    Multicellular heterotrophs Most have muscle and nervous tissue Most reproduce sexually, with a flagellated sperm and a large egg uniting to form a diploid zygote.

The diploid stage dominates the life cycle.

      The zygote is a fertilized egg.

The zygote of an animal undergoes a series of mitotic divisions called cleavage.

An eight-cell embryo is formed by three rounds of cell division.

Cleavage results in a multicellular stage called the blastula.

The blastula is typically a hollow ball of cells around a cavity called the blastocoel.

Most animals also undergo gastrulation , in which one end of the embryo folds inward to form two layers of embryonic tissue, ectoderm and endoderm; eventually fills in the blastocoel to form the gastrula.

Zygote

cleavage

8 cell stage

cleavage

Blastula Gastrulation

G

Gastrula Blastocoel

 The pouch formed by gastrulation is the archenteron



Cnidarians and Ctenophores are diploblastic.

 Ectoderm: Forms epidermal layer of the skin and nervous system  Endoderm : Forms the lining of the gut the liver and the lungs.

 Mesoderm forms muscles, bone, kidney, gonads, and connective tissue.

   Symmetry is the overall shape of the body.

If an animal is symmetrical, it can be divided along at least one plane into similar halves.

Animals that have no plane of symmetry are asymmetrical. SPONGES are asymmetrical.

   Simplest form of symmetry is spherical symmetry , in which body parts radiate out from a central point. In infinite number of planes can pass through the central point and divide the organism into halves. Unicellular protists.

Radial Symmetry : one main axis and any plane cutting through that axis divides the animal into similar parts. Cnidarians and Ctenophores, adult Echinoderms Bilateral Symmetry cephaliztion.

: Animal can be divided into a mirror “right” and “left” halves. Animals that move in one direction; Associated with

 Concentration of sensory equipment at one end (the anterior or head end) of the organism.

   Acoelomate animals, such as flatworms lack an enclosed, fluid-filled body cavity, the space is filled with masses of cells called mesenchyme.

Pseudocoelomate animals have a body cavity called a pseudcoel, in which many of the internal organs are suspended. It is enclosed by muscles only on the outside.

Coelomate animals have a body cavity that develops within the mesoderm. The coelom is enclosed on both sides by mesoderm.



Cushions the suspended organs



Acts as a hydrostatic skeleton



Enables internal organs to grow and move independently

     Many animals have bodies that are divided into segments.

Segments allow specialization of body parts.

Allow animals to alter shape of body to control movements and separate body cavity into compartments.

Arthropods and Annelids show segmentation.

Some animals have only internal segmentation, as in vertebrate embyos.

     Protostomes: Mouth forms from the blastopore Spiral, determinate cleavage Determinate means cells are commited to a specific fate as early as two cell stage Coelom forms from splits in the mesoderm      Deuterostomes Mouth forms from secondary opening Radial, indeterminate cleavage Indeterminate cleavage means cells are not fated until later in development Coelom forms from lining the archenteron out pockets of the mesoderm

      Lacks symmetry and true tissues?

Show radial symmetry and are diploblastic?

Have three tissue layers, but lack body cavity?

Show bilateral symmetry and have a psuedocoelom?

Have a true coelom and are protostomes?

Have a true coelom and are deuterostomes?