What is the relationship between sponges and Cnidaria?

Invertebrates, or animals without backbones, make up about 95 percent of the animal kingdom. Most invertebrate species live in aquatic or moist terrestrial habitats. There is great diversity within this group across the different phyla.

Abundant and diverse, sponges are the simplest of invertebrates. Sponges usually have no body symmetry and vary in size, shape, and color depending on the water temperature and their location. They are sessile, or stuck in one place, and are not able to move away from predators. Most live attached to the ocean floor or on another rocky, hard surface and remain there for their whole lifespan. Sponges lack bones, tissue, and organs, but they do have body parts to help them eat and digest. Sponges are filter feeders, which means they filter food out of the water that flows through their bodies. Their pores let water into their central cavity, and the flagella, or tiny hairs, keep water moving through the sponge. They have soft bodies that are protected by spicules (pointy spikes) of minerals made from calcium or silicon dioxide. Sponges reproduce both sexually and asexually.

Cnidarians, meaning stinging creatures, are invertebrates made up of jellyfish, anemones, and corals. They all have radial symmetry, which means their bodies are arranged around a central point. They are characterized by two basic body forms. The vase-shaped polyp is where the body forms the lower part of the animal and is tube-like in shape with tentacles that stick up from it. The bowl-shaped medusa is umbrella-shaped where the main body forms the top part of the animal and the tentacles hang beneath. Cnidarians use specialized cells called nematocyst in their tentacles to capture food and defend themselves. The venomous poison released paralyzes their prey, and the tentacles are used to haul their victim in. Cnidarians reproduce either sexually by releasing sperm or eggs or asexually through the process of budding. A genetically identical copy of the adult grows and eventually either falls off or stays on to form a colony.

Worms are classified into three major phyla, flatworms, roundworms, and segmented worms. Flatworms have long and flattened bodies. They are bilaterally symmetrical and have tissues and internal organ systems. Flatworms have one digestive tract and an incomplete digestive system. Some flatworms feed freely on organisms, while others are parasitic, living off or in a host. A tapeworm is an example of a parasitic flatworm. Roundworms have a digestive tract with two openings – one at the mouth and one at the anus. Their bodies are a tube within a tube. They can also be free-living or parasitic. They play an important role as decomposers in soil. Segmented worms, like earthworms and sandworms, are made up of many segments. They have a closed circulatory system where blood moves within the blood vessels. They have a nerve cord and a digestive tract with a mouth and an anus.

Review:

Identify three characteristics of sponges.
Compare a polyp to a medusa.
What is a closed circulatory system?

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Kingdom Animalia includes all organisms that develop from a hollow ball of cells called a blastula. Most animals have well-developed motility. The simplest animals include the sponges (Porifera) and the Cnidaria. Sponges are unsymmetrical or radially symmetrical, with many cell types but no distinct tissues; their bodies contain numerous pores and sharp protective spicules.

Coelenterates (phylum Cnidaria) are radially symmetrical, with two tissue layers (ectoderm and endoderm) surrounding an all-purpose gastrovascular cavity.

Phylum Porifera (sponges): Aquatic animals with radial symmetry or irregular shapes. Water enters by many incurrent pores that often lead to a central cavity. Water may exit by an excurrent opening (osculum). No distinct tissues, but many cell types:

Epidermal cells (pinacocytes): Outside lining
Porocytes: Barrel-shaped pore cells
Choanocytes: Flagellated "collar cells" that keep water flowing
Amoebocytes: Several kinds of amoeboid cells, including some that secrete sharp spicules.
Spicules: sharp needles or more complex shapes embedded within sponges, functioning in support and as a defense against predators. May be composed of silica, calcite, or horny protein.

Tissue-level organization: A tissue is a group of similar cells and their products, built together (structurally integrated) and working together (functionally integrated).
You may think of a sponge as built of a single tissue. Types of sponges include:

Calcispongia (or Calcarea): small sponges with needle-like spicules made of calcite (CaCO3 ). Symmetry usually radial.
Hyalospongia (or Hexactinellida): deep-water "glassy" sponges (often very beautiful) with spicules (usually six-pointed) made of silica (SiO2 ). Symmetry usually radial.
Demospongia: Horny or "true" sponges (about 90% of all known species) with spicules made mainly of horny protein ("spongin," similar to keratin) that often surrounds a needle-like core of silica. Irregular shapes (symmetry lost). Traditional "bath sponges" belong here.

Sponges and Cnidaria Phylum Cnidaria (coelenterates): Aquatic animals with two tissue layers (outer ectoderm and inner endoderm) separated by a jelly-like mesoglea; and an all-purpose gastrovascular cavity with a single opening (mouth). Tentacles surround mouth and have stinging cells (cnidocytes) containing stingers (nematocysts). Two major body forms:
Polyp: mouth directed upward, mesoglea thin, animal usually attached;
Medusa: free-swimming "jellyfish" with thick mesoglea; mouth directed downward.

Class Hydrozoa: Life cycle includes both asexual polyps and sexually reproducing medusae (usually small). Solitary or colonial; some colonies have many types of individuals interconnected.
Class Scyphozoa: Solitary "jellyfish" with dominant medusa stage.
Class Anthozoa: The largest class, including sea anemones and corals. Polyp stage dominant; no medusa. Mouth extends inward to form a tubular pharynx. Solitary or colonial.

Phylum Ctenophora ("comb jellies"): A small group of marine animals with biradial symmetry (like a two-armed pinwheel), 2 large tentacles, and 8 comb-like rows of cilia.

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Bilateral animals above the flatworm stage evolved a complete "assembly-line" digestive tract running from mouth to anus. Most also evolved body cavities. Analysis of RNA sequences allows scientists to divide bilateral animals into:

Protostomes— bilateral animals in which early cleavages are spiral and

determinate, and in which the mouth forms early from the blastopore. Protostomes are further divided into:

Lophotrochozoa, containing the Mollusca, Annelida, Bryozoa, etc.; and

Ecdysozoa, contining the Nematoda, Arthropoda and several smaller phyla. Deuterostomes— bilateral animals whose early cleavages are radial and indeterminate, and whose mouth forms at the other end from the blastopore. FROM THIS POINT ON, all remaining phyla share several important derived features:

A complete "assembly line" digestive tract (mouth to anus).
Some type of body cavity, either a pseudocoel (a persistent blastocoel) or a true coelom (surrounded with mesoderm throughout).

"Assembly line" digestion: Nearly all animals above the flatworm level have a complete digestive tract, with a separate entrance (mouth) and exit (anus). This allows food to be processed in stages, in the manner of an assmbly line, with different regions or organs specialized for different sequential steps or for different nutrients.

Evolution of body cavities: Fluid-filled body cavities, whatever their origin, are useful:

in support, as a hydrostatic skeleton
in burrowing, where inflation of the body cavity can swell and anchor part of the body, or else wedge forward and push sediment aside.

Because of their usefulness, body cavities have evolved many times, independently, and are often constructed differently in different phyla:

Some animals have a pseudocoel, lined with both endoderm and mesoderm, derived from persistence of the blastocoel cavity.
Other animals have a true coelom, lined with mesoderm throughout. This may be either an enterocoel, derived from outpouching of the gut (as in starfish), or a schizocoel, arising within the mesoderm by splitting (as in mammals).

Differences in the structure of the coelom are useful in distinguishing many phyla, but are a poor guide to relationships among phyla because body cavities have evolved repeatedly and independently.
Animal family tree

Evolution of Bilateria

Protostomes are bilateral animals sharing the following traits:

The opening to the embryonic archenteron becomes the mouth
(protostome means "first mouth").
Spiral cleavage, introducing an asymmetry in the 8-celled stage; the top 4 cells are rotated clockwise or counterclockwise with respect to the lower 4 cells.
Determinate cleavage, meaning that the cells destined to form the front left portion of the animal lose the ability to form structres on the right or the rear.

Deuterostome animals (considered later) have the opposite traits.

Phylogeny and classification of bilateral animals: Studies of ribosomal RNA sequences show evidence that bilateral animals evolved in three large groups (the first two are protostomes):

Lophotrochozoa: A large group that includes annelid worms, mollusks, and bryozoa, characterized in some cases by a ciliated feeding organ called a lophophore and in other cases by a ciliated larval stage called a trochophore.
Ecdysozoa: A group that includes the two largest phyla, Arthropoda and Nematoda, characterized by a hard outer covering that must be shed periodically during growth, using steroid hormones (ecdysones) to control the molting process.
Deuterostomes, including the chordates and echinoderms.

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The Lophotrochozoa are a diverse group of phyla. Most have a true coelom of the schizocoel type, but a few have only a pseudocoel. Ribosomal RNA sequences show these various phyla to be related. Phyla placed in this group include:

Phylum Rhynchocoela or Nemertea: "Proboscis worms" with a long, stick-like proboscis or "evert" that can be withdrawn by turning it inside out.
Phylum Rotifera: Rotifers, with a wheel-like crown of cilia at one end.
Phylum Acanthocephala: Spiny-headed, parasitic worms.
Phylum Annelida: Segmented worms, described below.
Several smaller phyla related to the Annelida:
1. Sipunculida: Marine worms with tentacles around the mouth.
2. Echiurida: Sausage-shaped worms with a tube-like extension in front of the mouth.
3. Chaetognatha: Arrow worms, with dart-shaped bodies, abundant in marine plankton.
4. Gastrotricha: Gastrotrichs.
5. Pogonophora: Deep-sea tube-dwelling worms.
Phylum Mollusca: Snails, clams, squids, etc., described below.
Four phyla (Entoprocta, Phoronida, Bryozoa, Brachiopoda) characterized by a ciliated feeding organ called a lophophore.

Animal family tree Lophophore-bearing phyla:
A Lophophore is a crown of ciliated tentacles that help gather suspended food particles. The cilia trap these particles and bring them to the mouth, a form of filter-feeding. The animal can withdraw its lophophore if conditions are muddy or if predators threaten.

Similarities of phoronids, bryozoans, and brachiopods:

All have lophophores.
True coelom, used as a hydrostatic skeleton.
Simple, U-shaped digestive tube, complete with mouth and anus.
Benthonic (bottom-dwelling), either mobile or sessile (attached).

Phylum Phoronida (phoronid worms): Tube-dwelling worms with a lophophore surrounding the mouth. Coelom is used in burrowing: muscle contraction builds up pressure in the coelom, which swells sideways and pushes sediment aside. Probably related to ancestry of Bryozoa and Brachiopoda.

Phylum Bryozoa ("moss animals"): Largest and most successful lophophorate group. Members quite varied. All are small, aquatic animals living in colonies. Many colonies are polymorphic, containing several dissimilar types of individuals. Ancestry is probably close to Phoronida.

Phylum Brachiopoda: Probably derived from phoronid ancestors by addition of a two-part shell as an aid in burrowing. Shell has two unequal valves; axis of symmetry bisects center of each valve. Valves are connected by muscles only (class Inarticulata) or by a hinge (class Articulata). Muscular stalk (pedicle) attaches animal to the bottom. Fossil record shows that brachiopods were more abundant and more diverse during the Paleozoic Era (530 million to 300 million years ago).

Phylum Entoprocta: A small group of sessile (attached) animals with a lophophore and a simple, U-shaped digestive tract. They differ from the previous 3 phyla in two ways: they have a pseudocoel instead of a true coelom, and their lophophore surrounds the anus as well as the mouth.

Evolution of Lophotrochozoa Phylum Mollusca (mollusks):

Animals with a true coelom of the schizocoel type, usually bearing a shell composed mostly of calcium carbonate and secreted by a mantle. The mantle is always withdrawn at the rear to form a mantle cavity, which contains anus and gills. Primitive mollusks and gastropods use a tongue-like radula with embedded teeth to scrape encrusted algae from rock surfaces.

Class Monoplacophora (primitive mollusks): Mollusks with a simple dome-shaped or low conical shell. Muscles, blood vessels, and other structures segmentally arranged. Digestive tract simple.
Class Gastropoda (snails and slugs): Body usually undergoes asymmetrical torsion (twisting and coiling). One-piece (univalve) shell, usually coiled. Most species herbivorous. Well-developed head, sense organs, and nervous system. Locomotion typically by creeping on a muscular foot.
Class Polyplacophora (chitons): Simple, flattened body, with shell divided into several overlapping plates that permit some flexibility. Head small but radula well-developed and used in feeding.
Class Bivalvia or Pelecypoda (clams, etc.): Body usually symmetrical, narrowly compressed from side to side. Two-piece (bivalve) shell; left and right halves are often mirror images (except at hinge). Many species filter-feed, straining small particles from the water. Head and sense organs poorly developed. Muscular foot hatchet-shaped (flattened side-to-side, like a wedge), often used in burrowing. (The old name "Pelecypoda" means "hatchet-foot".)
Class Scaphopoda (tusk-shells): Small mollusks with tusk-like shells. Mantle cavity runs for entire length of shell along posterior margin; water passes through mantle cavity, exiting through hole at the top.
Class Cephalopoda (octopuses, squids, nautiloids, etc.): Body usually symmetrical. One-piece shell is symmetrically curved or coiled in median plane, or often lost. Most species are actively swimming predators. Head very well-developed, with sense organs (especially eyes), brain, and beak. Muscular foot subdivided into numerous tentacles. Body doubled over, with mantle cavity (originally rear) tucked beneath head and opening forward. Frequent "ink glands" that secrete dark, inky fluid to confuse predators.

Phylum Annelida: Segmented worms. Complete digestive tract (with both mouth and anus) runs nearly the entire length of the body. Outer covering of chitin is thin, flexible, and prevents fluid loss. The true coelom and most internal organs are segmentally arranged. Blood cicrulates in closed vessels only. Advanced excretory organs (nephridia) are present. Some ability to regenerate missing parts after injury.
All annelids exhibit Metamerism, a division of the body into numerous similar segments.

Class Polychaeta: Largest group, mostly marine. Sense organs and nervous system highly developed; several setae (bristles) per segment.
Class Oligochaeta (earthworms): Poorly developed head; only one pair of setae per segment. Important to soil because their digestive wastes leave behind soil nutrients and their tunnels let air reach plant roots.
Class Hirudinea (leeches): Mostly parasitic, live in fresh water, attach to the outside of animals and suck blood. Leeches have degenerate anatomy: fewer sense organs, fewer segments, etc.

Locomotion in annelids (controlled separately in each segment):

Each segment contains a walled-off portion of the body cavity.
Muscles parallel to the body axis can shorten segments; these segments swell and anchor into the surrounding sand or soil.
Muscles perpendicular to the body axis will lengthen body segments and cause them to push forward.
Nervous system produces rhythmic waves of shortening and waves of lengthening among the segments.
Small bristles (setae) may help anchor the shortened segments.

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Phylum Nematoda (roundworms) are very small worms with tapering ends, living very abundantly in all environments. Some are free-living, but many are parasitic on plants or animals. Caenorhabditis elegans is a free-living nematode often used in biological research. Filaria is a genus of parasitic roundworms that causes elephantiasis and other human diseases. Trichina and Trichinella are parasitic roundworms that can enter human bodies through insufficiently cooked beef or pork.

Phylum Gordiacea or Nematomorpha: Very long, thin "horsehair worms."

Phylum Cephalorhyncha includes the groups Priapulida, Kinorhyncha, and Loricifera. They are all small and live in marine environments. All have a proboscis, covered with spines, that can either be withdrawn or turned inside out and everted.

Phylum Pentastomida: Endoparasites inside vertebrates, with 2 pairs of short, degenerate legs armed with claws.

Phylum Tardigrada: Tiny "water bears," with 8 short legs ending in claws.

Phylum Onychophora: An ancient group (Cambrian to Recent), closely related to arthropod ancestors. Segmented, wormlike body. Numerous short feet (1 per segment), ending in claws. Feet around mouth function in seizing and tearing food.

Ecdysozoa Phylum Arthropoda: Animals with a tough exoskeleton, often strengthened by calcium salts, and jointed legs with movable joints between rigid segments. Metamerism (segmental organization), but segments differ very much regionally. Mouthparts often derived from legs. Open circulatory system. Several anterior segments commonly coalesced into a head. Nervous system reminiscent of annelids, with ventral nerve cord, esophageal ring, and dorsal brain. Included groups:

Trilobites: Extinct, marine arthropods with numerous similar biramous (two-branched) appendages, each with a leg-like part and a feathery gill. Body divided into cephalon (head), thorax, and pygidium. May be ancestral to other arthropods (but experts disagree on this). Flat-bottomed shape shows that most trilobites were bottom-dwellers.
Crustacea: A largely marine group of arthropods, breathing by gills. Always two pairs of antennae. First post-oral segment has a pair of mandibles. Appendages biramous (two-branched), as in trilobites. Includes lobsters, crabs, shrimp, barnacles, and many other species.
Chelicerate groups: Originally marine arthropods, but one large group successfully invaded land environments. No antennae are present. First 2 pairs of appendages include a pair of chelicerae (piercing structures that may be used to inject venom) and a pair of pedipalps which may hold prey while the chelicerae pierce them. Many chelicerates are predators. Body usually divided into cephalothorax and abdomen (except in mites). Includes Pycnogonida (sea spiders), Xiphosura (horseshoe crabs), and Arachnida (scorpions, spiders, mites, etc.).
Uniramous "myriapods": Arthropods with elongate, worm-like bodies and many pairs of legs. Includes centipedes (Chilopoda), millipedes (Diplopoda), and two other groups (Pauropoda, Symphyla) allied to insects. All myriapods have similarities that they share with insects: a single pair of antennae; a pair of mandibles; uniramous appendages (having only a single branch); respiration using a tracheal system of air-tubes. Habitats usually terrestrial, often hiding beneath rocks and rotting logs.
Insects: The largest and most successful group of arthropods, including about 75% of all species in the entire animal kingdom. Mandibles, uniramous appendages, and tracheal systems as in myriapods. Division of the body into three portions: head (containing antennae, mouth, and compound eyes), thorax (typically with three pairs of walking legs and two pairs of wings), and abdomen or 7 or more segments (containing reproductive structures).

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Deuterostome characteristics: Embryological similarities shared by chordates, hemichordates, and echinoderms:

Radial cleavage: The 8-celled stage has 2 tiers of 4 cells each, with each cell directly below or above another.
Indeterminate cleavage: Cells separated in early embryonic stages can develop into an entire embryo.
Deuterostome condition: The embryo's blastopore becomes the posterior (tail) end. (Remember that in molluscs, arthropods, and other protostome phyla, the blastopore becomes the mouth.)

Phylum Echinodermata:
Animals with a unique water-vascular system, using sea water as a circulatory fluid. Several embryonic similarities to chordates, including a true coelom, which develops as an enterocoel. Change of symmetry in many cases, from a bilateral larva to a radial adult, typically in a 5-fold pattern. Protective plates or shells frequently made of calcium carbonate and armed with bumps or spines. High ability to regenerate lost parts.

Sessile (attached) echinoderms (Homalozoa and Crinozoa): Echinoderms that grow attached include crinoids (sea lilies) and a variety of extinct groups (blastoids, cystoids, carpoids, etc.). Many grow on stalks attached to the bottom. Body cup-shaped, open toward the top, with a mouth in the center of the top surface. Arm-like rays, in multiples of five, grow out and upward from the margins of the mouth. Each ray has a ciliated groove (the ambulacrum) that traps food particles and brings them to the mouth. The earliest fossil forms were irregular and lacked symmetry, but radial symmetry developed early, generally in a 5-fold pattern. Biologists believe that echinoderm ancestors were bilaterally symmetrical and that filter-feeding (filtering small particles of food from the water) made radial symmetry selectively advantageous. Attached echinoderms flourished mainly during Paleozoic times. Today, only a few crinoids remain; other attached echinoderms are extinct.

Free-moving echinoderms (Echinozoa and Asterozoa): Mostly bottom-feeding scavengers and predators that attack other invertebrates. The mouth, on the lower surface, faces downward. Branches of the water-vascular system may form foot-like podia, used in locomotion. Each of these podia has a suction-cup extension that can hold on and a water-filled bulb that controls water pressure (or suction).

Asterozoa: Body star-shaped, with protruding arms. Includes starfishes and brittle stars.
Echinozoa: Body globe-shaped, with no protruding arms. Includes sea cucumbers, sea urchins, and sand dollars. Deuterostomes

Chordata and Hemichordata:
Notochord: A stiff, flexible rod, forming the body axis. When muscles contract, it allows bending but prevents the body from collapsing like an accordion. In embryos, it induces the nervous system to form above it.

Gill slits: Openings from pharynx to either side, just behind mouth.

Hemichordata: Acorn worms and their relatives. All of them filter feed. Some use gill slits; others use tentacle-like feeding structures. Related to Chordata, but now usually treated as a separate phylum.

Phylum Chordata: Animals with a notochord, a series of gill slits, and a dorsal, hollow nerve cord developing from a neural tube. These traits may occur in larval stages, not always in adults.

Urochordata (tunicates or "sea squirts"): An actively swimming larva with well-developed notochord and nerve cord undergoes metamorphosis into a filter-feeding adult. The adult usually passes large amounts of water through a large gill basket.

Cephalochordata (sea lancets or amphioxus): Small, thin animals that filter feed by passing water through many gill slits. A notochord extends the entire length of the animal, including the head.

Vertebrata (vertebrates): Animals with a vertebral column or backbone that functionally replaces the notochord in adults, and a braincase that encloses and protects the brain. Examples: fishes, amphibians, reptiles, birds, and mammals.

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General characteristics of fishes:

Central nervous system protected by enclosure inside skeleton.
Free-swimming by means of side-to-side undulations of the body.
Gills used in respiration; but most fishes no longer filter feed.

Class Agnatha: Jawless fishes, often with a filter-feeding larval stage. Extinct forms were often heavily armored and continued to filter-feed as adults. Modern forms (cyclostomes) include lampreys and hagfishes, both eel-shaped and parasitic on other fishes: lampreys suck blood; hagfishes eat their way through the flesh of their victims.

Class Placodermi: An extinct group in which jaws first evolved. Paired fins also evolved in this group and are retained in all further vertebrate classes. Many placoderms were predators from 6 inches up to 50 feet long.

Class Chondrichthyes: Cartilaginous fishes, including sharks, skates, and rays. Bone is reduced to a series of tooth-like denticles embedded in the skin. The rest of the skeleton is made of cartilage only.

Illustrations

Class Osteichthyes: Bony fishes, including the vast majority of fishes. Scales and internal skeleton are both usually bony. A wide variety of sizes, shapes, and habits occurs in this group. One great subgroup has fins with ray-like supports but no internal muscles; a much smaller subgroup has fleshy, lobe-like fins with internal muscles.

Subclass Actinopterygii	Subclass Sarcopterygii
"Ray-finned" fishes: Flat fins, supported by bony rays with no internal muscles.	"Lobe-finned" fishes: Thick, fleshy fins supported by strong bones and internal muscles.
No internal nostrils or nasal passages.	Internal nostrils and nasal passages (choanae).
Lung transformed into swim bladder.	Lung usually maintained as a lung.
Thin, bony scales (cycloid or cternoid).	Thick, shiny "ganoid" scales (or cosmoid, derived from ganoid).

Origin of land vertebrates (tetrapods): The first tetrapods (amphibians) evolved from a group of bony fishes called Crossopterygians, who already had lungs and internal nostrils. The critical change transformed the fleshy fins into walking legs.

Class Amphibia: Eggs are laid in contact with fresh water, then fertilized externally. Larvae ("tadpoles") breathe with gills, then undergo metamorphosis into an adult, usually with lungs and legs. Living species always have slippery, moist skin. Examples: salamanders, newts, frogs, toads, and extinct labyrinthodonts.

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Embryonic membranes: Reptiles, birds, and mammals all have amniote eggs within which several specialized membranes develop from the embryo:

Archosaur traits include a diapsid skull condition (two openings in the temporal region of the skull), a third opening (the antorbital fenestra) in front of the eye, a bony sclerotic ring around the eye, and an early tendency toward bipedalism, which includes very strong hind limbs and a long, strong tail.

Aves (birds): Warm-blooded, feathered vertebrates ("glorified reptiles" or "glorified archosaurs").

Flight adaptations: Birds have adaptations for flying, including wings, feathers (modified scales), and good vision. Modern flying birds also have strong ribs and a rigid sternum (breastbone) with a keel.

Metabolism: A high rate of metabolism is needed for flight. Birds are warm-blooded, meaning that their metabolic rates and body temperatures are always rather high, regardless of external temperature. Downy feathers are part of an insulating layer that makes a high internal body temperature possible. The complete separation of oxygen-poor and oxygen-rich blood in the heart also increases metabolic efficiency.

Weight reduction: Modern birds have many adaptations that reduce weight, including reduction of the tail bones, loss of the teeth and lightening of the jaws, development of hollow air spaces in the arm bones, and loss of one ovary in female birds.

Archaeopteryx and the origin of birds: Archaeopteryx, the oldest bird, was preserved in a fine-grained Jurassic limestone. It had many reptile characteristics, including a long tail, slender ribs, weak sternum, small braincase, and jaws with teeth. However, it also had feathers and was probably capable of flight, and is, therefore, classified as a bird. Scientists now think that birds evolved from carnivorous dinosaurs.

Modern birds:

Modern flightless birds include the ostriches, rheas, moas, and kiwis.
Modern flying birds include owls, gulls, storks, pigeons, eagles, hawks, woodpeckers, and many perching birds (Passeriformes), which make up the largest group.

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Class Mammalia: Vertebrates covered with insulation, usually hair or fur (occasionally blubber).

Metabolic rates and body temperatures are kept high (homeothermy).

Glands in skin secrete sweat and oily secretions (sebum).

Young mammals are nursed by their mothers; milk is secreted by mammary glands, derived from sweat glands. Frequent parental care.

Normal standing posture keeps the body elevated from the ground, compared to the low-slung posture of amphibians and reptiles. Three tiny ossicles (malleus, incus, stapes) transmit sounds in middle ear. Four-chambered heart has complete separation of oxygen-rich and oxygen-poor blood. Only one bone, the dentary, makes up the lower jaw on each side. A muscular diaphragm is responsible for most breathing movements. A bony hard palate separates nasal cavity from oral cavity, allowing breathing and chewing at the same time. Teeth vary in shape with their position in the mouth and are restricted to only two waves of growth and replacement instead of many. Brain and braincase larger than in reptiles.

Origin of mammals: Mammals evolved during Triassic times from mammal-like reptiles (Synapsida). The transition involved changes in the teeth and tooth replacement, the replacement of one jaw hinge (between articular and quadrate bones) with another (between dentary and squamosal bones), and the conversion of the articular and quadrate bones into the malleus and incus.

Mammal origins

Monotremata: Egg-laying mammals. Example: platypus.

Marsupials: Pouched mammals. Examples: kangaroo, opossum, koala.

Placental mammals (Eutheria): Mammals in which the fetus is nourished in utero by a placenta. Includes the vast majority of mammals, arranged in over 30 orders, about half extinct and half with living members. Examples: shrews, mice, bats, rats, cows, deer, pigs, dogs, cats, monkeys, humans, whales, horses, elephants, rabbits.

Among the many orders of placental mammals are these:

Leptictida: small, ancestral placentals (extinct)
Edentata (Xenarthra): sloths, armadillos, anteaters, etc.
Insectivora (Lipotyphla): hedgehogs, shrews, moles, etc.
Scandentia: tree shrews (tupaiids)
Chiroptera: bats, the second largest mammalian order
Primates: lemurs, tarsiers, monkeys, apes, and humans
Rodentia: rodents (squirrels, beavers, mice, porcupines, etc.), the largest order of all
Lagomorpha: rabbits
Macroscelidea: "elephant-shrews" of Africa
Creodonta: early, extinct carnivores
Carnivora: dogs, cats, bears, racoons, weasels, hyaenas, seals, sea lions, etc.
Condylarthra: ancient hoofed mammals (extinct)
Tubulidentata: aardvarks
Artiodactyla: pigs, camels, deer, giraffes, cattle, goats, etc., with split hoofs
Cetacea: whales, dolphins, etc.
Six extinct orders of ungulates confined to South America before the Panama land bridge existed
Perissodactyla: horses, rhinoceroses, and tapirs
Hyracoidea: hyraxes
Proboscidea: elephants, mastodonts, etc.
Sirenia: manatees, etc.
. . and several other small orders

Mammal diversity

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Primate characteristics, mostly related to arboreal adaptations (life in the trees):

Lemuroidea or Strepsirhini: Lemurs, lorises, and galagos.

Tarsioidea: Tarsius and its extinct relatives.

Platyrrhina: New World monkeys and marmosets, with 3 premolars in each jaw, flat noses, and strong tails that aid in locomotion.

Catarrhina: Old World monkeys, apes (gibbons, orangutan, gorilla, chimpanzee) and humans, with 2 premolars in each jaw, protruding noses (nostrils opening downward), and reduced tails, native to Africa, Asia, and Europe.

Family Hominidae (humans): Catarrhine primates distinguished from apes

Habitual use of language

Origin of Hominidae: Approximately 5-6 million years ago when upright posture was attained. Human footprints at Laetoli, Kenya, are 4.1 million years old.

Evolutionary "dead ends": A number of hominid fossils are now considered to be evolutionary "dead ends," not ancestral to modern humans. These include Sahelanthropus, Ororrin, Kenyapithecus, Ardipithecus, and the large or "robust" Australopithecus robustus and A. boisei.

Australopithecus: The best-known early hominids, from South Africa and East Africa. Certain early species (Australopithecus anamensis, A. afarensis) may have been ancestral to Homo, but later species were not. One nearly complete skeleton of A. afarensis, nicknamed "Lucy," was only about 4 feet tall and walked upright.

Homo habilis: An East African contemporary of Australopithecus, from about 4 to 1.5 million years ago. Body size about 4 feet tall. Perhaps responsible for early stone tools.

Homo erectus: Lived in the middle Pleistocene, after the extinction of Australopithecus. Fossils known from Europe, Africa, Asia. In a cave near Beijing, China, heat-fractured rocks show that fire was used.

Homo sapiens: First appeared in the late part of the Ice Age. Taller skull than earlier species. Used more advanced tools. Invented agriculture around 8,000 years ago.

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Calories and Work: Energy is the ability to do work. It is measured in calories.
A calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius (or one degree Kelvin).
Food energy is usually measured in kilocalories. One kilocalorie, equal to 1000 calories, is the energy required to raise the temperature of a kilogram of water one degree Celsius. (IMPORTANT: the "calories" that you see on food labels are really kilocalories.)
(In the SI or Metric System, energy is measured in Joules; a calorie is equivelent to 4.184 Joules, and a kilocalorie is 4184. Joules.)
Basal metabolic rate (BMR) is measured in calories of energy used per minute, per gram or kilogram of body weight. "Basal" means that it is measured at rest (lying awake).
Any activity or exercise increases the metabolic rate above this basal value, by twofold or more for vigorous activity like running.
Metabolic rate and size: For animals of the same general shape, volume (and also mass)
are approximately proportional to the cube of body length. If Length (L) doubles, and width doubles, and height doubles, then volume and mass increase 8-fold (2x2x2 = 8). However, surface area increases in proportion to the square of body length. (A sphere of radius r has a surface area of 4πr2/3.)
The heat generated by muscle activity varies approximately in proportion to body mass, which is proportional to volume or to L3, but the rate of head LOSS across the body's surface varies with surface area and is thus proportional to L2. For this reason, an animal twice as large generates 8 times the internal heat, but only loses it 4 times as fast, and so is much more able to maintain its body temperature simply as a consequence of size. (The ratio of surface to volume is proportional to L2/L3, or 1/L.) This is thought to be one reason why many dinosaurs were so big— it was an adaptation to conserve heat. On the other hand, very small animals need to maintain a much higher metabolic rate (and therefore eat more in proportion to their body size) in order to compensate for rapid heat loss at small sizes.
This relationship helps explain the graph in the illustrations below.

Illustrations: Energy
ENERGY, TEMPERATURE, and ENVIRONMENTAL ADAPTATIONS:

BODY TEMPERATURE:

Poikilotherms: Most animals are poikilotherms, meaning that their body temperature varies and is usually close to that of their surroundings. Most of the time, they are "temperature conformers" whose body temperature matches their surroundings, but they may have behavioral mechanisms to regulate their body temperatures by sunning themselves to warm up of by burrowing underground or seeking ponds to avoid the dangers of overheating. Metabolism in poikilotherms varies with temperature: they tend to be slow and sluggish in the cold, and more active in warm conditions. Very few reptiles or amphibians live north of the U.S./Canada border. Fishes living in very cold waters often have special "anti-freeze" compounds in their blood and body fluids.
Homeotherms: Mammals and birds are homeotherms, meaning that their body temperature is fairly constant and usually warmer than their surroundings. To maintain higher temperatures, homeotherms must maintain much higher metabolic rates than poikilotherms of comparable size. Homeotherms must also have good body insulation (hair, fur, blubber, or feathers) to minimize heat loss. Some dinosaurs, pterosaurs (extinct flying reptiles), and synapsids (mammal-like reptiles) may have been homeotherms-- several dinosaurs had feathers, and pelycosaurs had sail-like membranes that may have been used to collect or to dissipate heat. Heat-regulating adaptations include:
- Sweating, which cools the body by evaporation to prevent overheating
- Counter-current blood flow to the limbs, which minimizes heat loss (see the accompanying illustration using the link above)
- Temporary hyperthermia in gazelles and camels, to minimize heat loss to hot surroundings
- Hibernation in small mammals

NUTRITION IN ANIMALS:

Calories and Work: Energy is the ability to do work. It is measured in calories.
A calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius (or one degree Kelvin).
Food energy is usually measured in kilocalories. One kilocalorie, equal to 1000 calories, is the energy required to raise the temperature of a kilogram of water one degree Celsius. (IMPORTANT: the "calories" that you see on food labels are really kilocalories.) (In the SI or Metric System, energy is measured in Joules; a calorie is equivelent to 4.184 Joules, and a kilocalorie is 4184. Joules.)
As explained above, small mammals need to eat more in proportion to their body weight, just to compensate for metabolic heat loss.
Carbohydrates: Carbohydrates include sugars and starches. The majority of caloric energy in most diets usually comes from carbohydrates.
- Monosaccharides are single-unit sugars, usually occurring in ring-like molecular shapes. Ribose and Deoxyribose are 5-carbon sugars that occur in nucleic acids like DNA and RNA.
  The nutritionally important monosaccharides are all 6-carbon sugars with the formula C6H12O6. They include glucose, fructose and galactose.
  
  All carbohydrates are digested into individual monosaccharides that the body then absorbs and uses for energy.
- Disaccharides are sugars containing two rings and usually 12 carbons. Sucrose (table sugar) is made of glucose and fructose units; it can be split into a moleculae of fructose and a molecule of glucose by adding water, a process called hydrolysis (literally, "splitting by water") that usually takes place with the help of an enzyme. The most familiar enzymes are named after the molecule that they split: sucrase splits sucrose into glucose plus fructose. Lactose is another common disaccharide. The enzyme lactase splits lactose into glucose plus galactose.
- Polysaccharides are made of many sugar units linked together as a polymer (meaning a chain of similar molecular units). Starch and cellulose are two polysaccharides, each made of multiple glucose units. Humans and many other mammals have enzymes to help digest starch, but no mammal has enzymes to digest cellulose without the help of symbiotic gut bacteria.
Amino acids and proteins: Amino acids all contain a central carbon, bonded to a hydrogen atom, a —COOH carboxyl group, an —NH2 amino group, and a "side chain" that differs from one amino acid to another. Twenty common amino acids occur in proteins; other (less common) amino acids can be made from these twenty. Eight of the twenty common amino acids are considered "essential" in human nutrition (nine in infants) because they are all needed to build proteins, but the body can convert these eight into all the others.
A protein is built of a chain (or polymer) of many amino acids. The simplest proteins are coiled into a helix (called an "alpha" helix) in which one twist of the coil includes three amino acid units. Most proteins bend or fold into compact and complex shapes by various physical and chemical forces. For example, some amino acids are positively charged, while others are negatively charged, and the positive and negative charges attract one another (while two positive charges or two negative charges will repel one another), bending the protein chain out of shape.
Proteins are digested by splitting them into individual amino acids. Once absorbed by the body, the individual amino acids are used to build proteins that the body needs, especially for growth and for tissue repair.
Lipids, meaning fat-soluble (or water-insoluble, nonpolar) compounds. The most common dietary lipids are triglycerides, made of a molecule of glycerol linked to three fatty acid chains. Gram for gram, lipids provide more caloric energy than carbohydrates. They are also used by the body to synthesize important membrane components, hormones, and certain brain chemicals. (Overconsumption, however, can lead to obesity, heart disease, diabetes, clogged blood vessels (atherosclerosis), and other problems.)
The above food components are called macronutrients-- things that the body needs in significant quantities. The vitamins and minerals, on the other hand, are called micronutrients because they are only needed in very small amounts, usually measured in milligrams (sometimes even micrograms).
Vitamins are complex organic compounds needed only in tiny amounts. Most vitamins function as parts of enzymes that get re-used at the molecular level, over and over again, so that a tiny amount can make a big difference.
Minerals are individual chemical elements (usually metallic elements) required in tiny amounts. In most cases, the minerals are important parts of molecules (like enzymes) that get re-used again and again, so a little bit goes a long way. Calcium and iron are two of the most important elements required in human diets. Eating a good VARIETY of foods is usually the best way to make sure you are getting all the important mineral nutruents. Milk products (including cheese and ice cream) are the best source of calcium; beans, eggs, and meats are among the best sources of iron.

DIGESTION IN ANIMALS:
Unicellular organisms ingest food by phagocytosis. Most digestion by lower animals is intracellular, but extracellular digestion (both mechanical and chemical) predominates among higher animals.

"Assembly line" (mouth-to-anus) digestion allows regional specialization to evolve, so that digestion happens in stages.

Intracellular digestion predominates in lower animals.
In phagocytosis, folds of the cell membrane engulf food material originally outside the cell, forming a vacuole. Chemical digestion then follows when these vacuoles fuse with lysosomes.

Extracellular digestion predominates in higher animals.

Mechanical digestion: Food is minced or crushed, exposing more surface area.
Chemical digestion: Food is broken down chemically with the help of enzymes.

"Cul-de-sac" versus "assembly line" digestion:

Among Cnidaria, digestive wastes can only go out the mouth. New food that is taken in will therefore include many wastes recently discarded (an inefficient process). Also, every portion of the digestive tract must carry out all phases of digestion, so there is no possibility of regional speciailzation (for different nutrients) or of sequential processing.
Among animals with a separate mouth and anus, digestion proceeds in stepwise fashion, as in an assembly line. Different regions can be adapted to handle different nutrients, or different steps in the sequential processing of complex nutrients like proteins.

TISSUE ORGANIZATION and the division of labor: In most multicellular organisms, cells are specialized to do different things (the "division of labor").

Cells of the same type are generally organized into tissues, defined as groups of similar cells (and their extracellular products) built together ("structurally integrated") and working together ("functionally integrated"). For example, the several tissues that line the gut (the digestive tube) are shown in one of the accompanying illustrations.

Illustrations: digestion

VERTEBRATE DIGESTIVE SYSTEMS:

Mouth: Major site of mechanical digestion (using teeth, etc.). Chemical digestion begins with amylase enzyme in saliva that breaks down starches. Food passes from mouth to stomach via the esophagus.

Stomach: Major site of protein digestion. Pepsin breaks proteins into small peptides. Hydrochloric acid (HCl) acidifies stomach contents; this helps pepsin, which works best at acidic pH (near 2.0). Mechanical digestion also occurs by contraction of 3 muscle layers, kneading food. Some species (like birds) have a large storage crop, followed by a strong muscular gizzard, specialized for mechanical digestion.

Liver: Secretes bile, containing bile pigments (derived from hemoglobin) and bile salts, which emulsify fats. Bile is stored in the gall bladder until needed.

Bile salts are soap-like ("amphipathic") molecules with a long nonpolar hydrocarbon chain and a polar (water-soluble) "head" at one end. Emulsification is a process in which these molecules coat each fat droplet with the polar "heads" while the nonpolar "tails" dissolve in the fat. Without emulsification, small fat droplets bump into one another and fuse to form bigger and bigger droplets. Emulsified droplets bounce off one another and remain small, without fusing. This increases their surface area, and thus the area over which they can be attacked by fat-digesting enzymes. Small intestine: Long and coiled, with large surface area.

Duodenum: Digestion of lipids, carbohydrates, and proteins, using enzymes secreted by the duodenum itself and by the pancreas. Lipids are emulsified by bile salts secreted by the liver.
Jejunum: Chemical digestion of most nutrients is completed here.
Ileum: Site of most absorption of digestion end-products.

Large (large-diameter) intestine: Includes:

Caecum (size varies), site of microbial digestion of cellulose in herbivores
Humans and carnivores have small caeca. Rodents, rabbits, horses, and other herbivores have large caeca that contain many millions of symbiotic bacteria that help digest plant cell walls, producing sugars. Many of the sugars are also absorbed by the lining of the caecum. Cows and other ruminant Artiodactyla are an exception: instead of a large caecum, they have a partitioned stomach, with a different bacterial flora in each of the four chambers.
Colon site of much water resorption

Anus: Undigested wastes are eliminated.

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Organismal Biology #31

GAS EXCHANGE and BODY FLUIDS

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ONLINE CLASSROOM VERSION

Flatworms and other flattened animals need no special organs for gas exchange because no cell is very far from a body surface. More complex animals use lungs, gills, or tracheal tubes.

Anatomy and physiology of gas exchange in land vertebrates:

Nostrils take air into nasal cavity, then into pharynx. Floor of pharynx opens behind mouth into larynx (voice-box); entrance to larynx is guarded by epiglottis.

The trachea, bronchi, and bronchioles form tree-like branchings within each lung.

The lungs have air sacs lined with box-like alveoli.

Air exchange: In inhalation (inspiration), diaphragm contracts and moves downward while intercostal muscles raise rib cage. In exhalation (expiration), muscles relax, rib cage falls, diaphragm springs upward.

Gas exchange in alveoli: Oxygen enters capillaries of lung through thin walls; CO2 leaves capillaries and diffuses into air sac.

Gas exchange within capillary blood: In lung alveoli, oxygen enters red blood cells, combines with hemoglobin, and is transported as HbO2 (oxyhemoglobin); bicarbonate ions enter blood cells and are split into water and CO2. The reverse occurs in body tissues: oxyhemoglobin breaks down to release oxygen; CO2 and water combine to form bicarbonate ions (HCO3–).

Gill systems: In fishes and many other aquatic animals, thin-walled arteries run through gills with direction of blood flow usually opposite to flow of water (counter-current exchange). Oxygen diffuses into these arteries; CO2 diffuses into surrounding water.

Insect tracheal systems: Air diffuses through many branched tubes (tracheae). Air movement is passive most of the time, but, when flying, rhythmic muscular contractions force air in and out.

Illustrations: Body fluids

Very small or very thin organisms need no special system for internal transport. Many invertebrates have an open system, with blood vessels opening into a general circulatory cavity or hemocoel. Vertebrates and annelids have a closed circulatory system: their hearts pump blood from atrium to ventricle and then through the major arteries; veins return blood to the heart.

Simple forms of transport:

Cytoplasmic streaming (cyclosis): Cytoplasm in all eucaryotic cells continually flows and changes direction.
Diffusion: Passive transport in all organisms, effective only at distances of a few cells. This may suffice for organisms in which each part is only a few cells away from a body surface, but larger animals need circulatory systems.

Open circulatory systems: Systems in which a body cavity or hemocoel contains most of the circulating fluid, as in insects.

The pumping action of a heart drives fluid forward through an aorta, then through a series of arteries. No veins exist; used blood seeps into sinuses that drain into the hemocoel.

Closed circulatory systems: Systems in which blood is everywhere contained in vessels, as in all vertebrates.

The heart may have 2 to 4 chambers. The heartbeat originates from a pacemaker at the sinoatrial node. Highest pressure, at maximum contraction, is called systole; lowest pressure is called diastole.
In mammals, the right atrium pumps oxygen-poor blood from the body's tissues into the right ventricle, which pumps it through the pulmonary arteries into the lungs. The left atrium meanwhile pumps oxygen-rich blood from the lungs into the left ventricle, which pumps it through the aorta for distribution throughout the body.
Arteries carry blood from the heart to the body's tissues.
Veins return the blood from the body's tissues back to the heart.
Valves in veins prevent the blood from flowing backward.
Vertebrate blood is always red because of the oxygen-carrying pigment hemoglobin, carried in red blood cells (erythrocytes).

OSMOREGULATION and EXCRETION:

Freshwater organisms tend to gain water across membrane surfaces and must actively get rid of it. Land and marine organisms tend to lose water; they must retain water and excrete salt. Vertebrate kidneys filter the blood first, then retrieve (resorb) useful molecules.

Osmotic pressure measures the level of dissolved ions in solution.
Hypotonic solutions (low osmotic pressure, few dissolved ions): Cells swell (or may burst) because water diffuses in. Freshwater organisms always gain water from hypotonic surroundings; they void lots of dilute urine and may actively take up some ions.
Hypertonic solutions (high osmotic pressure, many ions): water diffuses out; cells shrink. Marine and land animals lose water across membranes; they excrete concentrated urine or salt-rich fluids.
Isotonic solutions: Cells have the same concentration of dissolved ions. Water enters and leaves at the same rate, so cells stay the same size.
Simple excretory systems: Some freshwater protists pump water out by contractile vacuoles. Many small aquatic animals allow wastes to diffuse out. Flatworms have single-celled excretory tubules called flame cells.
Nephridial systems: Tubules (nephridia) drain coelomic fluid from the body cavity and exchange ions with small blood vessels nearby.
Vertebrate kidneys: Cortex (outer layer) contains mostly glomeruli and convoluted tubules; medulla (inner layer) is made of several medullary pyramids, which contain Henle's loops.
Kidney tubules: Blood plasma is filtered from a series of thin-walled blood vessels (the glomerulus) into Bowman's capsule. In the proximal convoluted tubule, the blood resorbs glucose and some ions. In mammals, Henle's loop resorbs water. In the distal convoluted tubule, more ions return to the blood. Collecting tubules finally concentrate the urine and drain into the renal pelvis, which drains into the ureter.
Nitrogen wastes: In mammals, the principal nitrogen waste is urea. Reptiles and insects excrete uric acid instead. Fishes and many other aquatic animals usually excrete ammonium salts.
Other organs of excretion: Lungs and gills get rid of CO2. Animals excrete salt and nitrogen wastes through the skin.