3.Animal Diversity and Symmetry

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Animal Diversity


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What is an animal?

Your text defines an animal as a eukaryotic,multicellular, heterotroph, that lacks a cell wall.

Eukaryotic- separates animals from bacteria and blue-

green algae (cyanobacteria)

Multicellular- separates animals from any protists

Heterotroph- separates animals from plants and some

protists

Lacks a cell wall - separates animals from plants.


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Other distinguishing features- Life cycles. Most adultstages are diploid which form gametes by meiosis.Gametes (egg and sperm) fuse to form a new individualwhich begins as a diploid zygote. The zygote divides bymitosis to an early stage referred to as a blastula, asingle layer of cells surrounding a hollow cavity. Furtherdevelopment results in a gastrula which looks like an

indented blastula. The gastrula develops into a saclikeembryo. Eventually three layers of cells form fromwhich the epidermis and nervous system, the digestivetract, and other organs develop. Some animals develop

through several morphological forms. The mostimmature form is the larvae which eventually developsinto an adult through these stages. Development of thistype is called metamorphosis.


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March through the phyla of the kingdom Animalia

Porifera. These are the sponges. There are about 5000

species. Most are marine but some (150) are freshwateraquatic organisms. Most feed on bacteria that flowthrough their porous bodies.


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Cnidaria. Radial symmetry, seen is some sponges, is a

hallmark of Cnidarians. These include Hydra which have apolyp body form and jellyfish which have a medusa body

form. The two traits that mark an animal as a cnidarian isradial symmetry and the presence of cnidocytes (stinging

cells) used to capture prey and defend the animal. Theseanimals have a digestive cavity and a gastrula stage whichare not seen in sponges. However, these animals areconsidered to be "tissue animals" which means thefunctions are achieved by tissue rather than organs,

which are organized layers of tissues (different tissues)that perform a function or functions.


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Platyhelminthes. The flatworms. These animals showbilateral symmetry or left and right are symmetrical.They do have a dorsal (back or top) that is differentfrom the ventral (front or bottom). These animalsinclude the planaria that you played with in high school

biology, flukes, and tapeworms. Many are parasitic.These organisms do not have a body cavity except forthe digestive tract itself. The body cavity on higherorganisms is called a coelom and it is lined by a layer of

tissue from the middle in development. It was theevolution of the coelom that allowed for thedevelopment of organs.


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Nematoda. The roundworms. These are the firstorganisms covered by a nonliving layer of tissue calledthe cuticle, that protects the organisms from dryingand being crushed. It is a "skin". This allows these to

live in the soil rather that wet environments. They havea pseudocoelom. An animal with a pseudocoelom hasa cavity lined with tissue, but the tissue is only incontact with the outer wall and doesn't surrond the

digestive tract as it does with the coelom. Many of thenematodes are parasitic.


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Mollusca. The mollusks. This is a very morphologicallydiverse group and yet they share a basic body plan. Thefour points to look for is the presence of a foot forlocomotion, a radula used to scrape up food, a mantle

covering the body, and the presence of a coelom. Theseanimals also have a primitive heart and circulatorysystem. The phylum is divided further into thegastropods (snails and slugs), the bivalves (clams,

oysters, mussels, and scallops), and the cephalopods(squids and octopus).


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Annelida. The segmented worms. Earthworms,

polychaetes, and leeches are members of this phylum.This group is distinguished by having a ringlike bodythat is repeated in many segments. Only the head isdistinguished from the other segments externally.


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Arthropoda. The arthropods. They are distinguishedby the presence of an exoskeleton made of chitin, and

jointed appendages used for walking, swimming,capturing prey or defending against predators. Theygrow in size by shedding the exoskeleton through a

process called molting. The four groups of arthropodsare the horseshoe crabs, the crustaceans (lobsters,barnacles, crayfish), arachids (spiders, scorpions,mites), and the wormlike arthropods (millipedes and

centipeds).


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Insecta. The insects. There are more species of insectsthan all other species combined. They are similar toarthropods in that they have an exoskeleton of chitin but

are distinguished in that they have three body segments,the head, thorax, and abdomen. Most have wings that

allow for flight.


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Echinodermata. The echinoderms. These animals have a

spiny skin, and endoskeleton, and a water vascular systemfor movement. This group includes starfish, sea urchin,and sea stars.


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Chordata. The chordates. We are chordates. Ourphylum is distinguished by (1) a dorsal hollow nervechord (2) a notochord, a flexible, longitudinal rodlocated between the digestive tract and the nerve cord(3) gill structures behind the mouth and (4) and postanal tail. Doesn't sound much like you does it. Youhave all four of these during embryonic development

and except for the gills, you have remnants as adults.

The chordates are first divided into two groups, theinvertebrates and the vertebrates. The invertebratesinclude the tunicates and the lanclets. The

vertebrates have an endoskeleton that includes askull and a backbone segmented into vertebrae. Mosthave a hinged jaw. The vertebrates include fish,amphibians, reptiles, birds, and mammals.


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Spinosaurus aegyptiacus


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The fish are divided into the cartilaginous fish called the

Chondrichthyes, and the bony fish called the Osteichthyes.The cartilaginous fish include the shark. The bony fishhave a protective flap over the gills which can be moved.

This allows the fish to move water across the gills withoutmoving.


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Amphibians have two stages in life. An embryonic stagein which they have gills and swim in water and an adultstage in which they breath air and usually spend at leastsome time on the dry land. They must return to the waterto lay they eggs so that the next generation can again

start in the water.


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Reptiles include snakes, lizards, turtles, etc. Unlike

amphibians they do not have a stage where they live andbreath water. They have an amniotic egg which is filled

with fluid and covered in a shell that allows the egg to belayed on land rather than in water.

The chordates up to this point are all ectotherms whichmeans they do not regulate their own temperature. Theirbodies get heat from the environment. Birds andmammals are endotherms. They generate their own body

heat through metabolism.


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Birds are in the class Aves. They are covered in feathersand most are capable of flight.

Mammals are distinguished by the presence ofmammary glands that produce milk to nourish theiryoung and by the presence of hair. We divide these intothree groups. The montremes are the egg laying

mammals (the Duck-billed platypus). The marsupials arethe pouched mammals that carry embryo in amarsupium on the mothers abdomen where it isattached to a nipple. The placentals carry their embryointernally in a placenta and nourish them by transferingnutrients from mothers blood to the embryo's bloodsystem within the placenta tissue. The placenta isderived from tissue from both mother and embryo.


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This section shows a phylogenetic tree which shows the

evolutionary relationships between these organisms. Youmight be surprised to note that the echinoderms are themost near to the chordates. This is because of the internalskeleton. Notice that we begin with protists and develop

tissue. The next division is based on symmetry, then thepresence of a pseudocoelom or coelom. The next divisionwe really didn't discuss but is based on whether thecoelom comes from cell masses or from a digestive tube.These are the protostomes or deuterostomes respectively.

The deuterostomes include only the echinoderms and thechordates.


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3.Animal Diversity and Symmetry

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