Chapter 17: Animal Evolution and Diversity
I. What makes an animal an animal?
A. 4 features, taken together, distinguish animals from other life
1. Eukaryotic: this feature unites them with all eukaryotes (plants, animals, fungi, and protists, and separates them from archaea and bacteria (prokaryotes)
2. Multicellular: separates animals from protists, generally unicellular; most plants, animals and fungi are multicellular
3. Heterotrophic: separates them from plants and algae (plantlike protists), Psing; groups them with fungi -they ingest food
4. Lack cell walls, distinguishes animals from plants, algae and fungi which have at least partial cell walls
B. Other features
1. Most animals take food into their bodies for nutrition, they generally have a digestive tract
2. Unique junctions between cells
3. Life cycles
a. except for haploid sperm and egg, all life stages of animals are diploid
b. male and female adults make haploid germ cells (gametes) by meiosis
c. egg and sperm fuse – zygote
d. early mitotic divisions – blastula (embryonic stage)
e. One side of blastula folds inward to form another embryonic stage – gastrula.
1). Gastrula now has an outer cell layer and an inner cell layer from infolding
2). inner layer becomes digestive system
3). outer layer becomes epidermis and nervous system
4). in most animals, a third layer forms between other 2 layers (endoderm) and becomes most other internal organs
5). many animals develop from gastrula into adults
6). some animals, like sea star, develop into 1 or more larval stages (intermediates). A larva is an immature individual that looks very different from adult, and doesn’t have capacity for sexual reproduction
7). Larvae undergo major change of body form, called metamorphosis, to become adults capable of sexual reproduction
C. Animals probably evolved from colonial protists
1. Oldest known fossil animals- 700 mya - Precambrian
2. Most modern animal phyla fossils from the Cambrian, 600 mya
3. No fossils show 1st animal - all too complex
4. Speculation about 1st animal
a. colonial protists - cells divide but don’t separate; colonies may have been hundreds or thousands of cells
b. hollow sphere, possibly. Floating aggregates of cells that ingested organic nutrients from the waters.
c. division of labor - locomotion, digestion, reproduction; at some point, division of labor- somatic vs. reproductive
d. still a long way from highly specialized, interdependent nature of true multicellularity; hollow colony must become layered
e. infolding may have produced a hollow colony for specialized feeding and digestive cells
f. infolding may have eventually eliminated hollow portion, paving way for more layers of cell, division of labor.
g. perhaps by this time, junctions may have formed btn cells for distributing nutrients and holding cells together
h. organism with this level of organization could be called primitive animal, or protoanimal (organism that gave rise to animals)
i. probably a crawler, not a floater, perhaps ate nutrients off ocean floor
D. Animal Phylogeny overview
1. symmetry
a. radial - Radial animals lack forward orientation, sit on surface of seafloor or drift, waiting for action to come to them. Bilateral animals - generally quite active and travel head first through environment. crawl, swim, run, walk, or fly. Sense organs contact environment first and help animal respond. Humans are bilateral. Why don’t we move head first through the environment?
b. bilateral - Most animals are bilaterally symmetrical - can be divided equally by a single cut into
mirror image right and left side
1. head end - anterior
a. head is prominent part of bilateral animals
1. sensory structures
2. brain
3. mouth
b. brain and sensory structures along with nerves = organ system = nervous system
2. tail end - posterior
3. dorsal side - back
4. ventral side - front or underside
5. lateral- side surfaces
2. body cavity Most animals have a body cavity, fluid filled space between digestive tract and body wall. Sponges, cnidarians and flatworms lack them. Fig. shows structures of 3 animals - color of cells indicates same tissue layers: outside covering region- blue, middle region (pink), and lining of digestive cavity (yellow). These tissue layers form from cell layers of gastrula in embryonic development
a. . acoelomates - flatworms lack a body cavity
b. pseudocoelomate - roundworms have a cavity called pseudocoelom (false, hollow),
a). an internal space in direct contact with wall of the digestive tract
b). the outer edge of pseudocoelom contacts muscle layer, part of body wall
c. coelomate - other animals have more complex body structure than either of the two above
a). body cavity - coelom - completely lined by a middle tissue layer, which extends from the body wall and wraps the digestive tract.
b). suspends organs from the body wall
Advantages of coelom:
1). more flexible- animals can crawl and burrow more easily
2). organs grow and move independently of outer body
3). fluids cushion organs, preventing injury; animals with skeletons would do serious damage to organs without fluids to protect organs during even mild exercise
4). fluids help circulate O2, waste and nutrients - amoeboid cells help in these functions
5). fluids provide structure to some animals (like earthworms).
c). structure related to function - small size of flatworms means they don't need a body cavity, all nutrients and O2 can diffuse in and waste out. Even long ones, like tapeworms, are very thin.
II. Invertebrates - Most animals are invertebrates, no vertebral column (backbone).
A. Sponges - Phylum Porifera
1. most are marine (oceanic), but some are freshwater
2. most live singly or in clusters formed by budding. Many can regenerate completely if fragmented into pieces
3. Vase- shaped or cylindrical sponges have radial symmetry - body arranged like pieces of a pie around radial axis. (any imaginary plane passed vertically through the body will produce mirror images
4. simple animals - generally, 3 loosely associated layers of cells
a. outside layer of flattened cells provides protection
b. middle body layer has motile cells called amoebocytes and skeletal components composed of a flexible protein called spongin or mineral containing particles.
c. third cell layer consisting of flagellated cells called choanocytes or collar cells, have collar like structure surrounding base of flagella. Aided by the flagella on choanocyte, water flows throughout the pores into the central cavity and then out of the sponge through its one large opening.
d. biologists believe sponges arose very early in animal evolution, from multicelled organisms
1). lack digestive tract
2). lack gastrula stage
3). 3 cell layers are not homologous to body layers of other animals
4). lack nerves and muscles
5). not true tissues
B. Phylum Cnidaria: hydras, jellies, sea anemones and corals. All have radial symmetry and cnidocytes
1. Radial symmetry is a hallmark of this phylum; any vertical cut will produce mirror images. Body forms:
a. polyps - columnar with radiating arms called tentacles
b. medusas - umbrella like, fringe of tentacles around umbrella - motile
2. some cnidarians have life cycles with both polyps and medusas
3. carnivores - use tentacles -capture small animals and protists, to push prey into mouth
a. polyp - mouth at top of column, at hub of radiating tentacles
b. medusa- mouth in center of underside of umbrella
4. in both body plans, mouth leads to digestive cavity called gastrovascular cavity
a. undigested food exits thru mouth, no anus; digestive system is said to be incomplete
b. cavity circulates body fluid that services internal cells
c. fluid in cavity provides body structure and support, like water in a balloon
5. All cnidarians have cnidocytes- specialized cells, found on surface of tentacles of polyps and medusas- fine thread coiled within a capsule. When discharged, thread can sting or capture prey; some big enough to catch fish
a. defense
b. prey capture
6. several features absent in sponges, present in other phyla:
a. digestive cavity
b. gastrula stage of development
c. presence of tissues. eg sea anemone has well developed nerve and muscle tissue, it can creep, swim, roll, burrow or somersault away from danger.
d. tissue animals - though some have organs, most cnidarians have tissues that perform vital functions.
C. Platyhelminthes- flatworms are simplest bilateral animals. Leaf or ribbonlike animals.
1. unusually simple bodies for bilaterals
2. incomplete digestive tract - no anus; most bilateral animals have a complete digestive system
3. flatworms lack an internal cavity; digestive cavity is only space in body, similar to cnidarians it is a gastrovascular cavity; most bilateral animals have another body space called the body cavity between digestive system and body wall.
4. 3 major groups of flatworms
a. freeliving (nonparasitic) flatworms - planaria
1). nervous system - dense clusters of nervous tissue (brain), eyespots, small nerves that branch throughout the body
2). digestive system - highly branched. mouth located not in head, but ventral surface; muscular tube protrudes through mouth and pulls food in. Planaria live on undersurfaces of rocks in ponds and streams. Using cilia, they crawl in search of food. Muscles allow them to twist and turn.
b. flukes - parasites. e.g. Schistosoma - female spends much of time in a groove in the male’s body; they copulate frequently producing over 1000 eggs/day. Females and males have suckers to attach to the inside of the blood vessels near the host’s intestines. Blood flukes infect humans and cause severe disease called schistosomiasis (blood fluke disease). It’s widespread in Africa, Southeast Asia and South America - 250 million people in 70 countries: severe abdominal pain and dysentery.
c. tapeworms are 3rd type of flatworms, also parasitic. Inhabit the digestive tracts of vertebrates, including reptiles, birds and mammals.
1). Long, ribbon like bodies with repeated parts.
2). no digestive tracts themselves - absorb partially digested nutrients directly
3). head is armed with suckers and teeth to attach to host.
4). behind head, a short neck generates the repeated parts; 5). youngest repeated part is behind the neck, oldest part at the posterior end of the worm.
6). Repeated parts filled with male and female reproductive parts.
7). parts at posterior end filled with eggs; break off, pass from body in feces.
8). Humans can become infected by a large tapeworm (20 m) Taeniarhynchus
D. Nematodes - roundworms -pseudocoelom and complete digestive tract
1. cylindrical worms with a finely tapered tail; head is more blunt.
2. tough, nonliving skin called cuticle, resists drying or crushing
3. pseudocoelomates
4. complete digestive tract; mouth near head, anus near tail. food travels one way; specialization in digestive tract - highly efficient
a. anterior region churns and mixes food w/enzymes
b. posterior region absorbs nutrients and disposes of waste
5. Nematodes are among the most numerous of animals in terms of number of individuals and number of species;
a. important decomposers, they live wherever there is rotting organic matter - soil, lakes, rivers, oceans
b. also parasites of plants and in body fluids of animals.
c. 90,000 species are known and likely that 10x exist; freeliving worms are most abundant
d. humans are host to at least 50 species, many of which cause major health problems
1). hookworms
2). heartworms in dogs
3). trichinella - trichinosis (NASTY!! worms may infect heart or brains)- undercooked pork may contain juveniles - cook meat until no longer pink
e. conservation of body form among nematodes - all look pretty much as described
E. Phylum Mollusca - snails, slugs, oysters, clams, octopus and squids are examples of great variety - body structure NOT conservative in this group!
1. most have soft bodies protected by hard shell
2. molluscus in latin means “soft”
3. basic body plan -
a. muscular “foot” - locomotion
b. mantle - outgrowth of body surface that protects animal
1). produces shell in clams and snails
2). also: respiration, waste disposal and sensory reception.
3). mantle cavity - created by mantle formation - may house a gill to extract O2 in water and dispose of fluid wastes
4). many molluscs have radula- a rasping tongue for scraping
c. true coelom - 3 small cavities
1). for heart
2). for reproductive organs
3). one forms part of kidney
d. circulatory system - distributes nutrients and water - has taken over the function of a large coelom
4. Different body features in different groups
a. gastropods - fresh water, salt water, terrestrial. In fact, this group has the only molluscs that live on land; 75,000 species
1). terrestrial snails, slugs lack gills; mantle cavity has evolved into a huge lung
2). most are marine; sea slugs are unusual mollusks, lacking a mantle, mantle cavity and shell
b. bivalves - (bi = double, valva = leaf of folding door); include clams, oysters, mussels, scallops.
1). have shells divided into 2 halves, hinged together
2). sedentary, live in sand or mud
3). muscular foot used for digging/anchoring
4). mucus coated gills to trap fine food particles
5). scallop - many eyes around mantle edges. can clap its shell shut and squirt water from its mantle cavity, jetting itself a short distance away
c. cephalopods (greek - kephale = head, pous = foot) - built for speed and agility
1). a few have large, heavy shells, but most have small internal shell (squid), or lack it all together (octopus)
2). marine predators - beaklike jaws and radula to crush or rip prey apart
3). mouth at base of foot, which is divided into numerous tentacles for catching and holding prey
4). squids -
a). draws water into mantle cavity, fires it out - fast sreamlined animal
b). large complex brain
c). among most complex sense organs in animal kingdom - lens (focus light) and retina (images)
5). all cephalopods have complex brains and sense organs, contribute to success as predators
6). Octopuses have larger, more complex brains, proportionate to body size than almost any animal. good learners!
F. Phylum Annelida - Segmentation - subdivisions of the body along its length; this feature played a major role in evolution of many complex animals. Phylum Annelida (annelus = ring) - segmented worms; 15000 species. Terrestrial (damp soil), freshwater and marine (some swim, most are bottom dwelling scavengers) habitats; separate head and tail; body segments all very similar
1. Earthworms are segmented (Fig. 18.10)
a. coelom is partitioned by walls
b. nervous system (yellow) has a cluster of cells in each segment
c. excretory organs (green) repeated in each segment
d. digestive tract is not segmented; it passes through the segment walls from the mouth to the anus.
e. circulatory system - dorsal and ventral blood vessels;
1. accessory hearts
2. main heart
2. Dragonfly - also segmented, though less uniformly than earthworm
a. segments most pronounced in abdomen.
b. head and thorax - fused segments
c. Each pair of legs and wings emerges from a segment in the thorax
3. Humans have segments, not homologous with earthworm’s or dragonfly’s
a. Backbone is repeated series of knobs called vertebrae, and muscles associated with vertebrae are segmented.
b. stomach muscles are segmented
4. Advantages of segmented bodies
a. great bodily flexibility
b. mobility; probably an adaptation for movement
5. Oligochaetes - Earthworms one of 3 large annelid groups
a. eats its way through soil extracting nutrients as soil passes thru digestive tube. Undigested material passes through anus at posterior end.
b. earthworms are incredibly valuable-
1). they till the soil and enrich it
2). they improve the texture; Darwin estimated that an acre of land (Britain) had about 50,000 earthworms that produced about 18 tons of feces/yr.
6. Polychaetes (gk- poly = many, chaeta = hair) most live on the seafloor; some freshwater or estuaries. Most annelids are polychaetes.
a. segmented appendages help worms wriggle
b. segmented appendages increase surface area for taking up O2 and dispose of wastes
c. many marine species live in tubes; extend feathery appendages to trap suspended food particles
d. tubes formed by sticky proteins secreted near mouth
7. Leeches are 3rd group of annelids; blood suckers, some are freeliving, eating snails and insects. Most leeches are fresh water; a few terrestrial species inhabit moist vegetation in the tropics
a. until this century, leeches were used to remove “bad blood” from patients
1). razor-like jaws cut through skin
2). saliva contains strong anesthetic
3). and strong anticoagulant - prevent heart attacks
b. leeches used on burn victims and in reattachments
G. Phylum Arthropoda -
1. nearly a million types of segmented animals - includes crayfish, lobsters, crabs, barnacles, spiders, ticks and insects.
2. arthropod population of earth is about a billion billion 1018 ! most successful phylum of animals ever, in terms of diversity, distribution and sheer numbers.
3. Fossils indicate intermediates btn annelids and arthropods - Cambrian, 550 mya. But molecular analysis shows an earlier ancestor of both, two different evolutionary lines.
4. Arthro = jointed, pod = foot; jointed appendages in this phylum. Appendages in lobster for walking, feeding, swimming, sensory reception and defense.
5. exoskeleton - hard external skeleton- chitin: polysaccharide, w/proteins
a. protection
b. attachment for muscles to move appendages
c. thick in some places thin in others
d. molting- periodic shedding of exoskeleton, secretion of new one
6. annelids have similar segments; most arthropods have distinct groups
lobster, e.g.
a. head- sensory parts: antennae, eyes and mouthparts
b. thorax-pincers for defense and 4 prs. legs for walking
c. abdomen- swimming appendages
7. horseshoe - living fossil - survived w/ little change for hundreds of millions of yrs. Only survivor of spiderlike arthros abundant in seas 300 mya
8. closest living relative of horseshoe crabs - arachnids: spiders, ticks, mites, scorpions. Most arachnids live on land.
a. Scorpions are nocturnal hunters. eat mainly insects and spiders
b. mites- thousands can live in a few square centimeters of carpet or in a dust bunny; not disease carriers; many people are allergic