Animals

What are animals?

• An animal is a multicellular eukaryote that:

o Has cells without cell walls.

o Obtains both carbon and energy from organic compounds.

o Has embryonic development.

o Is motile during part or all of its life cycle.

o Has a diploid sexual life cycle.

The Diploid Life Cycle of Animals

• A tissue consists of one or more cell types (and often the material between them) that collectively perform one or more specific tasks.

• An organ consists of two or more tissues organized in a specific way and having specific tasks.

• An organ or body system is a group of organs that work together to perform one or more functions.

Biosphere

Ecosystem

Community

Population

Organism

Body system

Organ

Tissue

Cell

• To investigate evolutionary relationships among animals, biologists rely upon many characteristics, especially those related to their embryonic development*:

o Type of body symmetry

o The number of primary tissue layers *

o The presence or absence of a coelom *

o The presence or absence of a complete digestive tract *

o The fate of the blastopore *

Body Symmetry

• Symmetry refers to correspondence in size, form, and arrangement of parts on opposite sides of a plane or around an axis. Some animals lack symmetry (they have asymmetry).

o The simplest animals (sponges) are, as a group, asymmetric.

• In radial symmetry, there are many planes that each can divide the body into two mirror-image halves; body parts are arranged regularly around a central axis.

o Animals with radial symmetry tend to be sedentary, which enables them to subsist on relatively small amounts of food.

o Radial symmetry allows them to capture food and respond to stimuli, regardless of the direction from which they come.

• In bilateral symmetry, there is only one plane that can divide the body into two mirror-image halves.

o Animals with bilateral symmetry can be mobile, which enables them to avoid predators and hunt for food.

o The end moving forward first (head) can have sensory organs to detect where the animal is going and a nerve center to process the sensory input; the opposite end (tail) can be modified for propulsion.

• Animal development begins with successive mitotic divisions of the zygote that produces a hollow, single-layered ball of about 100 to 1,000 cells called the blastula.

o However, the blastula is no larger than the original zygote.

Blastula

• In animals more complex than sponges, cells move into the interior of the blastula to produce the next stage of development called the gastrula.

o The pouch formed by this movement is called the primitive gut, which is the future digestive tract of the organism; the opening into the primitive gut is called the blastopore.

Gastrula

Sea urchin blastula to gastrula.

Primary Tissue Layers

• Gastrulation produces the first (primary) tissue layers of the embryo.

o Radially symmetrical animals have two layers: ectoderm at the surface

of the gastrula and endoderm enclosing the primitive gut.

o Bilaterally symmetrical animals have a third primary tissue layer between the ectoderm and endoderm, called the mesoderm.

o The mesoderm gives rise to muscle and most of the internal organs.

Coelom

• A coelom is a closed, fluid-filled body cavity that forms within the mesoderm, in which internal organs can develop.

o All bilaterally symmetrical animals have, or their ancestors probably had, a coelom.

• The advantages of having a coelom include:

o By cushioning the internal organs from external pressure, the organism is much less fragile.

o The organs are able to move, grow, and develop independently of the body wall.

o The coelom allows a rudimentary hydrostatic skeleton.

o Muscles surrounding the coelom contract to change its shape; the pressure of the fluid in the coelom produces movement.

• Digestion breaks down food, producing small molecules such as glucose and amino acids.

o Digestion within a cell is called intracellular. It is always chemical. o Digestion that occurs outside cells in a gut is called extracellular. It is

chemical and sometimes also mechanical.

Digestion

Extracellular digestion in Hydra.

• All animals that reach the gastrula stage have a gut in which food is digested; the small molecules produced are then absorbed into cells of the body.

o Either the gut is a sac with one opening, or it is a tube with two openings to the outside:

o A sac-like gut (gastrovascular cavity) is called an incomplete digestive system and was the first to evolve.

o A tubular gut (alimentary canal) is called a complete digestive system.

o All bilaterally symmetrical animals have, or their ancestors probably had, a tubular gut.

Type of Gut

• A tubular gut has distinct advantages over a sac-like gut:

o The animal can eat again before it has finished digesting its previous meal.

o A long tube can be regionally specialized for digestion, absorption of nutrients, and storage and elimination of indigestible wastes.

Protostome vs Deuterostome

• In animals with a complete digestive system, there are two ways the mouth can form during embryonic development:

o In protostome ("first opening, the mouth") development, the blastopore becomes the mouth and a second opening to the primitive gut becomes the anus.

o In deuterostome ("second opening, the mouth") development, the blastopore becomes the anus and a second opening becomes the mouth.

Types of Skeletons

• Animals use skeletons for support, movement, and/or protection. There are 3 major types:

o A hydrostatic skeleton is a fluid-filled, closed body compartment surrounded by two sets of muscles.

o Contraction by one set of muscles lengthens the compartment, causing the other set to relax.

o Contraction of the second set of muscles shortens the compartment, causing the first set to relax.

o Thus, the fluid gives the body enough rigidity to allow the muscles to work against one another.

o Animals with this type of skeleton include the earthworm and sea anemone.

o An exoskeleton is an external skeleton, such as a shell outside the body.

o An endoskeleton is an internal skeleton, such as bones within the body.

Animal Diversity

• Biologists today classify animals as they do other organisms – on the basis of their evolutionary relationships.

o They recognize about 35 major groups, or phyla (sing., phylum) of living animals.

o We will examine five phyla, which are each a clade, to sample the diversity of animals.

The Common Ancestor of Animals

• Choanoflagellates are free-living unicellular and colonial protists considered to be the closest living relatives of the animals.

o Choanoflagellates are similar to cells in modern sponges called choanocytes. Both have a single flagellum surrounded by a sticky, funnel-shaped "collar" that collects food particles.

Phylum Porifera

• The phylum Porifera ("pore bearer") consists of the sponges.

o They reach only the blastula stage of development.

o Therefore, they have no mouth, no gut, no muscles, and are the only animals without a nervous system.

o Because they have no tissues, each cell functions as an independent unit. But they are the only animals that, if separated into their component cells, will reassemble themselves.

o As a group, they are asymmetric and vary greatly in size and color.

o They are aquatic, mostly marine.

o Adults are sedentary but the young are motile.

The Poriferan Body Plan

• The sponge body plan is a hollow sac.

o The body wall consists of an outer layer of epidermal cells and an inner layer of collar cells (or choanocytes). The wall is perforated with tiny pores.

o The large central cavity has a single large opening at one end of the body.

• The beating of the flagella of choanocytes pushes water out of the central cavity through its single large opening. This flow pulls water into the central cavity through the pores.

CHOANOCYTES

• As water flows into the central cavity through the pores, food particles are trapped by the "collars" of the choanocytes.

o Straining food particles from the water is called filter-feeding.

o Food particles are absorbed by and digested within the collar cells, so digestion is strictly intracellular.

CHOANOCYTES

The collar cells of sponges, which resemble choanoflagellates.

• Gas exchange (oxygen and carbon dioxide) with the environment as well as the elimination of metabolic wastes, called excretion, occur by diffusion.

o Diffusion is only useful to very small (or thin) organisms where the diffusing substance does not have far to move.

Diffusion

Diffusion

Central cavity

• A firm network of tiny interlocking spicules located between the outer and inner layers of the body wall – an endoskeleton – supports the otherwise soft sponge body.

o Some spicules are formed of the mineralized substances calcium carbonate and silica, while others are made of an organic substance called spongin.

o Spongin skeletons were and are used as scrubbers in bathtubs, but the mineralized forms are much harder and are not as frequently used for commercial purposes.

Sponge spicule diversity. From van Soest et al. (2012)

Genetics: Genes Tell Us About Evolution https://vimeo.com/37224020

Phylum Cnidaria

• The phylum Cnidaria ("stinging nettle") includes the jellyfish, hydras, corals, and sea anemones.

o They reach the 2-layered gastrula stage of embryonic development.

o Thus, they have true tissues but no organs.

o They have radial symmetry.

o They are aquatic, mostly marine.

o Cnidarians are among the oldest animals in the fossil record.

The Cnidarian Body Plan

• The cnidarian body plan is a hollow sac: a central gastrovascular cavity and a mouth/anus encircled by tentacles. There are two forms of this body plan:

o The mouth/anus of the polyp faces upward and the organism is sessile. o The mouth/anus of the medusa faces downward and the organism

floats.

• The body wall of a cnidarian contains two layers of tissue and a middle, non-cellular layer:

o The epidermis is an outer protective layer derived from ectoderm. o The gastrodermis is an inner layer derived from endoderm. o The mesoglea is the middle, non-living layer that is jelly-like and thicker

in the medusa.

Cnidocytes

• Cnidarians are named for their unique, specialized "stinging" cells, called cnidocytes, located on their tentacles. Each cnidocyte can fire a tiny barbed spear called a nematocyst.

o Cnidocytes are used to capture prey and to thwart predators.

o The tentacles pull immobilized prey into the gastrovascular cavity, where secretory cells in the gastrodermis release enzymes to digest the food and other gastrodermal cells absorb the products. Thus, digestion is both extracellular and intracellular.

o Nutrients then diffuse to the rest of the body.

• Gas exchange and excretion also occur by diffusion.

• The water-filled gastrovascular cavity acts as a hydrostatic skeleton.

o When a cnidarian's mouth is closed, the gastrovascular cavity is a closed fluid-filled compartment, and if muscles surrounding the cavity contract, they'll change the shape of the space.

o Some polyps form a skeleton within or external to their tissues; some skeletons contain calcium carbonate, others are organic (of chitin or another carbohydrate), and some are both.

Nervous System

• The nervous system of cnidarians is a decentralized network of nerve cells ("nerve net").

o It is used to coordinate muscle contractions and to detect stimuli.

o It was probably within this group or a closely-related ancestor that nervous systems first evolved.

Cnidarian Body Plan https://vimeo.com/45320902

Bilateral Animals

• All the remaining animal phyla we will examine have the following characteristics:

o All have bilateral symmetry.

o All have a head with sensory organs.

o All have a gastrula with three primary tissue layers.

o All have, or their ancestors had, a coelom and a complete digestive tract.

• Bilateral animals are divided into the protostomes and the deuterostomes.

Phylum Mollusca

• Phylum Mollusca is one of the most diverse groups of animals on Earth.

o Although primarily aquatic, some molluscs live on land.

o They have tissues and organ systems.

o They are protostomes.

o Although molluscs have a coelom, it is reduced to a small cavity around the heart and reproductive organs (gonads).

o Examples of molluscs include snails (top), clams (middle), and squid (bottom).

The Molluscan Body Plan

• The body plan of molluscs consists of three basic parts (see next slide):

o The central visceral mass contains most of the internal organs. Since the body is thick, molluscs require specialized organs for circulation of nutrients and for excretion.

o The mantle is a covering that usually secretes a shell. A space between the visceral mass and the mantle, called the mantle cavity, contains gills for gas exchange.

o The muscular foot contains the motor organs.

ANATOMICAL DIAGRAM OF A HYPOTHETICAL ANCESTRAL MOLLUSC

The mantle and foot can be seen in the photograph above. The visceral mass is underneath the gill.

• All molluscs have (or their ancestors had) a hard shell made of calcium carbonate that functions as an exoskeleton.

o Slugs and octopuses have lost their shell, and squid and cuttlefish have a shell that has been greatly reduced in size and internalized.

Mollusc Diversity

• Phylum Mollusca contains three major clades:

o Gastropods, which includes snails and slugs. Gastropods have a coiled

body and shell and the head is well-developed with tentacles and eyes.

o Bivalves, which includes clams, oysters, mussels, and scallops. All have

shells composed of two pieces, known as valves.

o Cephalopods, which includes squid, cuttlefish, and octopuses. They are

fast-swimming, predatory molluscs with tentacles and sensitive,

camera-type eyes.

Circulatory Systems of Molluscs

• Since most molluscan cells are not in contact with the external environment, circulatory systems have evolved to transport nutrients, oxygen, carbon dioxide, and metabolic wastes. There are two types:

o In an open circulatory system, the blood leaves the system and flows freely within the tissues. This system is not very efficient because there is no blood pressure to move blood rapidly through the tissues, but it functions well enough for gastropods and bivalves, which have a sedentary lifestyle.

o Blood does not leave the blood vessels in a closed circulatory system. In this type of system, the heart can pump blood through the tissues rapidly, so it suits cephalopods, which have a more active lifestyle than other molluscs.

CLOSED CIRCULATORY SYSTEM

OPEN CIRCULATORY SYSTEM

Nervous System of Cephalopods

• The nervous system of cephalopods is so complex that they are considered to be the most intelligent of the invertebrates.

o All cephalopods are active predators and the ability to locate and capture prey often demands some sort of reasoning power.

The octopus has a mental capacity similar to that of a domestic cat.

Phylum Arthropoda

• More than 80% of named animal species are in phylum Arthropoda, and the largest group of arthropods are the insects.

o This phylum also includes the spiders and horseshoe crabs, millipedes and centipedes, and crustaceans.

o Like molluscs, arthropods are protostomes.

o Arthropods have well-developed nervous, (complete) digestive, and (open) circulatory systems.

The Arthropod Body Plan

• The basic body plan of arthropods consists of:

o A rigid exoskeleton made of chitin.

o Jointed appendages. Arthropods are the only invertebrates with this feature.

o Segmentation. The condition of being constructed of a linear series of repeating parts, each being formed in sequence in the embryo, from anterior to posterior.

A Hard Exoskeleton

• In addition to providing support and muscle attachment sites, the exoskeleton protects the arthropod from predators and water loss.

o However, it restricts growth, so periodic molting is necessary.

o The coelom has been reduced to small areas around the reproductive and excretory systems since it is no longer needed as a hydrostatic skeleton.

Pillbug molting.

Jointed Appendages

• "Arthropoda" means jointed leg.

o Appendages are structures that extend beyond the body wall.

o Regions of flexible, unhardened exoskeleton serve as joints.

o Body parts move when the muscles that attach to the inside of the hard exoskeleton on either side of the joint contract.

o Arthropod appendages have evolved a variety of functions, including walking, swimming, sensing, biting, stinging, manipulating food, chewing, and more.

Specialized Segments

• At some point in their lives, all arthropods have bodies that are internally and externally segmented.

o Each body segment tends to repeat the same set of structures (for example, a pair of legs, a set of breathing organs, and a set of nerves).

o In arthropods, sets of segments are usually grouped into a larger functional unit, such as the abdomen.

• The segments of insects fuse during development to form a body with three functional parts:

o A head bearing a single pair of antennae.

o A thorax bearing wings and three pairs of legs.

o An abdomen.

A Peacock Butterfly (Inachis io), before (top) and after (below)

• A large amount of animal diversity is built on two simple ideas: bodies made up of repeating units (or segments), and genetic programs for building structures.

o Homeotic (or "hox") genes specify how structures develop in different segments of the body. Early in development, different Hox genes are switched on in different segments, giving each an identity.

o A single Hox gene can regulate many other genes that work together to carry out a "program" during embryonic development— a program for building a leg or an antenna, for example.

o The evolutionary advantages of segmentation include:

o A small genetic change can easily add a segment to the body.

o Individual segments can be modified for different functions.

o Nearly every animal that's been examined has Hox genes, suggesting that they arose very early in animal evolution.

Just as Hox genes in arthropods direct segments to grow legs, wings, or antennae, Hox genes in

vertebrates direct segments to grow ribs (or not) or bones that fuse together to form a sacrum.

Matt Scott, Geneticist: How Genes Build Bodies https://vimeo.com/37403626

Phylum Chordata

• Phylum Chordata is the animal phylum with which everyone is most familiar, since it includes humans and other vertebrates – although not all chordates are vertebrates.

o The invertebrate chordates consist of lancelets (top↗) and tunicates (bottom↘).

o Chordates are deuterostomes.

o Chordates commonly have an endoskeleton.

The Chordate Body Plan

• At least as embryos, all chordates have these four features:

o A notochord: a cartilaginous semi-flexible rod running longitudinally along the "back".

o In invertebrate chordates, it serves as the endoskeleton.

o In vertebrates, the backbone replaces most of the notochord.

o A longitudinal, dorsal, hollow nerve cord: a bundle of nerve fibers that runs above and is supported by the notochord.

o Pairs of nerves branch from the nerve cord at intervals and connect to the muscles.

o Pharyngeal arches: a series of paired structures located in the throat region (pharynx) separated by clefts on the outside and pouches on the inside.

Human embryo (Carnegie Stage 14) with pharyngeal arches and clefts visible.

These structures were originally described in bird and mammal embryos in 1825 by Martin Heinrich Rathke, who used the term "gill clefts"; in 1844, Rudolf Wagner pointed out that "it was never imagined that the parts in question were proper gills; but they are vascular arches” like the vascular arches of the gills of fishes. Pharyngeal arches never function as gills in bird, mammal, or reptile embryos.

In fish, the cleft/pouches later perforate and do form gills, but in some other aquatic chordates they develop into a structure for filter-feeding.

o A postanal tail. Chordates, like many animals, typically have tails, but only chordates have tails behind their anus.

Human embryo (Carnegie Stage 14) with pharyngeal arches and post anal tail.

Segmentation

• Like arthropods, all chordates have segmented bodies.

o However, arthropods and chordates do not share a common ancestor that had this trait — it evolved independently in each lineage.

o Segmentation is evident in muscles, vertebrae, and ribs of the adult, although it often can be seen strongly only in embryos (segments are also called somites).

Vertebrate Chordates

• Most chordates are vertebrates, but vertebrates make up less than 5% of all animals.

o A brain and spinal cord develop from the dorsal nerve cord.

o The notochord is largely replaced during development by a series of vertebrae.

o The vertebral column encloses and protects the spinal cord.

o A cranium (skull) encloses and protects the brain.

• The earliest vertebrates were jawless fishes, like the modern lamprey.

o Their notochord persists for life, never being completely replaced by a vertebral column.

o Their skeleton is made of cartilage.

o Some species bore into the flesh of other fish to suck their blood, but most lampreys are not parasitic and never feed on other fish.

Lampreys have vertebrae but no jaws, and their skeleton is made of cartilage.

Jawless Fishes

• The evolution of jaws is an example of evolutionary modification of existing structures to perform new functions.

o Jaws probably evolved by expansion of rods of cartilage that had previously supported the anterior pharyngeal arches of early jawless vertebrates.

o They made many new feeding techniques possible, including grasping, biting, and suction feeding — opening up many evolutionary opportunities.

How did jaws evolve?

Jawed Fishes

• The earliest fishes with jaws split into two lineages: those with skeletons made of cartilage (cartilaginous fishes) and those with skeletons made of bone (bony fishes).

o Early jawed fish also had two sets of paired fins, pectoral and pelvic, which increased their ability to maneuver in the water.

o Bony fish evolved at the same time as the cartilaginous fish, about 400 million years ago.

o The fossil record shows that cartilaginous fish at first were more common, but after the end-Permian mass extinction, bony fish became more common.

Cartilaginous fishes were very diverse during the Permian period. However, after severe losses among cartilaginous fishes during the Middle Permian extinction, bony fishes

experienced a massive diversification in the subsequent Triassic period.

Cartilaginous Fishes

• Modern cartilaginous fishes include sharks, rays, and skates.

o An internal skeleton composed of cartilage, which is tough, flexible, but half as dense as bone, reduces the amount of energy needed to swim.

o Cartilaginous fishes maintain their position in the water in two ways:

o The tail and pectoral fins generate lift as the fish swims but this requires the fish to expend muscular energy to avoid sinking.

o The liver is filled with oil, reducing the overall density of the body, to provide buoyancy.

o Cartilaginous fishes were the first vertebrates to evolve internal fertilization.

Ray-Finned Fishes

• The bony fishes in turn split into two major lineages:

o The ray-finned fishes, which are the majority of fish living today.

o Their thin fins contain rod-like bones ("rays") that join the pectoral (shoulder) and pelvic (hip) girdles of the skeleton.

o The earliest ray-finned fishes lived in fresh-water and had both gills and a pair of pouched outgrowths from the pharynx which served as lungs. The lungs were inflated with air taken in through the mouth.

o After some ray-finned fishes migrated to the oceans, their lungs evolved into a swim bladder with which they could alter buoyancy.

o The lobe-finned fishes, which include the lungfishes and the coelacanths.

o They have fleshy fins with rays connected to central bones that are homologous to the humerus and the femur of tetrapods.

o Also, all lobe-fins have lungs of some sort.

Lobe-Finned Fishes

Amphibians

• The early lobe-finned fishes gave rise to a group of terrestrial animals called the amphibians.

• They were the first tetrapods, and were the second major group of animals to journey onto land, preceded by the insects.

o However, like the fishes, amphibians must still lay their eggs in water and therefore cannot live far from water.

The Amniotic Egg

• The breakthrough that allowed terrestrial vertebrates to complete their life cycles entirely on land was the evolution of the amniotic egg.

o Unlike the eggs of amphibians, most amniotic eggs have a shell that retains water and can be laid in a dry place.

o In most mammals, the embryo develops inside the mother's womb, or uterus, so in the course of evolution, the shell has been lost.

• All amniotes have four extraembryonic membranes during development:

o The yolk sac contains nutrients for the embryo (it had first evolved in the fishes);

o The amnion encloses the embryo in its own little "pond" (amniotic fluid);

o The allantois collects waste from the embryo;

o The chorion encloses the other three membranes.

https://www.youtube.com/watch?v=B6ckWpkuU

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