An Introduction to Animal Diversity

Page
3

Animal Body Symmetry

(a) Radial symmetry

(b) Bilateral symmetry

Slide 17

Body Plan -- 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.

Slide 18

Ectoderm is the germ layer covering the embryo’s surface.

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

Diploblastic animals have ectoderm and endoderm.

Triploblastic animals also have a middle mesoderm layer; these include all bilaterians.

Embryonic Germ Layers

Slide 19

Body Cavities 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.

A pseudocoelom is a body cavity derived from the mesoderm and endoderm. Triploblastic animals that possess a pseudocoelom are called pseudocoelomates.

Triploblastic animals that lack a body cavity are called acoelomates.

Slide 20

Triploblastic Animals Body Cavities

Coelom

Body covering

(from ectoderm)

Digestive tract

(from endoderm)

Tissue layer lining coelom

and suspending internal organs

(from mesoderm)

(a) Coelomate - true body cavity

Body covering

(from ectoderm)

Pseudocoelom

Digestive tract

(from endoderm)

Muscle layer

(from

mesoderm)

(b) Pseudocoelomate

Body covering

(from ectoderm)

Tissue-

filled region

(from

mesoderm)

Wall of digestive cavity

(from endoderm)

(c) Acoelomate - lack a body cavity

Slide 21

Cleavage: protostome 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.

Slide 22

Protostome

Development

molluscs, annelids

Deuterostome

Development

echinoderm, chordates

Eight-cell stage

Eight-cell stage