69. Phylogeny and development of digestive system..

Overview of the Digestive System

• The digestive system is essentially a continuous muscular tube running from the mouth to the anus or cloaca.
• It performs three key functions:
• Ingestion and breakdown of food (digestion)
• Absorption of nutrients into the bloodstream
• Elimination of undigested waste from the body
• Major components of this system include:
• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus or cloaca
• Several accessory organs assist digestion, such as:
• Teeth and tongue
• Salivary glands
• Liver
• Gallbladder
• Pancreas
• Digestion is mainly enzyme-driven. Enzymes, especially hydrolases, are produced in an inactive form known as zymogens. These are activated only when they reach their destination to prevent damage to the organs that produce them. For example, pepsin is secreted as pepsinogen and activated by stomach acid (HCl).

Phylogeny of the Digestive System (Evolutionary History)

1. Protochordates

• Filter feeders that use cilia to move food and water through their large pharynx
• Numerous slits in the pharynx trap food particles from seawater
• A liver-like structure forms as an outpouching of the intestine, located just behind the pharynx
• Gas exchange takes place efficiently across pharyngeal slit walls
• Due to small size and low activity, there is no need for a dedicated respiratory system

2. Vertebrates

• As vertebrate bodies increased in size and complexity, cilia-based food movement became insufficient
• This led to the evolution of a muscular pharynx, better suited for active feeding

3. Cyclostomes (Lampreys and Hagfishes)

• Oral cavity is lined with mucous glands
• Lampreys produce anticoagulant secretions to feed on host blood
• Both lampreys and hagfishes have a muscular tongue, which is not the same in origin as the mammalian tongue
• They possess cone-like structures resembling teeth, evolved independently, lacking dentin (an example of convergent evolution)
• Lampreys use their round suction-like mouth to attach to prey and rasp tissue
• Hagfishes use horn-like dental plates on their tongue to grip and tear food
• They have a short esophagus and a straight, compact intestine with a spiral fold to increase absorption surface
• A very basic pancreas is present

4. Cartilaginous Fishes

• Possess a well-developed pancreas
• The intestine features a spiral valve, increasing the area for nutrient absorption

5. Amphibians

• The transition to land brought new adaptations
• Evolution of salivary glands helped in moistening food to aid swallowing
• Tongue became muscular and mobile, aiding in food capture and manipulation
• Supported by a modified skeletal structure called the hyoid apparatus
• During metamorphosis, pharyngeal slits close off