2. C HAPTER 27: I NTRODUCTION TO A NIMALS Section 2: Animal Body Systems

3. T ISSUES AND O RGANS Digestion Single celled organisms and sponges digest their food within their body cells. All other animals digest their food extracellularity (outside of their body cells) within a digestive cavity.

4. T ISSUES AND O RGANS Simple animals, such as the hydra and flatworms, have a gastrovascular cavity, a digestive cavity with only one opening.

5. T ISSUES AND O RGANS Other animals have a digestive tract (gut) with two openings, a mouth and an anus.

6. T ISSUES AND O RGANS Respiration In simple animals, oxygen gas and carbon dioxide gas are exchanged directly with the environment by diffusion.

7. T ISSUES AND O RGANS The uptake of oxygen and the release of carbon dioxide, called respiration, can take place only across a moist surface.

8. T ISSUES AND O RGANS Some aquatic (and a few terrestrial) animals respire with gills – very thin projections of tissue that are rich in blood vessels.

9. T ISSUES AND O RGANS In more advanced animals, lungs are the respiratory organs used to transfer oxygen into the blood and remove carbon dioxide from blood.

10. T ISSUES AND O RGANS Circulation In complex animals, oxygen and nutrients must be transported to the body cells by a circulatory system.

11. T ISSUES AND O RGANS Two types of circulatory systems: Open circulatory system: heart pumps fluid containing oxygen and nutrients through a series of vessels out into the body cavity.

12. T ISSUES AND O RGANS Closed circulatory system: a heart pumps blood through a system of blood vessels.

13. T ISSUE AND O RGANS Conduction of Nerve Impulses Nerve cells (neurons) are specialized for carrying messages in form of electrical impulses. Bilaterally symmetric animals have clusters of neurons called ganglia.

14. T ISSUE AND O RGANS More complex invertbrates, such as the grasshopper, have brains with sensory structures.

15. T ISSUE AND O RGANS Support Many soft-bodied invertebrates have a hydrostatic skeletal systems. Hydrostatic skeleton - consists of water that is contained under pressure in a closed cavity.

16. T ISSUE AND O RGANS Other invertebrates, such as insects, have an exoskeleton, which is a rigid external skeleton that encases the body of an animal.

17. TISSUE AND O RGANS An endoskeleton is composed of a hard material, such as bone, embedded within an animal.

18. T ISSUE AND O RGANS Excretion The term excretion refers to the removal of wastes produced by cellular metabolism.

19. T ISSUE AND O RGANS Simple aquatic invertebrates and some fishes excrete ammonia into the water through their skin or gills by diffusion.

20. T ISSUE AND O RGANS Other animals, especially terrestrial animals, convert ammonia to nontoxic chemicals, like urea. As the excretory system eliminates these wastes, water and other useful substances are returned to the body.

21. R EPRODUCTIVE S TRATEGIES Asexual Reproduction Reproduction that does not involve the fusion of two gametes is called asexual reproduction. An unusual method of asexual reproduction is parthenogenesis, in which a new individual develops from an unfertilized egg.

22. R EPRODUCTIVE STRATEGIES Animals that reproduce asexually are usually able to also reproduce sexually.

23. R EPRODUCTIVE S TRATEGIES Sexual Reproduction In sexual reproduction, a new individual is formed by union of a male and a female gamete.

24. R EPRODUCTIVE S TRATEGIES Gametes are produced in the sex organs. Males have testes that produce sperm. Males produce sperm until death.

25. R EPRODUCTIVE S TRATEGIES Females have ovaries that produce eggs. At birth, females have produced all the eggs they will ever have.

26. R EPRODUCTIVE S TRATEGIES Some species of animals, called hermaphrodites, have both testes and ovaries.

27. R EPRODUCTIVE S TRATEGIES Most aquatic animals simply release the male and female gametes near one another in the water, where fertilization occurs. This is known as external fertilization.

28. R EPRODUCTIVE S TRATEGIES Most terrestrial animals sexually reproduce by means of internal fertilization. Internal fertilization occurs when the sperm and egg unite inside of the female’s body.