41.0 Introduction

1. Animals Are Among the Most Abundant Living Things
1. Noncoelomates Are Simple Animals that Lack a Coelom Body Cavity fig 41.1
2. Basic Animal Body Plan Evolved First in these Animals

1. Some General Features of Animals
1. Animals Depend on Other Organisms for Nourishment
1. Feed directly or indirectly on plants, algae or autotrophic bacteria
2. Animals are mobile in their search for food fig 41.2
3. Food is ingested and digested in an internal cavity

2. Major Characteristics of Animals
1. Multicellular heterotrophs
1. Encompass all animal species
2. Unicellular heterotrophs, "Protozoa," are classified as Protists

2. Diverse in form
1. Most animals are invertebrates
2. Only 1% of all species are vertebrates
3. Include 35 phyla
1. Size ranges from microscopic forms to enormous whales
2. Most are marine, some are freshwater, few are terrestrial
3. Arthropods, mollusks and chordates dominate the land

3. No cell walls
1. Lack cell walls and are relatively flexible
2. Cells are generally organized into tissues, except for sponges
3. Tissue: Collection of cells specialized to perform specific function

4. Active movement
1. Directly related to flexibility of cells
2. Flying is the most specialized form of locomotion

5. Sexual reproduction
1. Nonmotile eggs are much larger than motile sperm
2. Cells formed by meiosis function directly as gametes
3. Haploid cells fuse directly to form zygote
4. There is no animal counterpart to plant gametophyte or sporophyte

6. Embryonic development
1. Zygote becomes an adult through process of embryonic development
2. Zygote divides mitotically forming solid ball of cells, the morula
3. Divides further to form a hollow ball of cells, a blastula
4. This ball folds inward to form a hollow sac, a gastrula
5. Opening of sac called the blastopore
6. Cells subsequently grow and move in relation to one another
7. Details differ from one phylum to another, but provide clues regarding their evolutionary relationships

2. The Classification of Animals
1. Divided into Two Subkingdoms fig 41.3
1. Parazoa
1. Lack definite symmetry
2. Neither tissues nor organs are present
3. Consist primarily of sponges, phylum Porifera

2. Eumetazoa
1. Possess definite shape and symmetry
2. Have tissues organized into organs and organ systems
3. Include all other animals, 35 phyla

2. Comparison of Parazoa and Eumetazoa
1. Eumetazoa are generally more complex than sponges
2. Radially symmetrical Eumetazoa form two layers, are diploblastic
1. Outer ectoderm
2. Inner endoderm

3. Bilaterally symmetrical Eumetazoa form three distinct embryonic cell layers
1. Ectoderm, endoderm and mesoderm in between them
2. Layers differentiate into tissues of the adult
3. Sponges lack tissue layers

3. Subgrouping of Animals tbl 41.1
1. "Primitive" or "lower" invertebrates
1. Have less complex tissue organization
2. Comprise 14 phyla

2. Four major phyla
3. Porifera: No tissue organization = sponges
1. Cnidaria: Radially symmetrical = jelly fish, hydroids, sea anemones, corals
2. Platyhelminthes: Bilaterally symmetrical = flatworms
3. Nematoda: Free-living and parasitic forms = roundworms

3. Four Key Transitions in Body Plan
1. Radial Versus Bilateral Symmetry
1. Sponges are simple animals that lack definite symmetry, grow as irregular masses
2. Radial symmetry
1. Exemplified by two phyla
1. Cnidaria: Hydroids, jellyfish, sea anemones and corals
2. Ctenophora: Comb jellies

2. Body parts arranged around a central axis
3. Division through any plane of axis produces approximate mirror images fig 41.4a

3. Bilateral symmetry
1. All other animals are fundamentally bilaterally symmetrical
2. Bilateral organisms exhibit right and left halves, mirror images fig 41.4b
1. Possess dorsal (top) and ventral (bottom) halves
2. Differentiate anterior (front) and posterior (back)

4. Allows for differential adaptation of various parts of body
1. Different organs located in different parts of body
2. Echinoderm adults radially symmetrical, larvae bilaterally symmetrical
3. More efficient in seeking food and avoiding predators
4. Evolved various sensory organs generally grouped at head end

5. Nerve system composed of major longitudinal nerve cords
1. Grouped around anterior end of body
2. Evolution of cephalization

2. Pseudocoelom Versus Coelom
1. Importance of a body cavity
1. Coelom allows digestive tract to be longer than animal's body length
1. Allows for storage of undigested food, limits exposure to predators
2. Longer exposure of food to enzymes improves digestion
3. Tube-within-a-tube design allows for more flexibility and greater mobility

2. Internal body cavity provides space for expansion of gonads
1. Allows for accumulation of eggs and sperm
2. Advanced phyla able to evolve diverse reproductive strategies
3. Large numbers of gametes stored and released under favorable conditions

2. Kinds of body cavity fig 41.5
1. Acoelomate: Possess no body cavity
2. Pseudocoelomate: Body cavity located between mesoderm, endoderm
3. Coelomate: Body cavity entirely within mesoderm
1. Body cavity called coelom, animals called coelomates
2. Gut and internal organs suspended in coelom
3. Coelom surrounded by epithelium layer, derived from mesoderm
1. Parietal peritoneum lines outer wall
2. Visceral peritoneum lines internal organs within cavity

4. Requires development of sophisticated circulatory system
1. Network of vessels carries fluid, blood, to all parts of body
2. Blood carries nutrients and oxygen to tissues
3. Removes wastes and carbon dioxide from tissues
4. Circulation effected by contraction of muscular hearts
5. Open circulatory system: Blood mixes with body fluid
6. Closed circulatory system: Blood separate from body fluid

5. Supports various evolutionary relationships
1. Acoelomates could give rise to coelomates or be derived from them
2. Pseudocoelomate phyla could all have different origins

3. Advantages of a coelom
1. Success of coelomate body cavity stems from embryonic development
2. During primary induction primary tissues interact with each other
3. Coelomate body plan allows necessary contact between mesoderm and endoderm
1. Permits development of localized portions of digestive tract, i.e. stomach
2. Mesoderm and endoderm separated by body cavity in pseudocoelomates
3. Limits developmental interactions

3. Nonsegmented Versus Segmented Bodies
1. Body built from series of similar segments
1. Like prefabricated building
2. Segmentation obvious in mesoderm early on
3. Later reflected in endoderm and ectoderm

2. Advantages to early embryonic segmentation
1. Repetition of organ systems less lethal if one segment damaged
2. Locomotion more effective when segments can move independently

3. Segmentation important to organization of all advanced animals
1. Many arthropod segments are fused, apparent in embryologic development
2. Vertebrate backbone and muscle areas are segmented

4. Protostomes Versus Deuterostomes
1. Coelomates characterized into two groups by embryology
1. Dissimilar echinoderms and chordates share key embryological features
2. Four phyla share common ancestry, are deuterostomes
3. Remaining coelomates are protostomes
4. Deuterostomes clearly derived from protostomes early in their evolution

2. Protostome development fig 41.6
1. Invagination in blastula forms blastopore
2. Mouth develops from blastopore
3. Anus develops at other end of embryo

3. Deuterostome development
1. Anus forms from blastopore
2. Mouth develops at other end

4. Present two different cleavage patterns
5. Differences in developmental fate of cells
1. No single cell of protostome can develop into complete adult
2. Any cell of deuterostome can become an adult

1. The Sponges
1. General Biology of Sponges
1. Primarily marine species, fewer freshwater varieties fig 41.7
2. Few radially symmetrical, but most lack any symmetry
3. Many are colonial, all are sessile as adults
4. Cellular organization
1. Little coordination among cells
2. Simple mass of cells in a gelatinous matrix
3. Cells are specialized for different functions

5. Filter feeders
1. Water flows through system of pores and canals
2. Water forced through passageways
3. Water forced out through a larger pore called the osculum fig 41.8

6. Basic structure fig 41.9
1. Choanocytes
1. Specialized flagellated cells that face inward
2. Line internal cavity or specialized chambers in large sponges

2. Epithelial layer of flattened cells, frequently contractile in nature
3. Mesohyl
1. Intermediate gelatinous layer with amoeboid cells
2. May possess minute, needles called spicules
3. May possess fibrous spongin protein network

2. The Choanocyte
1. Structurally resembles a protist with a single flagellum
1. Independent beating of flagella creates water currents
2. Used to acquire food and oxygen and expel wastes
3. Body cavity inner wall may be convoluted to increase surface area

2. Microstructure
1. Base of flagellum surrounded by collar of hair-like projections
2. Strands of collar connected by microfibrils
3. Food particles in water filtered by collar
4. Food digested by collar cell or adjacent amoeboid cell

3. Reproduction in Sponges
1. Frequent reproduction by fragmentation
2. Sexual reproduction via production of egg and sperm
1. Larval sponges undergo development within adults
2. Have external choanocytes when released
3. Exist as free-swimming planktonic form for a short time
4. Settle on a suitable substrate to begin a sessile adult life

2. Radially Symmetrical Animals
1. Eumetazoans Have Definite Shape and Symmetry
1. Possess two distinct cell layers in embryo form
1. Outer ectoderm, inner endoderm
2. Give rise to basic body plan
3. Ectoderm produces body epidermal covering and nervous system
4. Endoderm produces gastrodermis, digestive tissue
5. Mesoglea lies between epidermis and gastrodermis, contains muscles

2. Eumetazoans divided into two main groups, one is radially symmetrical
1. Cnidaria: Hydroids, jellyfish, sea anemones and corals
2. Ctenophora: Comb jellies

2. The Cnidarians
1. Nearly all are marine, only a few are freshwater
2. Basically gelatinous, have tissues but no organs
3. Carnivores, capture food with tentacles that surround mouth
4. Exhibit two body forms fig 41.10
1. Polyp: Cylindrical, generally attached to a substrate
1. Solitary or colonial
2. Mouth faces away from substrate, generally upward
3. May form hard internal or external skeleton

2. Medusa: Umbrella-shaped, free-floating
1. Mouth faces substrate, generally downward
2. Possess a thick jelly-like mesoglea, between epidermis and gastrodermis

3. May exist in polyp or medusa forms only or alternate between the two phases
4. Both phases are diploid

5. Reproduction
1. Polyps reproduce asexually by budding, form polyps or medusae
2. Sexual reproduction produces fertilized eggs
3. Develops into a free swimming, multicellular, ciliated planula larva

6. Evolutionary advancement: Development of an internal digestive cavity fig 41.11
1. Digestive enzymes secreted into a primitive gut
2. Food broken into smaller particles
3. Particles further digested by cells lining gut
4. Enable cnidarians to digest food larger than individual cells
5. Undigested food particles exit gut through mouth

7. Organization of tissues
1. Nerve cells organized into nets to coordinate muscle contraction
2. No blood vessels
3. No respiratory system
4. No specialized internal cavity

8. Cnidaria possess cnidocytes
1. Structures specialized for food capture and defense
2. Located on tentacles, sometimes the body surface
3. Each cnidocyte contains a harpoon-like nematocyst
4. Propelled by water pressure
5. Protein toxin injected into prey
1. Portuguese man-of-war possesses powerful neurotoxins
2. Stings of other jellyfish can be severely painful

3. Classes of Cnidarians
1. Class Hydrozoa: The hydroids fig 41.12
1. Have both polyp and medusa forms
2. Mostly marine, colonial forms like Obelia
3. Example: Portuguese man-of-war
4. Example: Freshwater Hydra
1. Atypical, has polyp form only
2. Readily glides on basal disk or somersaults
3. May float to surface if detached from substrate

2. Class Scyphozoa: The jellyfish fig 41.13
1. Conspicuous medusae alternate with inconspicuous polyp forms
2. Medusa are bell-shaped, tentacles hang around margins
3. Outer epithelial layer contains contractile epitheliomuscular cells
4. Separate male and female individuals produce planulae
5. Polyps can reproduce asexually, may be suppressed in forms that live in open ocean

3. Class Cubozoa: The box jellyfish
1. Previously contained within class Scyphozoa fig 41.14
2. Medusa is box-shaped, polyps are inconspicuous or unknown
3. Tentacle found at each corner of box
4. Strong swimmers, voracious predators
5. Stings of some species fatal to humans

4. Class Anthozoa: The sea anemones and corals fig 41.15
1. Solitary and colonial marine organisms
2. Cylindrical "plantlike" body with tuft of hollow tentacles
3. Live primarily in shallow warm waters, harbor photosynthetic algae
4. Exclusively polyp form
5. Sea anemones are soft-bodied
6. Corals secrete hard or protein skeletons that comprise coral reefs
1. Waters that support corals are nutrient poor
2. Corals are abundant due to algae within them

4. The Ctenophorans (Comb Jellies) fig 41.16
1. Relationship to Cnidarians
1. Traditionally thought to be closely related
2. Recent research questions this assumption, structurally more complex

2. Have anal pores, water and substances pass completely through body
1. Abundant in the open ocean
2. Transparent spherical to ribbon-shaped forms, few centimeters long
3. Have two long retractable tentacles
4. Possess eight comb-like plates of fused cilia for locomotion
5. Many are luminescent

1. Solid Worms
1. Phylum Platyhelminthes: The Flatworms
1. General biology fig 41.17
1. Dorsoventrally flattened bodies, have definite head at anterior end
2. Bodies are solid (acoelomate), gut is the only internal cavity
3. Ribbon-shaped, softbodied, flattened dorsoventrally (top to bottom)
1. Have definite head at anterior end
2. Possess organs fig 41.18

4. Many species are parasitic others species are free-living carnivores or scavengers
5. Move via ciliated epithelial cells on lower surface

2. Organ systems of flatworms
1. Digestive system is branched with a single opening
1. Cannot feed continuously
2. Gut also functions to transport food
3. Partial extracellular digestion, also phagocytosis
4. Tapeworms lack digestive system, bathed in nutrients

2. Excretory system of fine tubules with bulb-like flame cells
1. Primarily regulate water balance
2. Excretion evolved secondarily

3. Lack a circulatory system, food and oxygen transported via diffusion
4. Simple nervous system with longitudinal nerve cords, primitive brain
5. Free living forms possess sensory systems on side of head
1. Detect chemicals and fluid movements associated with food
2. Eye spots are light sensitive, pigmented cups

6. Generally more active than radially symmetrical invertebrates
7. Reproductive systems are complex
1. Most flatworms are hermaphroditic with internal fertilization
2. Fertilized eggs deposited in cocoons, hatch into miniature adults
3. Asexually reproduce by fragmentation followed by regeneration

3. Class Turbellaria: The turbellarians
1. Free living organisms, found in water and moist habitats
2. Example: Dugesia, the common planarian fig 41.19a

4. Class Trematoda: The flukes
1. Parasitic forms have epithelium and gut linings resistant to host digestive enzymes
2. Lack sensory and locomotive adaptations of free-living forms
3. Take food in through mouth
4. Have complex life cycles involving one, two or more hosts
5. Example: Clonorchis sinensis, human liver fluke fig 41.19b
1. Eggs containing miracidium larva passed out in feces
2. Ingested by snail, transformed into sporocyst
3. Sporocysts produce rediae
4. Nonciliated redia give rise to cercariae
5. Tadpolelike cercariae released in water, are free-swimming
6. Bore into muscles of fish, turn into metacercariae
7. Humans eat fish, cysts dissolve, flukes migrate to liver

6. Example: Schistosoma blood flukes fig 41.13
1. Life cycle
2. Disease schistosomiasis is spreading through the tropics
1. Control via breaking life cycle
2. Investigations into effects on immune reaction, develop vaccine

5. Class Cestoda: Tapeworms
1. Extremely specialized parasitic organisms fig 41.18
2. Absorb food through outer body wall
3. Bodies divides into scolex, neck and reproductive proglottids
1. Proglottids formed continuously from region behind neck
2. Eggs toward end mature, become fertilized
3. Embryos emerge from end proglottids, leave host in feces

4. Example: Taenia saginata, beef tapeworm (often found in humans)

2. Phylum Nemertea: The Ribbon Worms fig 41.21
1. Mostly marine, free-living ribbon-shaped or thread-shaped worms
2. Have a long, muscular, retractable proboscis for capturing prey
3. Simplest organisms that possess complete digestive system
4. Simplest animals with closed circulatory system

1. The Pseudocoelomates
1. Pseudocoelomate Animals fig 41.10
1. Possess internal body cavity
2. Include seven phyla, Nematoda contains the most members
3. Pseudocoel serves as a hydrostatic skeleton against which muscles contract
4. Lack defined circulatory systems
5. Have complete, one-way digestive tract

2. Phylum Nematoda: The Roundworms
1. Include nematodes, eelworms and roundworms
2. Ubiquitous and abundant in marine, freshwater and terrestrial habitats fig 41.22
3. Most are microscopic in size, parasitic and live in soil
4. General biology of nematodes fig 41.23
1. Bilaterally symmetrical, cylindrical, unsegmented worms
2. Covered by thick, flexible cuticle that is molted periodically
3. Have longitudinal muscles located beneath the epidermis
1. Pull against cuticle and pseudocoel
2. Results in side-to-side whipping movement

4. Specialized digestive system with piercing stylets, mouth, pharynx and anus
5. Completely lack cilia or flagella
6. Excretory systems of canals or glands not dependent on cilia
7. Reproduction sexual, generally the sexes are separate
8. Development is simple, precise
1. Caenorhabditid elegans composed of 1000 cells
2. Fate of each cell completely mapped out

3. Many nematodes parasitize humans
1. Example: Trichinella, pig intestinal roundworm fig 41.24
2. Trichinosis may occur if pork eaten raw or undercooked
3. Worms may also infect bears and be transmitted to humans

4. Phylum Rotifera: Rotifers fig 41.25
1. Microscopic animals found in aquatic and soil habitats
2. Have crown of cilia at heads for feeding and locomotion
3. Have muscular pharynx with grinding jaws inside
4. Have flame cells like flatworms to control osmotic pressure
5. Sexual reproduction with separate sexes
6. Some species possess only females and reproduce solely by parthenogenesis

5. A New Phylum: Cycliophora fig 41.26
1. Tiny organism with circular mouth surrounded by ring of fine cilia
2. Suck up stray food particles from lobsters on which they live
1. Most of life spent as symbiont on lobster, reproduces asexually there
2. When lobster molts, cyliophoran begins bizarre sexual reproduction
1. Dwarf males emerge, only brains and reproductive organs
2. Males seek out females, fertilizes eggs
3. Eggs produce free-swimming individuals that seek out new lobster

3. Evolutionary relationship unclear, may be related to rotifers or lophophorates