Animals

 

What is an Animal?

Animals are:

Eukaryotic

Multicellular

Heterotrophic organisms that obtain nutrients by ingestion

Able to digest their food within their bodies

Animal cells lack the cell walls that provide strong support in the bodies of plants and fungi.

Most animals:

Have muscle cells

Have nerve cells that control the muscles

Are diploid

Reproduce sexually

Proceed through a series of typically similar developmental stages

 

Early Animals and the Cambrian Explosion

Animals probably evolved from a colonial flagellated protist that lived in Precambrian seas about 600–700 million years ago.

At the beginning of the Cambrian period, 542 million years ago, animals underwent a rapid diversification.

During a span of about 15 million years:

All major animal body plans we see today evolved

Many of these animals seem bizarre

 

Animal Phylogeny

Biologists categorize animals by:

General features of body structure

More recently, using genetic data

One major branch point distinguishes sponges from all other animals because, unlike more complex animals, sponges lack true tissues.

A second major evolutionary split is based on body symmetry.

Radial symmetry refers to animals that are identical all around a central axis.

Bilateral symmetry exists where there is only one way to split the animal into equal halves.

Animals also vary according to the presence and type of body cavity, a fluid-filled space separating the digestive tract from the outer body wall.

There are differences in how the body cavity forms.

If the body cavity is not completely lined by tissue derived from mesoderm, it is a pseudocoelom.

A true coelom is completely lined by tissue derived from mesoderm.

 

Sponges

Sponges represent multiple phyla.

Sponges include sessile animals that lack true tissues and that were once believed to be plants.

The body of a sponge resembles a sac perforated with holes.

Choanocyte cells draw water through the walls of the sponge where food is collected.

 

Cnidarians

Cnidarians (phylum Cnidaria) are characterized by:

The presence of body tissues

Radial symmetry

Tentacles with stinging cells

The basic body plan of a cnidarian is a sac with a gastrovascular cavity, a central digestive compartment with only one opening.

The body plan has two variations:

The sessile polyp

The floating medusa

Cnidarians are carnivores that use tentacles, armed with cnidocytes (“stinging cells”), to capture prey.

 

Molluscs

Molluscs (phylum Mollusca) are represented by soft-bodied animals, usually protected by a hard shell.

Many molluscs feed by using a file-like organ called a radula to scrape up food.

The body of a mollusc has three main parts:

A muscular foot used for movement

A visceral mass housing most of the internal organs

A mantle, which secretes the shell if present

The three major groups of molluscs are:

Gastropods, protected by a single, spiraled shell.

Bivalves, with a shell divided into two halves hinged together.

Cephalopods, which typically lack an external shell and are built for speed and agility.

 

Flatworms

Flatworms (phylum Platyhelminthes) are the simplest bilateral animals.

Flatworms include forms that are:

Parasites or

Free-living in marine, freshwater, or damp habitats

The gastrovascular cavity of flatworms is highly branched

Provides an extensive surface area for absorption of nutrients

 

Annelids

Annelids (phylum Annelida) have:

Body segmentation, a subdivision of the body along its length into a series of repeated parts

A coelom

A complete digestive tract with two openings (a mouth and anus), and one-way movement of food.

The three main groups of annelids are:

Earthworms, which eat their way through soil

Polychaetes, marine worms with segmental appendages for movement and gas exchange

Leeches, typically free-living carnivores but with some bloodsucking forms

 

Roundworms

Roundworms (phylum Nematoda) are:

Cylindrical in shape, tapered at both ends

The most diverse and widespread of all animals

Roundworms (also called nematodes) are:

Important decomposers

Dangerous parasites in plants, humans, and other animals

 

Arthropods

Arthropods (phylum Arthropoda) are named for their jointed appendages.

There are about one million arthropod species identified, mostly insects.

Arthropods are a very diverse and successful group, occurring in nearly all habitats in the biosphere.

There are four main groups of arthropods.

General Characteristics of Arthropods

Arthropods are segmented animals with specialized segments and appendages for an efficient division of labor among body regions.

The body of arthropods is completely covered by an exoskeleton, an external skeleton that provides:

Protection

Points of attachment for the muscles that move appendages

Arachnids:

Live on land

Usually have four pairs of walking legs and a specialized pair of feeding appendages

Include spiders, scorpions, ticks, and mites

Crustaceans:

Are nearly all aquatic

Have multiple pairs of specialized appendages

Include crabs, lobsters, crayfish, shrimps, and barnacles

Millipedes:

Eat decaying plant matter

Have two pairs of short legs per body segment

Centipedes:

Are terrestrial carnivores with poison claws

Have one pair of short legs per body segment

Insects typically have a three-part body:

Head

Thorax

Abdomen

The insect head usually bears:

A pair of sensory antennae

A pair of eyes

The mouthparts are adapted for particular kinds of eating.

Flight is one key to the great success of insects.

Insects outnumber all other forms of life combined.

Insects live in:

Almost every terrestrial habitat

Freshwater

The air

Many insects undergo metamorphosis in their development.

Young insects may:

Appear to be smaller forms of the adult or change from a larval form to something much different as an adult

 

Echinoderms

Echinoderms (phylum Echinodermata):

Lack body segments

Typically show radial symmetry as adults but bilateral symmetry as larvae

Have an endoskeleton

Have a water vascular system that facilitates movement and gas exchange

Echinoderms are a very diverse group.

 

Characteristics of Chordates

Chordates (phylum Chordata) all share four key features that appear in the embryo and sometimes the adult:

A dorsal, hollow nerve cord

A notochord

Pharyngeal slits

A post-anal tail

Another chordate characteristic is body segmentation, apparent in the:

Backbone of vertebrates

Segmental muscles of all chordates

Chordates consists of three groups of invertebrates:

Lancelets are bladelike animals without a cranium.

Tunicates, or sea squirts, also lack a cranium.

Hagfishes are eel-like forms that have a cranium.

All other chordates are vertebrates.

 

Fishes

The first vertebrates were aquatic and probably evolved during the early Cambrian period, about 542 million years ago. They:

Lacked jaws 

Are represented today by hagfishes

Lampreys:

Are vertebrates

Have a cranium

But lack jaws

The two major groups of living fishes are the:

Cartilaginous fishes (sharks and rays) with a flexible skeleton made of cartilage

And bony fishes with a skeleton reinforced by hard calcium salts

Bony fishes include:

Ray-finned fishes

Lungfishes

Lobe-finned fishes

Cartilaginous and bony fishes have a lateral line system that detects minor vibrations in the water.

To provide lift off the bottom:

Cartilaginous fish must swim but

Bony fish have swim bladders, gas-filled sacs that make them buoyant

 

Amphibians

Amphibians:

Exhibit a mixture of aquatic and terrestrial adaptations

Usually need water to reproduce

Typically undergo metamorphosis from an aquatic larva to a terrestrial adult

Amphibians:

Were the first vertebrates to colonize land

Descended from fishes that had lungs and fins with muscles

Terrestrial vertebrates are collectively called tetrapods, which means “four feet.”

Tetrapods include:

Amphibians

Reptiles

Mammals

 

Reptiles

Reptiles (including birds) and mammals are amniotes, which produce amniotic eggs that consist of a fluid-filled shell inside of which the embryo develops.

Reptiles include:

Snakes

Lizards

Turtles

Crocodiles

Alligators

Birds

Reptile adaptations to living on land include:

Amniotic eggs

Scaled, waterproof skin

Non-bird reptiles are ectotherms, sometimes referred to as “cold-blooded,” which means that they obtain their body heat from the environment.

A non-bird reptile can survive on less than 10% of the calories required by a bird or mammal of equivalent size.

Reptiles diversified extensively during the Mesozoic era.

Dinosaurs were the largest animals ever to live on land.

Recent genetic evidence shows that birds evolved from a lineage of small, two-legged dinosaurs during the great reptilian radiation of the Mesozoic era.

Birds have many adaptations that make them lighter in flight:

Honeycombed bones

One instead of two ovaries

A beak instead of teeth

Unlike other reptiles, birds are endotherms, maintaining a warmer and steady body temperature.

Birds wings adapted for flight are airfoils, powered by breast muscles anchored to a keel-like breastbone.

 

Mammals

The first true mammals:

Arose about 200 million years ago

Were probably small, nocturnal insect-eaters

Most mammals are terrestrial although dolphins, porpoises, and whales are totally aquatic.

Mammalian hallmarks are:

Hair

Mammary glands that produce milk, which nourishes the young

There are three major groups of mammals:

Monotremes, egg-laying mammals

Marsupials, pouched mammals with a placenta

And eutherians, placental mammals

Eutherian placentas provide more intimate and long-lasting association between the mother and her developing young than do marsupial placentas.

 

The Evolution of Primates

Primates evolved from insect-eating mammals during the late Cretaceous period.

Primates are distinguished by characteristics that were shaped by the demands of living in trees. These characteristics include:

Limber shoulder joints

Eyes in front of the face

Excellent eye-hand coordination

Extensive parental care

Taxonomists divide the primates into three main groups.

The first group of primates includes:

Lorises

Pottos

Lemurs

Tarsiers form the second group.

The third group, anthropoids, includes monkeys.

Hominoids consist of humans and their ape relatives.

The Emergence of Humankind

Humans and chimpanzees have shared a common African ancestry for all but the last 5–7 million years.

 

Some Common Misconceptions

Chimpanzees and humans represent two divergent branches of the anthropoid tree that each evolved from a common, less specialized ancestor.

Our ancestors were not chimpanzees or any other modern apes.

Human evolution is not a ladder with a parade of fossil hominids (members of the human family) leading directly to modern humans.

Instead, human evolution is more like a multi-branched bush than a ladder.

At times in hominid history, several different human species coexisted.