Lecture 28

                                                          November 9, 2001



pp 589-598


I. Introduction

A. Animal life began in Precambrian seas.

B. The first animals exploited previously untapped re­sources and entered new adaptive zones leading to the development of many diverse forms.

C. Animal life on Earth today is the result of 600 million years of evolution.

 D.More than one million extant species have been described. Estimates of the total number of animal species range from 10-30+ million.

E.There are about 35 animal phyla. Most are primarily marine.

F.Invertebrates are animals without backbones that constitute 95% of all animal species.

G.In the next series of lectures we will explore the diversity of invertebrates and the levels of organization that they represent.

 II. What is an Animal?

The following characteristics help to distinguish animals.

  A.Animals are multicellular, heterotrophic, eukaryotes.

1.Most ingest other organisms.

2.Others are symbiotic - demonstrating relationships that range from host-parasite interactions to mutualistic bonds.

B.Carbohydrate reserves are stored as glycogen, rather than starch as in plants.

 C.Animals lack a cell wall. They have unique types of intercellular junctions:tight junctions, desmosomes and gap junc­tions.

D.Two types of tissue are unique to animals (when compared to other kingdoms):muscle                    and nerve.

E.Features of the life history also distinguish animals.

1.During development the zygote forms a blastula (often a hollow ball of cells).

2.The blastula undergoes reorganization and produces a gastrula that forms embryonic tissues of the adult body.

3.Two general forms of development are seen:(1) direct development in which the young resemble the adult, and (2) development of a larval stage, which is very different from the adult organism.

F. Animals inhabit all environments of the biosphere.

1. The greatest number of animal taxa are marine.

2. Only a few taxa have been successful in terrestrial environments: land snails, athropods and vertebrates.

 III.    Comparative Anatomy and Embryology Provide Clues to Animal Phylogeny

A.The Precambrian ancestor from which animals arose was proba­bly a choanoflagellate.

B.By the early Cambrian Period all the main animal phyla were represented.

 IV. Three Significant Evolutionary Levels of Organization

Tissue development, symmetry, coelom development.

A. Sponges

(Phylum Porifera) represent a unique taxon characterized by a cellular level  of organization.

1.They are basically aggregates of cells, lacking true tissues.

2.Because of their uniqueness, they are often placed in a separate taxon from other animal phyla called Parazoa. All other animal phyla are Eumetazoa.

  B.Cnidaria represent the most primitive eumetazoans. They are at the tissue level of organization.

1. Cnidarians, such as hydra and jellyfish, exhibit radial symmetry.

2. They have no head or rear end. Instead the mouth opening identifies the oral surface and opposite it is the aboral surface.

3. Cnidaria are diploblastic. There are two embryonic germ layers present:(1)ectoderm, which produces the outer tissue layer, and (2) endoderm, which produces the inner tissue layer.

Platyhelminthes represent the most primitive bilateral organisms. They are at the organ level of organization.

1. Platyhelminthes, such as Dugesia, exhibit bilateral symmetry. A bilateral animal is characterized by an anterior and posterior end, as well as a top (dorsal) and bottom (ventral) surface.

2. Associated with bilateral symmetry is cephalization. Cephalization involves the concentration of nerve tissue and sensory organs at the anterior end.

3. Bilaterality and concomitant cephalization produce an animal that is more aware of its environment (as it moves forward) and exhibits a more active life style.

4. Platyhelminthes are triploblastic. There are three embryonic germ layers present:(1) ectoderm, (2) endoderm, and (3) mesoderm.

5. Flatworms do not have a true body cavity between the gut and outer body wall. They are  referred to as acoelomate, literally without a body cavity.

V. Coelom Development

  A.Animals with a body cavity are referred to as coelomates.

  B.A true coelom is a fluid-filled body cavity completely lined by tissue of mesodermal origin.

 C.The coelom confers certain advantages.

1. Its fluid cushions suspended organs and protects them.

2 It enhances processing of nitrogenous wastes (excretion).

3. It enhances circulation.

4. It serves for development and storage of gametes.

5. In some animals it serves as a hydrostatic skeleton.


D.Animals with a body cavity usually have some form of blood vascular system.

  E.Coelomate phyla are divided into two distinct evolutionary lineages:

1.Protostomes include the mollusks, annelids, and arthropods.

2.Deuterostomes include echinoderms and chordates.


VI. The Protostome-Deuterostome Split

Fundamental differences in developmental processes separate deuterostomes from protostomes.

A.Cleavage patterns

1.Protostomes - spiral, determinate cleavage

2. Deuterostomes - radial, indeterminate cleavage

 B.Blastopore fate

1.Protostome mouth develops from blastopore.

2.Deuterostome - mouth is derived from a secondary opening, and the anus usually develops           from the blastopore.

C.Coelom formation

1.Protostomes - solid masses of mesoderm split to form the coelom - schizocoelous development.

2.Deuterostomes - mesoderm pockets from the wall of the archenteron form the coelom - enterocoelous development.


Read your notes and these notes.

Know the characteristics of an animal. How are animals different from the rest of kingdoms?

Know the three levels of evolution in animals.

Know what is meant by the following terms: blastula, gastrula, direct development, larval stage, Parazoa, Eumatazoa, diploblastic, triploblastic, cephalization, acoelomate, coelom.

Know the functions of the coelom.

Know the differences between protostome and deuterostome development.