Introduction to Kingdom Animalia

Defining Animals

1. Animals are multicellular, heterotrophic eukaryotes.

2. Animals generally store their carbohydrate reserves as glycogen

3. Animal cells possess a number of unique cellular structures, e.g., gap junctions - intracellular junctions that allow the passage of materials between cells

4. Animals possess special tissues that are responsible for impulse conduction (nervous tissue) and movement (muscle tissue).

5. Most animals reproduce sexually, with the diploid stage dominating the life cycle

Animal Phylogeny• Origin of most animal phyla and major body plans took place in Precambrian era and early Cambrian period of the Paleozoic era (circa 500-600 mya)• Most systematists agree that the animal kingdom is monophyletic• By the early Cambrian period (about 550 mya), virtually all known animal phyla had evolved from the first animals of the late Precambrian

Trends in Animal Evolution

1. The first trend was a shift from a body plan called radial symmetry to a body plan referred to as bilateral symmetry• Radial symmetry - a circular body plan having a central axis from which structures radiate outward• Bilateral symmetry - a body plan in which the right and left sides of the body are mirror images of each other; central longitudinal plane divides the body into 2 equal but opposite halves

Trends in Animal Evolution cont.

2. A trend toward increasing cephalization - the development of a distinct head, which has associated with it a brain and various kinds of sensory structures

3. A change from a simple sac-like body with a single opening at one end to a more complex, elongated body containing a tube called the "gut" with openings at both ends

4. A change from a tube enclosed in solid tissue toward suspension of the tube in a fluid filled space

5. An increase in body segmentation - the development of a series of body units, each containing similar sets of muscles, blood vessels, nerves. etc.

Major Events in Animal Phylogeny

Major Events in Animal Phylogeny cont.

1. The Parazoa-Eumetazoa Split

Two basic kinds of animals evolved early in animal evolution.

A. Parazoa ("beside the animal") - animals that lack true tissues. e.g., Sponges

B. Eumetazoa - animals with well defined tissue layers, e.g., essentially all other animals.

Major Events in Animal Phylogeny cont.

2. The Radiata-Bilateria Split

• The eumetazoa are divided into 2 major branches depending on the type of body symmetry •Some organisms exhibit radial symmetry, and are called the Radiata• Other eumetazoa exhibit bilateral symmetry, and are called the Bilateria

2. Radiata-Bilateria Split cont.• The Radiata-Bilateria split is also defined by the differences in the number of germ layers that are formed in the embryo during gastrulation• The Radiata are referred to as diploblastic - two germ layers form

• Ectoderm - covering the surface of the embryo; gives rise to the epidermis and in some phyla the nervous system.• Endoderm - inner most germ layer; lines the primitive gut; gives rise to the lining of the digestive tract and associated organs, such as the liver and lungs of vertebrates

• The Bilateria are triploblastic; in addition to the ectoderm and endoderm they produce a third germ layer, the mesoderm

• Mesoderm - germ layer between the ectoderm and the endoderm; gives rise to muscles and to most other organs

Major Events (Branch Points) in Animal Phylogeny con’t

3. The Acoelomate-Coelomate Split

A. Acoelomates - animals with solid bodies; there is no body cavity between the gut (endoderm) and the outer body wall. • The other 2 body plans are often referred to as a tube within a tube body plan; a fluid filled sac separate the gut from the outer body wall.• A second important difference between acoelomates and animals having a body cavity is that animals with a body cavity have some sort of blood vascular system

3. Acoelomate-Coelomate Split cont.

B. Pseudocoelomates - animals in which the body cavity is not completely lined with mesodermal tissue • The body cavity is called a pseudocoelom

C. Coelomates - animals that have a fluid filled body cavity that is completely lined with tissue that is derived from the mesoderm.• This kind of body cavity is called the coelom

Animal Phylogeny cont.

4. The Protostome-Deuterstome Split

Coelomates, can be divided into 2 distinct groups: protostomes and deuterostomes.They are distinguished based upon fundamental differences in early development, including cleavage, fate of the blastopore, and coelom formation

4. The Protostome-Deuterstome Split con’t

A. Cleavage

Protostomes - Spiral, determinate cleavage

Deuterostomes - Radial, indeterminate cleavage

4. The Protostome-Deuterstome Split cont.

B. Fate of the Blastopore

• During gastrulation, the rudimentary gut or archenteron forms; it has a single opening called the blastopore• A second opening forms later at the opposite end of the archenteron to produce a digestive tube with a mouth and anus• Protostomes - the blastopore becomes the mouth•Deuterstomes - the blastopore becomes the anus

4. The Protostome-Deuterstome Split cont.

C. Coelom Formation

• Protostomes - coelom formation is called schizocoelous development; coelom forms by splitting of mesoderm• Deuterostomes - coelom formation is called enterocoelous development; coelom forms as outpockets from the endoderm