Lesson 24 Lesson Outline: Phylogenetic Trends in Digestive System Form and Function Objectives: Throughout the course what you need to master is an understanding of: 1) the form and function of structures, 2) the phylogenetic and ontogenetic origins of structures, and 3) the extend to which various structures are homologous, analogous and/or homoplastic. References: Chapter 12: 265-291 Reading for Next Lesson: Chapter 13: 292-313
Exercise #1 system: List the basic functions of the various components of the digestive Processes: Ingestion Storage Mechanical Digestion churning Propulsion swallowing peristalsis Chemical Digestion Absorption Defecation Functional Concepts Digestion actually occurs outside the body. Substances are mechanically and chemically broken down. Substances then enter the body by transport across mucosal cells of the alimentary canal. Digestion consists of the breakdown of substances in a controlled area outside (but within) the body so that the products of the breakdown can be efficiently absorbed. From your notes, you should be able to go through each major subdivision of the digestive system and describe their functions in detail
Exercise #2 Describe the evolutionary trends that we see in the digestive systems of the different vertebrate groups: At its simplest in the protochordates, the digestive system was a tube. These animals used cilia to create a water current that drew water in through the mouth and over the gills where it could be strained by the gills. Food particles adhered to mucous lining the gills, and were trapped. The trapped food and mucous was then transported to the primitive gut where the bolus of food was moved along by peristalsis, and chemical and mechanical digestion took place, and the digested substances were absorbed. In the cyclostomes, the alimentary canal is a straight tube leading from mouth to anus without coils, or major bends. The ciliated esophagus runs directly from the pharynx to the intestine. There is no distinct stomach present (remember they eat detritus and blood, and small particulate matter rasped from prey). In the Chondricthys we see the evolution of jaws, teeth, and active predation. This allowed these fishes to eat large pieces of food. This requires not just jaws, but teeth. The teeth evolve from the surface armour. In the Chondricthyes teeth develop from the dermal denticles. Initially they evolved not for chewing, but for grasping. This leads to the evolution of a more complex digestive systems including: - a stomach in which to store and mechanically digest the food - a more definitive pancreas to aid in the chemical digestion of food - a well developed intestine for absorption of food. Beginning with the gnathostome fishes, there is considerable variation in the design of the alimentary canal reflecting the changes in diet that are seen in individual groups. The size and length of the stomach, small intestine, and large intestine reflects the diet of the animal and amount of surface area required for digestion, and absorption of nutrients and reabsorption of water.
Exercise #3 Comparisons For the following structures: Case 1 Based on the anatomy that you see, what can you say about the lifestyles of the animals possessing these digestive structures?
Case 2 Based on the anatomy that you see, what can you say about the lifestyles of the animals possessing these digestive structures?
Exercise #4: Mammals are the only vertebrates that secrete digestive enzymes into their saliva and begin the process of chemical digestion in the mouth. Can you link this to: 1) the evolution of cheeks and lips and 2) the evolutionary changes seen in the pectoral and pelvic limbs /girdles and the movement of the limbs underneath the body?