• Several characteristics, taken together, sufficiently define the group– Heterotrophs– Reproduce sexually, with the diploid stage usually

dominating the life cycle– multicellular eukaryotes– lack cell walls– Their bodies are held together by structural proteins such as

collagen– Nervous tissue and muscle tissue are unique, defining

characteristics of animals– Tissues are group of cells that have a common structure,

function or both

Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers (with some exceptions)

ZygoteCleavage

Eight-cellstage

Figure 32.2-1

After a sperm fertilizes an egg, the zygote undergoes rapid cell division called cleavageCleavage leads to formation of a multicellular, hollow blastulaThe blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues

Reproduction and Development

ZygoteCleavage

Cleavage

Eight-cellstage

Blastula

Blastocoel

Cross sectionof blastula

Figure 32.2-2

ZygoteCleavage

Cleavage

Eight-cellstage

Blastula

Blastocoel

Cross sectionof blastula

Gastrulation

Cross sectionof gastrula

Blastocoel

Endoderm

Ectoderm

Archenteron

Blastopore

Figure 32.2-3

More Animal Characteristics

• Many animals have at least one larval stage

i. A larva is sexually immature and morphologically distinct from the adults

ii. A juvenile resembles an adult, but is not yet sexually mature

• Most animals, and only animals, have Hox genes that regulate the development of body form

i. Although the Hox family of genes has been highly conserved, it can produce a wide diversity of animal morphology

OTHEREUKARYOTES

Choanoflagellates

SpongesAn

imals

Other animals

Individualchoanoflagellate

Collar cell(choanocyte)

Figure 32.3

Animals can be characterized by “body plans”

• Symmetry – radial symmetry, with no front and back, or left or right• Radial animals are often sessile or planktonic – Two-sided symmetry is called bilateral symmetry

• Bilaterally symmetrical animals have a dorsal (top) and a vental (bottom) side A right and left side Anterior (head) and posterior (tail) ends Cephalization, the development of a head

(a) Radial symmetry

(b) Bilateral symmetry

Figure 32.7

Tissues

• Animal body plans also vary according to the organization of the animal’s tissues

• Tissues are collections of specialized cells isolated from other tissues by membranous layers

• During development, three germ layers give rise to the tissues and organs of the animal embryo

1. Ectotherm is the germ layer covering the embryo’s surface

2. Endoderm is the innermost germ layer and lines the developing digestive tube, called the archenteron

3. Mesoderm lies between the ectoderm and endoderm

Body Cavities

• Sponges and a few other groups lack true tissues• Diploblastic animals have ectoderm and endoderm These include cnidarians and comb jellies • Triploblastic animals also mesoderm; these include

all bilaterians Most triploblastic animals possess a body cavity A true body cavity is called a coelom and is derived

from mesoderm– Coelomates are animals that possess a true coelom

• These include flatworms, arthropods, vertebrates and others

(a) Coelomate

Coelom

Digestive tract(from endoderm)

Body covering(from ectoderm)

Tissue layerlining coelomand suspendinginternal organs(from mesoderm)

(b) PseudocoelomateBody covering(from ectoderm)

Pseudocoelom Muscle layer(frommesoderm)

Digestive tract(from endoderm)

(c) Acoelomate

Body covering(from ectoderm)

Wall of digestive cavity(from endoderm)

Tissue-filled region(frommesoderm)

Figure 32.8

A pseudocoelom is a body cavity derived from the mesoderm and endoderm

Triploblastic animals that lack a body cavity are called acoelomates

Coeloms

• The coelom is a cavity entirely surrounded by mesoderm.

• A coelom provides a tube-within-a-tube arrangement which has many advantages:

Allows visceral organs to grow independently of the body wall

Fluid-filled coelom acts as a hydrostatic skeleton in some animals (e.g. earthworms)

In mammals, the pericardial, peritoneal, and pleural cavities are formed from the coelom

Protostome and Deuterostome Development

• Based on early development, many animals can be categorized as having protostome development or deuterostome development– In protostome development, cleavage is spiral and determinate– In deuterostome development, cleavage is radial and indeterminate – With indeterminate cleavage, each cell in the early stages of cleavage retains the capacity to develop into a complete

embryo – Indeterminate cleavage makes possible identical twins, and embryonic stem cells

In some embryos, the daughter blastomeres are either above or to the side of each other. This is said to be radial-type symmetry. In some embryos, the daughter blastomeres are not direclty over or beside each other. They are tilted to the left or right 45 degrees. This latter cleavage symmetry is said to be spiral.

(a) Cleavage

(b) Coelom formation

(c) Fate of the blastopore

Key

Ectoderm

Mesoderm

Endoderm

Protostome development(examples: molluscs,

annelids)

Deuterostome development(examples: echinoderms,

chordates)

Eight-cell stage Eight-cell stage

Spiral and determinate Radial and indeterminate

Archenteron

Coelom

Coelom

Blastopore BlastoporeMesoderm Mesoderm

Folds of archenteronform coelom.

Solid masses of mesodermsplit and form coelom.

Anus

Anus

Mouth

Mouth

Digestive tube

Mouth develops from blastopore. Anus develops from blastopore.

Figure 32.9

New views of animal phylogeny are emerging from molecular data

• Zoologists recognize about three dozen animal phyla

• Phylogenies now combine morphological, molecular, and fossil data

• Current debate in animal systematics has led to the development of multiple hypotheses about the relationships among animal groups

ANCESTRALCOLONIALFLAGELLATE D

eutero

stom

iaP

roto

stom

ia

Bilateria

Eu

metazo

a

Metazo

a

Porifera

Cnidaria

Ctenophora

Ectoprocta

Brachiopoda

Echinodermata

Chordata

Platyhelminthes

Rotifera

Mollusca

Annelida

Arthropoda

Nematoda

Figure 32.10

One hypothesis of animal phylogeny is based mainly on morphological and developmental comparisons

ANCESTRALCOLONIALFLAGELLATE

Deu

terosto

mia L

op

ho

troch

ozo

a

Bilateria

Eu

metazo

a

Metazo

a

Ecd

ysozo

a

Porifera

Ctenophora

Cnidaria

Acoela

Echinodermata

Chordata

Platyhelminthes

Rotifera

Ectoprocta

Brachiopoda

Mollusca

Annelida

Nematoda

Arthropoda

Figure 32.11

One hypothesis of animal phylogeny is based mainly on molecular data

Points of Agreement

1. All animals share a common ancestor

2. Sponges are basal animals

3. Eumetazoa is a clade of animals (eumetazoans) with true tissues

4. Most animal phyla belong to the clade Bilateria, and some are bilarians

5. Chordates and some other phyla belong to the clade Deuterostomia

Progress in Resolving Bilaterian Relationships

• The morphology-based tree divides bilaterians into two clades: deuterostomes and protostomes

• In contrast, recent molecular studies indicate three bilaterian clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa

• Ecdysozoans shed their exoskeletons through a process called ecdysis

Figure 33.2

ANCESTRALPROTIST

Commonancestor ofall animals

Porifera

Cnidaria

Lophotrochozoa

Ecdysozoa

Deuterostomia

Eu

metazo

a Bilateria

Invertebrates are animals that lack a backbone that account for 95% of known animal species

Figure 33.3aPorifera (5,500 species)

A sponge

Cnidaria (10,000 species)

A jelly

Acoela (400 species)

Acoel flatworms (LM)

1.5 mm

Placozoa (1 species)

0.5 mm

A placozoan (LM)

Ctenophora (100 species)

A ctenophore, or comb jelly

Platyhelminthes(20,000 species)

A marine flatworm

Acanthocephala(1,100 species)

Curvedhooks

An acanthocephalan (LM)

Mollusca(93,000 species)

An octopus

A ribbon worm

A cycliophoran(colorized SEM) A marine annelid

100 m

Lophotrochozoa

Nemertea(900 species)

Cycliophora(1 species)

Ectoprocts A rotifer (LM)

A brachiopod

0.1

mm

Annelida(16,500 species)

Ectoprocta(4,500 species)

Rotifera(1,800 species)

Brachiopoda(335 species)

Figure 33.3b

Loricifera (10 species)

50 m

Ecdysozoa

A loriciferan (LM)

Priapula (16 species) Onychophora (110 species)

A priapulan

Nematoda(25,000 species)

Tardigrada(800 species)

Arthropoda(1,000,000 species)

An onychophoran

A roundworm(colored SEM)

Tardigrades(colorized SEM)

A scorpion (an arachnid)

100 m

Figure 33.3c

• 12.1. Advent of Multicellularity

• A. Advantages

• 1. Nature’s experiments with larger organisms without cellular differentiation are limited.

• 2. Increasing the size of a cell causes problems of exchange; multicellularity avoids surface-to-mass problems.

• 3.cell assemblages in sponges are distinct from other metazoans, but molecular evidence shows common ancestry

•B. Form and Function•1. Body openings consist of small incurrent pores or ostia and a few excurrent oscula.

•2. Openings are connected by a system of canals; water passes from ostia to osculum.

•3. Choanocytes or flagellated collar cells line some of the canals.

–a. They keep the current flowing by beating of flagella.

–b. They trap and phagocytize food particles passing by.

•4. The framework of the sponge is composed of needle-like calcareous or siliceous spicules or organic spongin fibers.

Phylum Porifera

A. General Features

• Porifera means "pore-bearing"; their sac-like bodies are perforated by many pores.

• They are sessile and depend on water currents to bring in food and oxygen and carry away wastes.

• Their body is a mass of cells embedded in gelatinous matrix and stiffened by spicules of calcium carbonate or silica and collagen.

• They have no organs or tissues; cells are somewhat independent.

• Being sessile, they have no nervous or sense organs and have simplest of contractile elements

• They are aside from the mainstream of animal evolution and thus they are often called Parazoa

• Most of the 5000 species are marine, about 150 are freshwater

• Morphology changes with substratum, calmness of water etc…

• Sponges are ancient (fossils extend to Cambrian Period

Lophotochozoa• The clade Lophotrochozoa was identified by

molecular data• Some develop a lophophore for feeding,

others pass through a trochopore larval stage and a few have neither feature

• Ex: flatworms, rotifers, ectopracts, brachiopods, molluscs, annelids

Phylum Cnidaria• Two forms – Polyp and medussaPolyps = sessileMedusa = free swimming• Cnidocytes = stinging cells on tentacles -

carnivores• Gastrovascular cavity = central body cavity

Phylum Platyhelminthes• Flatworms• Diffusion replaces body systemGas exchange takes place across the surface,

and protonephridia regulate the osmotic balance• Reproduce asexually

by fission• Reproduce sexually

by cross fertilization• flukes and tapeworms

Phylum Nematoda

• Roundworms• Some are parasitic Hookworms = drink blood of GI tract Trichirella found in pig muscle Filarial roundworms infect lymphatic

system

Phylum Annelida

• Segmented worms Closed circulatory system Five pair of hearts Pharynx draws in food Crops store food Gizzard grinds food Intestine absorbs nutrients Rest is passed through the anus

Phylum Arthropoda• Dominant animals wrt numbers

– exoskeleton made of chitin– efficient gas exchange– Well developed sensory system– Well developed nervous system– Well developed circulatory system

Phylum Mollusca

• Shells of calcium carbonate– mantle lays down the shell

• Open circulatory system( except for cephalopods)

• Radula tongue made of chitin used to scrape for food

• Bivalve named for number of shells• About three-quarters of all living species of

molluscs are gastropods

Phylum Echinodermata• Water-vascular system for locomotion,

respiration and food acquisition• Lack circulatory system • Have regenerative capabilities