous matrix called mesohyl. Pinacocytes
These cells form the pinacoderm; they are flat epitheliallike cells. Pinacocytes are somewhat contractile. Some are myocytes that help regulate flow of water.
Figure 12.8
Choanocytes These are oval cells with one end embedded in mesohyl. The exposed end has a flagellum surrounded by a collar. A collar is made of adjacent microvilli forming a fine filtering device to strain food. Particles too large to enter the collar are trapped in mucous and moved to the choanocyte where they are phagocytized. Food engulfed by choanocytes is passed to neighboring archaeocytes for digestion.
Fig. 12.10
Archaeocytes These cells move about in the mesohyl. They phagocytize particles in the pinacoderm. They can differentiate into any other type of cell. Those called sclerocytes secrete spicules. Spongocytes secrete spongin. Collencytes secrete fibrillar collagen. Lophocytes secrete lots of collagen but may look like collencytes.
Types of Skeletons Collagen fibrils are found throughout intercellular matrix of sponges. Various Demospongiae secrete a form of collagen called spongin. Demospongiae also secrete siliceous spicules. Calcareous sponges secrete spicules of crystalline calcium carbonate. Glass sponges have siliceous spicules with six rays. Spicule patterns are important taxonomic features.
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Sponge Physiology Filtration Rates Leuconia, a small sponge, has 81,000 incurrent canals. It would have more than two million flagellated chambers. Expulsion of water carries wastes some distance away. Some large sponges can filter 1500 liters of water a day.
Sponge Physiology (continued) Particles are filtered nonselectively; choanocytes phagocytize 80%. Digestion is completely intracellular, primarily by archaeocytes. There are no excretory or respiratory organs; diffusion suffices. The only movements are very slow opening and closing of pores; nerve cells have not been demonstrated.
Asexual Reproduction External buds are small individuals that break off after attaining a certain size. Internal buds or gemmules are formed by archaeocytes that collect in mesohyl and are coated with tough spongin and spicules; they survive drought, freezing, etc.
Sexual Reproduction Most are monoecious with both male and female sex cells in one individual. Sperm arise from transformed choanocytes. In some Demospongiae and Calcarea, oocytes develop from choanocytes; others derive them from archaeocytes. Sponges provide nourishment to the zygote until it is released as a ciliated larva.
Sexual Reproduction (continued) In some, when one sponge releases sperm, they enter the pores of another. Choanocytes phagocytize the sperm and transfer them to carrier cells that carry sperm through mesohyl to oocytes. Some release both sperm and oocytes into water.
Development The freeswimming larva of sponges is a solid parenchymula.
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Calcarea and some Demospongiae have strange development.
A hollow amphiblastula develops with flagellated cells toward the interior. The blastula then turns inside out (inversion). Flagellated cells or micromeres of the larva are at one end; larger non flagellated macromeres are at the other end. Macromeres overgrow the micromeres. Flagellated micromeres become choanocytes, archaeocytes and collencytes; nonflagellated cells give rise to pinacoderm and sclerocytes.
Regeneration and Somatic Embryonogenesis
Sponges can regenerate wounded portions. Sponge fragments aggregate into new structures, this is somatic embryogenesis.
Representative Classes Class Calcarea (Calcispongiae) Class Hexactinellida (Hyalospongiae) Class Demospongiae
Class Calcarea These are calcareous sponges with spicules of calcium carbonate. The spicules are straight or have three or four rays. Most are small sponges with tubular or vase shapes. Asconoid, syconoid and leuconoid forms all occur.
Class Hexactinellida These are glass sponges with sixrayed spicules of silica. Nearly all are deepsea forms; most are radially symmetrical. Root spicules attach them to the substrate. Their spicular network forms a network; a trabecular net of living tissue made of a fusion of pseudopodia of archaeocytes forms the chambers opening to the spongocoel.
Class Hexactinellida (continued) They lack a pinacoderm or gelatinous mesohyll and myocytes are absent. The body of hexactinellids is composed of a single, continuous synctial tissue called a trabecular reticulum. (This is the largest continuous syncytial tissue known in Metazoa.) Collar bodies do not participate in phagocytosis; that process is accomplished by the primary and secondary reticula.
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Class Hexactinellida (conclusion) Chambers appear to correspond to both syconoid and leuconoid types. They are adapted to a deepwater habitat with a large and easy flow of water. Some scientists advocate placing hexactinellids in a subphylum separate from other sponges.
Class Demospongiae This class contains 95% of living sponge species. Spicules are siliceous but not six rayed; they may be absent or bound together by spongin. All are leuconoid and all are marine except for Spongillidae, the freshwater sponges. Freshwater sponges flourish in summer and die in late autumn, leaving gemmules. Marine demosponges are highly varied in color and shape. Bath sponges belong to a group that lacks siliceous spicules but have spongin skeletons.
