Chapter 29- Comparing Invertebrates

I. Invertebrate Evolution

A. Origin of the Invertebrates

1. Invertebrates fossils have been found to date between 610-570 million years ago.

2. Now molecular paleontology studies the fossils DNA.

3. The Ediacaran fossils include some of the earliest and most primitive animals.

4. The Cambrian period, 544 million years ago has many different fossils.

5. Example: Burgess Shale animals.

B. Invertebrate Phylogeny

1. Some features evolved over time also.

2. These features include tissues, and organs, patters of early development, body symmetry, cephalization, segmentation and the formation of three germ layers and a coelom.

C. Evolutionary Trends

1. Specialized cells, tissues and organs.

2. Body symmetry- Sponges lack symmetry BUT all other invertebrates have some type of body symmetry.

3. Cephalization-Invertebrates with cephalization can respond to the environment in more sophisticated ways than can simpler invertebrates.

4. Segmentation

5. Coelom formation- most complex animals have a true coelom that is lined completely with tissue derived from the mesoderm.

6. Embryological development- Some of them are deuterostomes, meaning they have an anus.

II. Form and function in invertebrates

A. Feeding and Digestion

1. Intracellular and extracellular digestion- the simplest animals break down food primarily through intracellular digestion, but more complex animals use extracellular digestion.

2. Extracellular digestion- Food is broken down outside the cells in a digestive cavity and then absorbed into the body cavity.

B. Respiration

1. Respiratory organs have large surface areas that are in contact with the air or water. Also, for diffusion to occur the respiratory surfaces must be moist.

C. Circulation

1. Most complex animals move blood through their bodies using one or more hearts and either an open or closed circulatory system.

2. Open circulatory- blood is only partially contained within a system of blood vessels.

3. Closed circulatory- a heart or something like it forces blood through vessels within the body.

D. Response-invertebrates show three trends in the evolution of the nervous system: centralization, cephalization and specialization.

1. Cephalization- where if any sense organs the animal has are located at the front of the head.

2. Specialization- cells with particular cells.

E. Movement and Support

1. Invertebrates have one of three main kinds of skeletal systems: hydrostatic skeleton, exoskeleton, or endoskeleton.

2. Hydrostatic skeleton- muscles are surrounded by a fluid filled body cavity.

3. Exoskeleton- External skeleton.

4. Endoskeleton- A structural support located inside the body.

F. Sexual and Asexual Reproduction.

1. Most invertebrates reproduce sexually during at least part of their life cycle. Depending on environmental conditions, many invertebrates may also reproduce asexually.

2. External fertilization- fertilization outside of the body.

3. Internal fertilization- fertilization inside of the body.

The End