Evolution of Digestive System

  

Sponges:	They are sessile filter feeders. This means that they remain in one place as an adult and the food they acquire filters through the pores. Microscopic food particles brought by the water are engulfed by collar cells and are digested by them on food vacuoles or they are passed to the amoeboid cells for digestion.
Hydra/Polyp (Cnidarians):	The cells of gastrodermis secrete digestive juices that pour into a central cavity. The enzymes begin the digestive process, which is completed within food vacuoles when small pieces of prey are engulfed by cells of gastrodermis.
Flatworm/Tapeworm (Planarians):	The digestive organ is tripartite (having 3 branches) and ramifies throughout the body. It begins with a pharynx (throat) which ejects from the mouth to suck food particles into the digestive organ.
Roundworm:	The head of a nematode has a few tiny sense organs and a mouth opening into a muscular pharynx where food (mostly bacteria and detritus) is pulled in and crushed.
Molluscs
Clam (Bivalve):	The clam is a filter feeder. Food particles and water enter the mantle cavity by way of the incurrent siphon, a posterior opening between the 2 valves. Mucous secretions cause smaller particles to adhere to the gills and cilia action sweeps them toward the mouth. The digestive system includes a mouth, a stomach and an intestine which coils about in the visceral mass and then goes right through the heart before ending in an anus. The anus empties at an excurrent siphon, which lies just above the incurrent siphon.
Snail (Gastropod):	Snails have sets of jaws inside their mouth used to cut off bits of food. Just behind the jaws the digestive tract is swollen to form a large buccal mass with muscles attached. This area is covered by the radula, the snail version of the human tongue. Radula moves back and forth very rapidly to grind up pieces of food. It wears away with use, but is continuously replaced since it is formed in a radular sac at the end of buccal mass and grows constantly like human finger nail. The teeth are fastened to radula in rows. Snails may have up to thousands of individual teeth with tiny cutting points called cusps. The oesophagus leaves the buccal mass and passes from the foot into visceral mass within shell to form a crop. Behind the crop is a dilated stomach, which is followed by the long intestine, whose posterior end is dilated to form the rectum. The anus opens into the mantle cavity near the edge of the mantel and the shell.
Octopus (Cephalopod):	Octopuses catch prey with their arms and then kill it by biting with their horny beat like jaws and the radula or tooth ribbon, paralysing the prey with a nerve poison and softening the flesh. They then suck of the flesh.
Annelids (Earthworm):	The earthworm feeds on leaves or any other organic matter living or dead that can be taken conveniently into its mouth along with dirt. Food drawn into the mouth by action of the muscular pharynx is stored in crops and is ground up in a thick muscular gizzard. Digestion and absorption occur in long intestine, whose dorsal surface is expanded by a typhlorde that allows additional surface area for absorption. Notice that the tube with in a tube plan has allowed specialisation of digestive system to occur.
Arthropods (Insect):	The mouth contains grinding mouth pats. Food in the mouth is mixed with saliva, which contains enzymes that begin the process of digestion. The crop functions in storage. Upon leaving the crop, food enters the gizzard where it is ground into smaller particles. Most chemical digestion occurs in the stomach. Absorption of nutrients occurs in the stomach and pouches that are attached to the stomach called gastric caeca. Absorbed nutrients move into hemocoel. The intestine functions mostly to absorb water.
Echinoderms (Starfish):	Starfishes feed on molluscs. When a starfish attaches a clam, it arches its body over the shell and by the concerted action of the tube feet, forces the clam to open. Then it everts a portion of its stomach to digest the contents of the clam. The mouth of a starfish opens into a narrow oesophagus which in turn leads to an expanded stomach. The stomach has 2 portions: the sac like cardiac which can be everted as described and the narrower pyloric which is connected to a short intestine. The anus opens on the aboral or upper end of the animal.
Chordates:	The digestive system is very complex in vertebrates. It consists of the gastrointestinal tract (GIT), an external tube extending from the mouth to anus, through which the swallowing, digestion and assimilation of food and elimination of waste products are accomplished. The system includes large digestive glands, liver and pancreas.
Fishes:	The mouths shape is good clue to what fish eat. The larger it is bigger the prey it can consume. Fish have a sense of taste and may sample items before swallowing if they are not obvious prey items. The stomach and intestines break down (digest) food and absorb nutrients. Fish such as bau that are piscivorous (eat other fish) have fairly short intestines because such food is easy to chemically break down and digest. Fish such as tailpia that are herbivorous (eat plants) require longer intestines because plant matter is usually tough and fibrous and more difficult to break down into usable components. The function of pyloric caeca is not entirely understood, but it is known to secrete enzymes that aid in digestion, may function to absorb digested food or do both. The liver has a number of functions. It assists in digestion by secreting enzymes that breakdown fats and also serves as a storage area for fats and carbohydrates. The liver is also important in destruction of old blood cells and it maintains proper blood chemistry as well as playing a role in nitrogen (waste) excretion.
Amphibians:	The frog’s mouth is where digestion begins. It is equipped with feeble, practically useless teeth. These are present only in the upper jaw. The frog’s tongue is highly specialised. Normally the tip of its tongue is folded backward toward the throat. From this position the frog can flick it out rapidly to grasp any passing prey. To better hold this prey, the tongue is sticky. Food passes from frog’s mouth into stomach by way of oesophagus. From the stomach the food moves into small intestine where most of digestion occurs. Large digestive glands, the liver and the pancreas are attached to digestive system by ducts. A gall bladder is also present.
Reptiles:	Except for most snakes, reptiles have a cacum. The stomach of crocodilians has two compartments. The 1st is similar to the glandular stomach of mammals. All reptiles have a gall bladder. The liver of many reptiles contains melanin and can have black spots or streaks. Reptiles generally have little subcutaneous fat and store fat in discrete masses (fat bodies) in caudal abdomen.
Birds:	Birds digest food quickly. They cannot afford the extra weight. They have no teeth; breakdown of food occurs in the gizzard – sometimes birds swallow rocks to assist the process. The crop stores food; mother brings regurgitate food stored in the crop to their babies. Waste exists through the cloaca and sodo eggs.
Mammals:	The digestive tract is a tube with coils which begins at the mouth and ends at a closed anus. It processes food which moves by peristalsis (waves of involuntary muscle contractions) through the process of digestion, absorption and elimination. The general pattern is to have an oral cavity, pharynx, oesophagus, stomach and intestine. Accessory organs are pancreas, liver and galls bladder which arise as invaginations from embryonic digestive tract.
Primates:	Most primates are nearly or exclusively herbivorous but their digestive tract does not show the high degree of morphological specialisation seen in many other herbivorous. Even some of the smallest primates, which until recently were believed to be carnivorous, subsist on plant food. In humans the large intestine is relatively less voluminous than in apes (which are predominantly plant eater) but nevertheless, humans are surprisingly effective at digesting cellulose.