Download Lesson 24 Lesson Outline: Phylogenetic Trends in Digestive System

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?