Digestive System Evolution

Introduction: All living organisms require food for various reasons. The necessity of food is due to satisfying three basic needs: fuel for cellular work in the body, organic materials used in biosynthesis (carbon skeletons), and essential nutrients, which are substances which an animal cannot make on its own. ATP used in respiration requires energy, which comes from carbohydrates, fats, and proteins. All animals need ATP in order to survive, and therefore must digest food in order to obtain this energy.

The adaptations of animals over many years has caused there to be three different types of dietary categories: herbivores, carnivores, and omnivores. All animals fit into these categories, and the same goes for the means of ingestion. Suspension-feeders, substrate-feeders, deposit-feeders, fluid-feeders, and bulk-feeders are the main groups of animal mechanisms for which they injest food. Animals have adjusted to fit into these different categories over time due to their environment and living conditions. Certain animals may need more protein to live, depending on how or where they are living. Others may need different types of vitamins. This is exactly why there is a diverse and vast number of digestive systems which coordinate with individual animals.

The digestive system includes four main "stages" of food processing: ingestion, digestion, absorption, and elimination. All organisms must partake in these stages of digestion in order to fully process their food into energy.

Digestion occurs in all different animals. Yet, they are all unified with the fact that digestion occurs in specialized compartments within the organism. Organisms could have intracellular digestion or extracellular digestion, but both contain a digestive compartment to keep the enzymes from going to other parts of the body. Most all animals similarly have complete digestive tracts, which are a series of tubes making somewhat of a "pathway" through the body.

Check out the rest for more detailed examples of different organisms' digestive systems!


Definition of a Digestive System: The organs in the body that break down and absorb food. Organs that make up the digestive system are the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. (Research Consortium)

Mouth: The digestive tract begins in the mouth of an organism. Chewing provides mechanical digestion, while saliva excreted into the mouth provides as chemical digestion. Glands in the mouth (salivary) along with the tongue help out with digestion, even though they are not part of the digestive system (BARC).

Teeth, which take part in the mechanical digestion, have been developed by organisms to fit their needs based on their diets. In general, carnivores have primarily incisors, herbivores have molars, and omnivores have a combination of both.



Source: http://www.ccs.k12.in.us/chsHAP/Web-Pig---open-mouth.gif
Source: http://www.ccs.k12.in.us/chsHAP/Web-Pig---open-mouth.gif

Esophagus: Muscular tube portion of the digestive system that leads from the mouth to the stomach. It is interesting to note how the pig esophagus' location in relation to other organs is similar to that of a humans, but is oriented horizontally rather than vertically.

Stomach: A muscular, sac-like organ that serves as the principle organ of digestion that uses digestive enzymes, such as pepsin, to break down proteins.


Small Intestine: Organ of the lower digestive system that includes three parts: duodenum, jejunum, and ileum. This organ is where most of the absorption and digestion occurs.
Duodenum: The first part of the small intestine that links the small intestine to the stomach.
Jejunum: The longest part of the small intestine that connect the duodenum to the ileum.
Ileum: final part of the small intestine that connects to the large intestine.

WOWZERS! They certainly don't call it the "small" intestine because of its length. This intestine is 10'2" long! This allows the organ to absorb as much nutrients as possible (increased surface area).
intestines.jpg IMG_0450_2.JPG

Large Intestine: Thick, tube-like organ of the lower digestive system that includes the cecum, colon, rectum, and anal canal. Water recycling, nutrient absorption, and waste processing occur in the large intestine.
Cecum: The pouch-like start of the large intestine that links the large intestine and small intestine.
Colon: The muscular tube section of the large intestine where water is absorbed.
Rectum: The last 5-8 inches (in humans) of the large intestine.
Anal Canal: Opening at the end of the digestive tract where excretion is expelled.
IMG_0299.JPG
This is the fetal pig's Large Intestine, still ravelled up


stomach,%20large,%20small.JPG
This diagram shows the location of the stomach, the large intestine and the small intestine.

Large_Intestine.JPG
This image is a cross section of a pig large intestine, and one can clearly see the tubular shape. This is the lower portion of the large intesting and has been separated completely from the small intestine. The beginning of the rectum is acutely noticeable.

Unity and Diversity:
Digestive Systems in Other Organisms


Mussel
source: http://kentsimmons.uwinnipeg.ca/16cm05/1116/33-21-ClamAnatomy-L.jpg
source: http://kentsimmons.uwinnipeg.ca/16cm05/1116/33-21-ClamAnatomy-L.jpg

source: http://kentsimmons.uwinnipeg.ca/16cm05/1116/33-21-ClamAnatomy-L.jpg

Mussels are a prime example of digestive system diversity as they are considered to be filter-feeders, which means that they take in water and their gills are used as to strain organic and inorganic particles from the water. Each gill sheet contains a cilia; the cilia sweep the particles toward the food groove, found on the edge of the gill filaments. From here the food particles are swept to the labial palps where food is sorted into edible and inedible. Edible particles are swept to the mouth - a thin-walled sac with a gasteric shield - while inedible particles are added to mucus and sent to the edge of the mantle. In the stomach, particles are digested through chemical and mechanical digestion. This type of dual digestion is also seen in the human mouth as food is chewed but salivary enzymes also work to break down food. Like the starfish, human and fetal pig, the mussels uses digetive enzymes to break down food particles. Also like the human, fetal pig and starfish, the mussel has an intestine. As in these other organisms, the intestine is responsible for carrying the digested food and waste products to the anus.

Starfish
source: http://faculty.baruch.cuny.edu/jwahlert/bio1003/images/starfish_arm.jpg
source: http://faculty.baruch.cuny.edu/jwahlert/bio1003/images/starfish_arm.jpg

source: http://faculty.baruch.cuny.edu/jwahlert/bio1003/images/starfish_arm.jpg
source: http://www.k-state.edu/organismic/images/starfish_mouth_cardiac_stomach.jpg
source: http://www.k-state.edu/organismic/images/starfish_mouth_cardiac_stomach.jpg

source: http://www.k-state.edu/organismic/images/starfish_mouth_cardiac_stomach.jpg

Food is brought into the stomach through the mouth or cardiac stomach. The cardiac stomach is similar to the human stomach as it centrally located within the organisms body. Despite the word "stomach" in its name, the cardiac stomach is more similar to the heart of the fetal pig, frog or human. However, some starfish are able to extend their cardiac stomachs through the mouth to digest food outside the body, using secreted enzymes. This exterior extension of the stomach is a quality unlike anything else we have examined in the other organisms. Suspension-feeding starfish use their tube feet to pass food into the mouth. The cardiac stomach is connected to a pyloric stomach that is located directly above it. The pylroic stomach is connected to the anus and to the pyloric ducts and pyloric cecum. The materials are then sent to each ray (limb of the starfish). Each ray then has its own digestive system including digestive glands. Digestive glands are one uniting trait found in most of the organisms we have dissected. The starfish has an interesting feature called the hemal system. The hemal system is responsible for distributing nutrients from the digestive tract. Hemal channels run down the rays.

Grasshopper
source: http://universe-review.ca/I10-82-grasshopper.jpg
source: http://universe-review.ca/I10-82-grasshopper.jpg

source: http://universe-review.ca/I10-82-grasshopper.jpg

Like the crayfish, the grasshopper's digestive tract has three major parts - the foregut, the midgut and the hindgut, just as the entire animal is divided into three different parts. The foregut is essentially the mouth region, while the hindgut is the anus. Food is brought from the pharynx to the crop (large storage organ) by the esophagus. As in humans, the esophagus in grasshoppers is a muscular tube. The crop stores food that has already been chewed up. The stomach of the grasshopper has six digestive glands as well as a gastric caecae – an organ that produces gastric enzymes. Most of the digestion occurs in the stomach. Digestion occurs in the intestine, which eventually leads to the rectum.The digestive tract of the grasshopper is remarkably similar to that of the crayfish as they are both arthropods (have segemnted bodies).

Perch
source :http://io.uwinnipeg.ca/~simmons/images/lb8fig1b.gif
source :http://io.uwinnipeg.ca/~simmons/images/lb8fig1b.gif

source :http://io.uwinnipeg.ca/~simmons/images/lb8fig1b.gif

source: http://bioweb.wku.edu/faculty/McDaniel/fish1.png
source: http://bioweb.wku.edu/faculty/McDaniel/fish1.png

source: http://bioweb.wku.edu/faculty/McDaniel/fish1.png


As in other organisms we have dissected, in a perch's digestive system the food is ingested through the mouth and then broken down by enzymes in the esophagus. This is different than the human espophagus as no food is broken down there. From the esophagus the food is passed to the stomach where it is further broken down by digestive enzymes. In many fish, such as perch, the food is broken down even further in finger-like pouches called the pyloric caeca, which secrete more digestive enzymes and absorb nutrients from the digested food. Starfish also have pyloric caeca. The intestines complete the digestive system of a perch where food bi-products are excreted.

Crayfish
external image digestive_system_l.gif
external image digestive_system_l.gif

Like many other organisms we have dissected, the crayfish has digestive glands which secrete enzymes to aid in the process of digestion. Similar to the grasshopper, the crayfish has a foregut, midgut and hindgut. The foregut, which is basically an enlarged stomach and mouth region, is responsible for grinding the food eaten by the crayfish. As in the human mouth, both mechanical and chemical digestion occur throughout the foregut. Next, the food travels to the midgut. Interestingly, in a human embryo there is a portion called the midgut; this is where the intestines are derived from.Eventually, food travels to the hindgut. This portion, responsible for salt and water regulation, leads to the anus.