Architectural Pattern of an Animal

Chapter 9

The Appearance of Major Body Plans

The Cambrian explosion marks the earliest fossil appearance of all major groups of living animals plus some groups that are only known from fossils. All major body plans

appeared at this time. Result of extensive

selection. Are limiting

determinant of future adaptational variants.

Body Plans

One way in which zoologists categorize the diversity of animals is according to general features of morphology and development.

A group of animal species that share the same level of organizational complexity is known as a grade.

Hierarchical Organization – 5 Levels

Protoplasmic grade of organization – protists are the simplest eukaryotes, but they still carry out life functions and show division of labor among the various cell structures.

Metazoans are multicellular animals that have cells specialized for particular functions. This is the Cellular grade of organization. Shown by the simplest metazoans –

Volvox, sponges.

Hierarchical Organization – 5 Levels

Cell-tissue Grade – Usually, specialized cells are grouped together and perform their common function as a coordinated unit, a tissue. Jellyfish

Tissue-organ Grade – Tissues are then assembled into organs like the heart (primarily muscle tissue, but connective, nervous, and epithelial also present). Flatworms

Hierarchical Organization – 5 Levels

Organ-system grade – In the highest level of organization, organs work together as organ systems like the circulatory system.

Animal Body Plans

Body plans are constrained by ancestry, major features may become modified, but are rarely lost.

Animal body plans differ in their grade of organization, body symmetry, number of germ layers, and type of body cavity.

Symmetry

Spherical symmetry occurs when any plane passing through the center divides the body into mirror image halves. Mostly found

among the protists.

Symmetry

Radial symmetry applies when more than two planes passing through the longitudinal axis can divide the organism into mirror image halves. Jellyfish

Biradial symmetry – two planes will divide the organism. Comb jellies

Radiata

The Cnidarians (jellyfish, corals & sea anemones) and Ctenophores (comb jellies), the radial or biradial animals, comprise the Radiata. No front/back Weak swimmers Can interact with

environment in all directions.

Symmetry

Bilateral symmetry is found in organisms where one plane can pass through the organism dividing it into right and left halves. Better for directional

movement. Monophyletic group

called Bilateria.

Cephalization

Bilateral symmetry is associated with cephalization, differentiation of a head.Nervous tissue, sense organs, and often the

mouth are located in the head.Advantages for organisms moving head first

– directional movement.Elongation along anteroposterior axis.

Regions of a Bilaterally Symmetrical Animal

Anterior-posterior (transverse plane)

Dorsal-ventral (frontal plane)

Left-right (sagittal plane)

Proximal-distal Medial-lateral

Body Cavities

Examples of body cavities include the gut, blastocoel, and a pseudocoel or coelom.The gut forms from the archenteron during

gastrulation.The blastocoel persists in some, but

usually fills with mesoderm.Pseudocoel and coelom are fluid filled

body cavities that cushion organs and provide support.

Body Cavities

In protostomes, mesoderm forms as endodermal cells from near the blastopore migrate into the blastocoel. Three body plans

possible: Acoelomate (no body

cavity) Pseudocoelomate (body

cavity between endoderm & mesoderm)

Coelomate (body cavity surrounded by mesoderm)

Body Cavities

Coeloms surrounded by mesoderm can arise in two ways: Schizocoely – mesodermal

cells fill the blastocoel, forming a solid band of tissue around the gut, then a space opens inside the mesodermal band.

Enterocoely – portions of the gut lining form pockets that pinch off and form a ring of mesoderm.

Developmental Patterns

Sponges develop only to blastula stage, then reorganize to form adult.

Gastrulation allows animals to proceed to tissue level organization. Diploblastic – 2 germ

layers Cnidarians,

Ctenophores Triploblastic – 3 germ

layers

Other Key Features of Body Plans

In some organisms, the gut does not form a complete tube.Waste must come back out the mouth.Food must be digested & waste expelled

before eating again.A complete gut forms a tube within a tube

body plan.

Other Key Features of Body Plans

Segmentation is a serial repetition of similar body segments along the body. Each segment is a

metamere or somite. May include external &

internal components. Obscured in many

animals, like humans. Permits greater body

mobility and complexity of structure & function.

9-20

Components of Metazoan Bodies

Extracellular Components - noncellular components of metazoan animals:Body fluidsExtracellular structural elements

Tissue Structure and Function

A tissue is a group of similar cells specialized for performing a common function.

Different types of tissues have different structures that are suited to their functions.

Tissues are classified into four main categories: Epithelial Connective Muscle Nervous

Cellular Components - Tissues

Epithelial Tissue

Epithelial tissue covers the outside of the body and lines organs and cavities within the body. Squamous, cuboidal,

columnar. Simple vs. stratified.

Connective Tissue

Connective tissue functions mainly to bind and support other tissues. Contains sparsely packed cells

scattered throughout an extracellular matrix.

Muscle Tissue

Muscle tissue is composed of long cells called muscle fibers capable of contracting in response to nerve signals. Smooth Skeletal Cardiac

Nervous Tissue

Nervous tissue senses stimuli and transmits signals throughout the animal.

A neuron (nerve cell) receive signals at the dendrites and send them out via the axons.

Complexity and Body Size

Increased complexity allows for an increase in body size.

Larger size decreases the surface area to volume ratio. Necessitates complex

systems for respiration, nutrition, and excretion – diffusion not adequate.

Buffers environmental fluctuation.

Escape predators.

Complexity and Body Size

Cost of maintaining body temperature is less per gram of body weight than in small animals.

Energy costs of moving a gram of body weight over a given distance less for larger animals.