The Digestive System

By Michaela, Amelia, Gabby


  • Fetal Pig (Mammalia)
  • Frog (Amphibian)

The digestive system's purpose in all organism is to breakdown consumed food and to convert it into nutrients which is organism can use to function as a whole. The two types of vertebrates that we observed were that of the fetal pig and frog. All vertebrates share similar characteristics of the digestive system. The digestive system in vertebrates is made up a tract consisting of several different organs with different purposes, and additional accessory organs which produce hormones and enzymes that are vital in the process of digestion. There are some varying structures in vertebrates' digestive tract, but they all relate to the organism's surroundings and assist in the most efficient way to consume and breakdown food into nutrients.

Salivary Glands:


The salivary glands are commonly located on both sides of the cranium. The glands produce salivary amylase, an enzyme that begins the breakdown of starch to glucose. The enzyme travels down tubules from the gland into the mouth when it is acitvated by a hormone that senses the consumption of food. The salivary gland is an example of an accessory because it does not directly interact with the food.
Fetal Pig: The above image illustrates the salivary gland on the right side of the pig. Along with salivary gland, the Stensen's duct is evident, which brings the enzyme to the mouth.


This is where the consumption of food begins in a vertebrate. Both chemical and mechanical digestion take place in the mouth. The salivary amylase from the salivary glands combines with the food and begins the chemical breakdown and the teeth combine the consumed food with the salivary amylase to create a bolus. Teeth will vary for each organism, depending on their diet. For example, most herbivores have flat molars for grinding plants and breaking up some of the cellulose in the plants, while carnivores have sharp canine teeth to help tear at meat and assist in killing their prey.
Fetal Pig: In the above image the fetal pig's tongue, hard palate (roof of the mouth), and canine teeth are shown. The tongue assists in swallowing along with the hard palate. The canine teeth are just starting to develop, but given more time the pig would generate more molars because its diet consists mainly of vegetation.
Fish: The mouth of the fish are minute and almost invisible to the naked eye. This is because of the fish's diet and the manner in which it consumes food. Fish very rarely have to attack their prey or tear it apart to eat it, which would eliminate the need for canine teeth. Also, fish do not consume large leafy vegetation and thus would not need molars. In most cases fish consume their food without chewing it because it is so small and the food follows the next step in the process of digestion.

Esophagus: The esopagus connects the mouth to the stomach and carries the food bolus through a smooth muscle tube with muscle contractions known as peristalsis.

Stomach: The stomach is where the bolus from the mouth is mechanically and chemically digested. Food is temporarily stored in the stomach as it is also mechanically digested through peristalsis. Chemically, like the human stomach, cheif cells of the stomach lining secrete pepsinogen, an inactive form the enzyme pepsinigin. The parietal cells in the stomcah wall secrete HCl which activates the pepsinogen into pepsin to be used in the hydrolsis of proteins. The folds in the stomach wall are called rugae and are used to increase the surface area of the stomach wall so that there are more cells available to secrete HCl and pepsinogen.
Fish: As observed during dissection the stomach of the fish was practically closed, but had many folds in it. The folds in the stomach allow the fish's stomach to expand when it has consumed a full meal and not becoming uncomfortable.


The pancreas is an accessory organ to the digestive system. The pancreas secretes important digestive enzymes into the small intestine as well as hormones like insulin and glucagon directly into the blood.
Fetal Pig: The pancreas of the pig contains two sections, one that runs along the duodenum and one that runs across the duodenum.
Dieseases: Damage to the liver could result in diabetes in humans. Because the pancreas controls the blood levels in the circulatory system through hormones activating the glucagon in the liver, damage to the "control center" would cause low or high blood sugar levels. These blood levels can be deadly and cause uncomfortable symptoms, such as dizziness, numbness in the limbs and temporary loss of vision.



The liver is also an accessory organ. It is responsible for synthesizing bile, to break down lipids, as well storing excess energy by converting glucose to glucagon.
Fetal Pig and Fish: As observed through diessection and the above images, both the fetal pig and fish have a liver. One difference that should be taken note of is the difference in size between the two livers. The liver of the fetal pig takes up a majority of the pig's abdonminal cavity, while the fish's liver is small in size. The difference in sizes relate to the diets of the fish and liver and also the other functions of the liver. First, the diet of the fetal pig involves more lipids(fats) than the fish and thus more bile is needed to break down the lipids. Another function of the liver than would explain the size difference is the storage of glucagon. The fetal pig would need to store more because of its range of activity. A pig moves with a varying amount of energy output, while a fish stays, for the most part, at a constant rate of motion. Because the fish does not need bursts of energy, then it does not need stored glucagon.

Small Intestine:


Small Intestine: The stomach is connected to the small intestine by the pyloric sphincter. This connective region helps regulate the transfer of chime into the small intestine. The small intestine conducts most of the “body’s enzymatic hydrolysis of macromolecules and most absorption of nutrients into the blood.” (Campbell) Nutrients go through the lumen in the small intestine while passing through many small microscopic finger-like structures called microvilli. In the central part of each villi is a blood vessels and a lacteal. Because there are so little components within the villi, the nutrients passing through the intestine only has to pass through the inner skin of the intestine and past the lacteal to reach the blood vessels thus leaving little division between the bloodstream and the nutrients. Due to the folded like structure of villi, there are many tightly packed within the small intestine increasing the surface area of the inside to allow as much absorption as possible through the inner intestine. Another factor to enhance digestion is the length of the small intestine. In herbivores the small intestine is often found to be longer than in carnivores because the herbivores need to spend more time digesting because of the tough cell walls they ingest. After material passes through the small intestine having most of its nutrients digested and absorbed, it moves to the large intestine through the cecum.
Fetal Pig: The right two images above are key examples of the extenuating length of the fetal pig's intestine. The intestine measured to about 328 cm long which in comparison to the length of the pig (36.5 cm) is really long! As discussed before the microvilli within the intestine help increase the surface area in ensure maximum absorption of nutrients however, another way to increase this surface area is through length. The vein like structure in the top left small intestine set of photos is the mesentary of the small intestine which is where the blood vessels are in which nutrients is absorbed into. In the picture with the blue gloves the right side is what the small intestine appears like inside the pig and the right side is what the intestine looks like stretched out. Often in textbooks we see the intestine stretched out in a long tube like seen in the floor picutre but, they are infact a compilation of twisted knots loosely held together by a thin connective membrane, known as the mesentary. Because of this condensed state, the body is able to then fit a longer organ inside and not take up too much space.

Cecum: The cecum in humans has virtually little function (was used thousands of years ago when the human diet consists more of vegetation) and is very short and small. This pouch-like structure of the colon is found between the Illeocal valve and the ascending colon and is really consdiered to be the first part of the large intestine. In humans, the cecum has virtually little function and is very short and small acting as mearly a passage-way into the rest of the colon. However the cecum in a herbivore like a koala helps with the break down of their extensive plant diet and has enzymes in it that “functions as a fermentation chamber where symbiotic bacteria convert the shredded leaves into a more nutritious diet” (Campbell). Therefore, the cecum found in a koala is often more extensive and much longer, and known to be up to to 2m long. Because humans can not really digest many plant materials like cellulose that herbivores use their cecum to digest, human cecum's and especially carnivore cecum's have a really short cecum that is even sometimes entirely replaced by an appendix.

Large Intestine:


Large Intestine: The large intestine, also known by it's greek name as the colon, is largerly responsible for removing and reabsorbing water and electrolytes from the ileal contents (liquid material) passing through that came from the small intestine at the ileocecal valve. As the materials passes through the digestive tract from peristalsis, they become less and less useful because of the re-absorption of its water and nutrients. Thus towards the end of the large intestine little nutrients is left and the material that passed through is mainly waste. This waste becomes increasingly solid because of the constant loss of water and is called feces. Feces mainly contain cellulose which has no caloric value to animals because of inability to digest. Feces also have bacteria- many of which are harmless and even helpful in the colon. After feces has reached the end of the large intestine, they are stored in the rectum until they are excreted from the animal.
Fetal Pig: The large intestine was coiled and held together with a thin-membrane like substance called mesentary, as mentioned earlier in the small intenstine, in order to fit into the body. The top picture on the left shows the large intestine still connected to the cecum of the pig however, at this point the small intestine had been removed. Like the small intestine the large intestine was knotted up and as seen on the top right picture, was never fully unknotted and strung out. It is very hard to find the cecum in the fetal pig because thier diet of nutrients from the mother has little use for a cecum therefore it was very small.
Fish: The large intestine in the dissected fish is on the bottom picture of the group and is the beige tube running along the bottom of a dark yellow fat indicated by the scalpel. The intestine acts similarily as the fetal pig's and completes the same function.

Rectum- The rectum is located after the large intestine and is where the feces, after having a majority of its moisture reabsorped into the body, is stored until is it released from the body.

Anus- The anus is where the feces is released from the body as a form of waste.


Frog: In addition to the fetal pig and fish, we also dissected a frog. The digestive system of the frog was overall very similar to that of the fetal pig. The main differences we found during dissection was the anatomy of the frog's organs. The fetal pig had an obvious separation of the thoracic cavity and the abdominal cavity. In contrast, the frog's organs were concentrated in one cavity (the liver of the frog was surrounding the heart and lungs). The frog's digestive system also lacked the cecum that was present in the fetal pig. This is because of the frog's diet and its minimal consumption of cellulose. Overall, the frog's digestive anatomy was very similar to that of the fetal pig.


- Starfish (Echinodermata)

- Mussel (Mollusca)

- Grasshoppers (Romalea)

- Crayfish (Arthropod)

Starfish (Echinodermata):


Similar to vertebrates, starfish have a stomach in the center of the body. However, starfish are unique in that they have two stomachs, the cardiac stomach and the phyloric stomach. The cardiac stomach, shown in the picture on the left, can be pushed out of the body to digest larger animals, like clams, and then engulf the food. Once engulfed, the digested food moves to the pyloric stomach which is located on top of the cardiac stomach. Then, the digestive glands, called phyloric ceca, secrete digestive enzymes in each arm of the starfish. These glands also absorb the food to be absorbed and used in the starfish body. Because the phyloric ceca (shown in the arms of the starfish in both pictures) extend down the length of each arm, the need for an advanced circulatory system is greatly reduced.

Mussel (Mollusca):

The digestive system of mussels is relatively similar to that of a vertebrate with a mouth, stomach, intestine, and anus. The mussel is a filter feeder and obtains food using their gills as a net for catching whatever material flows through. Once the edible material is removed from the water, it is moved ot the food grove and then filtered to discover the edible material, which constists mainly of micro-algae, micro-invertebrates, and bacteria. The food is then moved to the mouth, down the esphogus, and into the stomach. The stomach does both mechanical and chemical digestion to break the food into smaller particles. The stomach is relatively small because as a filter feeder not much food is ever needed to be stored, and as a mussel not much energy is required to survive. The bivalves, which surround the stomach, secrete digestive-enzymes, along with the digestive glands. Once the food is broken down enough, is is carried by ciliary to digestive diverticulum. Digestive cells in the digestive diverticulum move the food into food vacuoles within the cells so that the cells can use the nutrients. The undigested particels and waster travel through the intestine to the rectum and then out of the anus.

Grasshoppers (Romalea)

Grasshopper: The grasshopper ingests it's food through its mouth and swallows the food down the esophagus where the beginning of digestive juices are secreted like saliva. Next the chewed food moves into the crop where it is stored. Food also moves through the gizzard that has thick muscular walls that further help the process of mechanically breaking up the food. The gizzard is an organ unique to mostly birds and insects and is often described as having a horny like texture used to grind the food the mouth didn't fully break down. Continuing in the digestive tract, the chewed food moves into the stomach passing through the gastric caecae which is a compilation of six digestive glands that produce digestive enzymes to aid the stomach in the breaking apart of the chewed food. Depending on the diet of the grasshopper helps determine the types and amount of enzymes released. The stomach of the insect is where most of the absorption of nutrients take place for the rest of he digestive tract besides the crop is lined with chiton. After the food moves through the stomach it is transfered through the intestine and out the malpighian tubules.

Crayfish (Arthropod)

Crayfish: The crayfish has a digestion system similar to that of it's fellow invertibrates having a foregut, midgut, and hindgut sections for the food to pass through to. Also food passes through the mouth, stomach, digestive gland, intestine and anus very similarly to the grasshopper. One aspect unique to the crayfish's digestive system is the enlarged stomach in the foregut. Located in the stomach of the crayfish are chitenous teeth which forms a "gastric mill" that both mechanically and chemically grinds up the ingested food of the animal. By looking both at the disected animal and the labeled diagram, its easy to see that the stomach takes up almost the entire foregut of the animal. Another organ unique to the digestive system of the crayfish is the green gland
that can be found right behind the brain of the crayfish. This gland helps filter fluid waste from the blood and though may not entirely be associated with the digestive system, it does effect the nutrients and filtration of nutrients from the foods that the animal ingests.

As demonstrated through examples of invertebrates, the digestive system of an invertebrate has many more variations than that of a vertebrate. An additional example of a digestive system of an invertebrate is that of a sponge. A sponge has a gastrovascular cavity where food is consumed into the cavity and then directly absorbed into the organism. All these differences in digestive systems are a result of the different environments organisms live in. They must adapt in order to survive and breakdown the sources of food around them. The diversity of digestive systems reflect the diversity of environments throughout the world.

In the invertebrate Hydra the inner gastrodermis is specialized for digestion. Enzymes released from gland cells into the gastrovascular cavity initiate digestion, which is completed intracellularly after small food particles are taken into the nutritive cells of the gastrodermis by phagocytosis (Campbell 800). [From Reproductive/Excretory System]

Fish (Chordata)

When food enters the fish's body it goes down a short and expandable esophagus that is lined with muscle. This allows it to swallow large objects. The food then enters a stomach that is a bent muscular tube. Here, gastric glands release substances to break down the food to prepare it to be digested. After the stomach the food moves into the intestine where enzymes that have been secreted by the pancreas digest the food. Here most of the absorption takes place. The length of the intestine varies between types of fish. In herbivore fish it is usually long and coiled and in carnivorous fish it is short. This is due to the amount of time that it takes for the intestine to break down the food. After this the waste then leaves the body through the anus.

This shows the digestive system of the fish. Unlike many of the organisms that we dissected, the fish only has one intestine as opposed to a large and a small intestine.

--The Circulatory System Group